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               THE MAXC MICROPROCESSOR
                       MAXC 0.1

                    March 2, 1972


                   B u t 1 er Lampson
                        Ed Fiala
                    E d McCreight
                    Chuck Thacker




           Xerox Palo A l t o Research Center
                   3180 P o r t e r Drive
                P a l o Alto, CA 94304
--,                                                                                                                      -
              ,   I
      f




                      MICROPROCESSOR / Lampson,              e t al.                       MAXC 8.1 / Page 2
                      Xerox P a l o Alto Research C e n t e r                              March 2, 1972


          C           1.0 Overview
                            1.1 N o t a t i o n
                      20 C o n t r o l
                       .
                            2.1   Interrupts
                            2.2   Flag Register
                      3.0 Ar it hme t ic/Lo g i c Sect i o n
                            3.1 R e g i s t e r Banks
                            3.2   P and Q R e g i s t e r s ; C y c l e and Mask
                            3.3   A r i t h m e t i c and Logic O p e r a t i o n s
                            3 . 4 Communication w i t h t h e Bus
                      4 . 0 Local Memories
                            4.1   Scratch Pad Memory
                            4.2   D i s p a t c h Memory
                            4.3   Map
                            44
                             .    Instruction Memory
                      5 . 0 Memory I n t e r f a c e
              .       6.0 Maintenance I n t e r f a c e ,
                      7 . 0 Disk C o n t r o l


                      Appendix      A     Summary of M i c r o i n s t r u c t i o n Bits
                      Appendix      B     Summary of Branch C o n d i t i o n s
                      Appendix      C     Summary of P r i m a r y a n d Secondary F u n c t i o n s
                      Appendix      D     Summary of Bus S o u r c e s and D e s t i n a t i o n s
                      Appendix      E     Summary of F l a g R e g i s t e r B i t s
                      Appen3ix      F     Summary o f System Maintenance I n t e r f a c e I n s t r u c t i o n s   .
                                          ( N o t written yet)
                      Appendix G          More Than You R e a l l y Wanted t o Xnaw About D i s k Control
                      Table   1..         I n s t r u c t i o n S e q u e n c i n g and S t a c k Actions
                      Table   2.          P Input S e l e c t i o n
                      Table   3.          Q Input Selection
                      Table   4..         ALU F u n c t i o n s
                      Table   5 0 .       KSET-, KCSET-, and KSTAT Bus B i t s

                      Figure 1.           MAXC Processor O r g a n i z a t i o n




          c
         MICROPROCESSOR / Lampson, e t al,                                           MAXC 8,1 / Page 3
         X e r o x P a l 0 A l t o Research Center                                  ,March 2, 1 9 7 2



    c.   1 0 Overview
          .                                            \




                 The MAXC m i c r o p r o c e s s o r i s i n t e n d e d t o be a r e a s o n a b l y
         g e n e r a l p u r p o s e p r o c e s s o r , c u s t o m i z e d t o some e x t e n t f o r PDP-18
         emulation.              It w i l l be u s e d a s a c e n t r a l F r o c e s s o r and d i s c
         c o n t r o l l e r i n t h e MAXC s y s t e m ,                   Physically, the          processor
         o c c u p i e s 24 c a r d p o s i t i o n s i n t w o Augat c a r d cages (19'1 x 8,7tt) ,
         a n d t h e d i s c c o n t r o l o c c u p i e s 8 card p o s i t i o n s i n a t h i r d c a g e .
         F i g u r e 1 i s a l o g i c a l block d i a g r a m of the p r o c e s s o r ,                It is
         o r g a n i z e d a r o u n d a 3 6 - b i t b u s , on which a l l t r a n s f e r s b e t w e e n
         s u b s e c t i o n s of t h e machine Occur, Data t r a n s f e r s t o a n d from
         this b u s a n d a l l o t h e r f u n c t i o n s i n t h e machine a r e under c o n t r o l
         of a 7 2 - b i t m i c r o i n s t r u c t i o n W o r d ,          A machine may be c o n f i g u r e d
         w i t h e i t h e r 1 0 2 4 or 2 0 4 8 words of i n s t r u c t i o n memory.

                 Two f i e l d s in e v e r y m i c r o i n s t r u c t i o n s p e c i f y a b u s s o u r c e ,
         w h i c h l o a d s d a t a o n t o t h e b u s , and a b u s d e s t i n a t i o n which reads,
         and u s u a l l y stores, t h e d a t a , Sometimes a s i n g l e v a l u e o f t h e
         s o u r c e o r d e s t i n a t i o n f i e l d may s p e c i f y a d d i t i o n a l o p e r a t i o n s , o r
         s e v e r a l d i f f e r e n t s o u r c e o r d e s t i n a t i o n v a l u e s may s p e c i f y t h e
         same b u s o p e r a t i o n s , T f r e s e p e c u l i a r i t i e s a r e s p e c i f i e d i n t h e
         a p p r o p r i a t e s e c t i o n of this manual.            The s o u r c e s a n d d e s t i n a t i o n s
         a r e l i s t e d a n d t h e i r p r o p e r t i e s summarized i n Appendix D                      m      In
         g e n e r a l a n y s o u r c e may be s e n t t o a n y d e s t i n a t i o n , w i t h t h e
         f o l 1 o w i n g " e x c e p t i o n : a s l o w s o u r c e may n o t be s e n t t o a s l o w
    c    destination.
         S l o w s o u r c e s are:
                        a l o c a l memory
                        NOT F
                        t h e ALU;
                        KSTAT a n d KUNXT i n t h e d i s k i n t e r f a c e

         s l o w d e s t i n a t i o n s are:
                        a l o c a l memory
                        Y i f the next i n s t r u c t i o n c o n t a i n s PQRCYY
                        Q if t h e n e x t i n s t r u c t i o n c o n t a i n s QODE o r QEVEN

                 T h e r e a r e also t w o f u n c t i o n f i e l d s F l a n d F2 w h i c h i n v o k e
         v a r i o u s a c t i o n s supplementary t o the source-destination                 scheme ,
         These actions a r e s p e c i f i e d w h e r e a p p r o p r i a t e t h r o u g h o u t t h e
         manual a n d summarized i n Appendix C.
                 Th'e m a c h i n e i s SYnChrOnOuS, w i t h a c y c l e t i m e of 150 n s .        The
         t e c h n o l o g y w i t h which t h e p r o c e s s o r i s implemented is 74H TTL;
         ICgs a r e mounted on w i r e - w r a p cards, and t h e back p a n e l s a r e a l s o
         wire-wrapped.              An e x c e p t i o n i s t h e 1 0 2 4 x 1 9 - b i t memory c a r d
         w h i c h i s u s e d f o r the i n s t r u c t i o n , d i s p a t c h , map, a n d s c r a t c h
~   c.   m e m x i e s ; t h i s card i s a p r i n t e d c i r c u i t . A l l cables e x i t t h e
         processor            from the rear edges of t h e cards.                         N o special
    MICROPROCESSOR / Lampson, e t a l .                                       MAXC 8,1 / Page 4
    Xerox P a l o Alto R e s e a r c h C e n t e r                            March 2, 1972


    mechanical p r o v i s i o n s are r e q u i r e d f o r c a b l i n g . The p r o c e s s o r i s
    cooled b y a f a n u n i t which mounts i m m e d i a t e l y below t h e p r o c e s s o r
    c a r d cage, a n d powered by a power s u p p l y mounted on t h e bottom o f
    the cabinet.
           The e x t e r n a l i n t e r f a c e s t o t h e p r o c e s s o r are shown d a s h e d i n
    F i g u r e 1, and c o n s i s t of the f o l l o w i n g :
           1,        8 disc u n i t cables, which c o n n e c t t h e d i s c control
                     p o r t i o n of the p r o c e s s o r t o 8 2 3 1 4 or . 3 3 3 8 t p e d i s c
                     files;

           2.        2 memory p o r t cables, which c o n n e c t t o t w o p o r t s of t h e
                     MAXC memory system.        T h i s memory is a 512K              (expandable
                     t o l V 2 4 K ) x 4 0 b i t (+8 e r r o r c o r r e c t i o n and d e t e c t i o n
                     b i t s ) dynamic MOS system.       Access time a n d c y c l e t i m e
                     a r e 880 ns,       One p o r t i s u s e d f o r d i s k t r a n s f e r s , t h e
                     second for CPU t r a n s f e r s -
                     One i n t e r p r o c e s s o r communication cable (labeled "TO
                     NOVA").        T h i s i n t e r f a c e has two E u n c t i c n s .
                     a.    I t carries i n t e r p r o c e s s o r ccmmunication strobes
                           between all p r o c e s s o r s o f the MAXC system.                 All
                           normal           communication b e t w e n processors o c c u r s
                           t h r o u g h memory, a n d these s t r o b e s serve t o i n d i c a t e
                           t h e p r e s e n c e of messages i n mailbox l o c a t i o n s known
                           t o a l l processors.
                     b.    I t i s c o n n e c t e d t o a c o n t r o l l i n g minicomputer (Data
                           G e n e r a l Nova), which h a s t h e t a s k of monitoring t h e
                           s y s t e m for error's and abnormal. c o n d i t i o n s .         This
                           i n t e r f a c e i s used. for debugging microcode i n t h e
                           processor under c o n t r o l of a debugger i n t h e Nova.
                           The control memory of t h e microFrocessor i s loaded
                           v i a t h i s i n t e r f a c e a t s t a r t up, d u r i n g debugging,
                           and when errors o c c u r d u r i n g normal o p e r a t i o n .

    1.1 N o t a t i o n
                      i n t h i s document a r e i n decimal unless followed
           A l l .numbers
    by a      i n which case t h e y are o c t a l .
                B,                                        Thus, 10 = 12B.
    Arithmetic is 2s complement.

           Names f o r f i e l d s i n t h e m i c r o i n s t r u c t i c n are in Appendix A,
    Registers, memories and d a t a paths are named L, R, P, Q, X, AC,
    Y,     B    (bus)       S    ( s c r a t c h p a d ) I D ( d i s p a t c h ) MAP, I ( i n s t r u c t i o n
    memory) , NPC, STACK, IMA ( i n s t r u c t i o n memcry a d d r e s s ) , MAR, MDR,
    MDRL      (low 4 bits of the 4 0 b i t memory word) BALUBC ( b u s and A L U
    branch c o n d i t i o n s ) 8 F ( f l a g register) 8                   ( o u t p u t of arithmetic-
c   l o g i c u n i t ) , G, H, J, K t o (F r e g i s t e r b i t s )       .
       MICROPROCESSOR / Lampson, e t a l .                                        MAXC 8.1 / Page 5
       Xerox Palo A l t o Research C e n t e r                                    March 2 , 1 9 7 2


               B i t s i n registers             (and on d a t a ' p a t h s l i k e B and ALU) a r e
       r e f e r e n c e d by i n t e g e r s i n b r a c k e t s f o l l o w i n g t h e r e g i s t e r name,
       c o u n t i n g from t h e l e f t a s t h o u g h the r e g i s t e r (or p a t h ) were 36
       b i t s wide.        Numbering r e g i s t e r s i n t h i s way i s compatible w i t h
       PDP-10 d o c u m e n t a t i o n ( i t would otherwise b e better t o number from
       t h e : r i g h t ) , Thus B[0) i s t h e sign b i t of t h e b u s , Y[27 J i s t h e
       s i g n b i t of the 9 - b i t Y r e g i s t e r , and B C 9 - 1 2 7 i s t h e AC f i e l d of
       a PDP-10 i n s t r u c t i o n on t h e bus. For 4 0 - b i t r e g i s t e r s l i k e MDR,
       the e x t r a 4 b i t s a r e MDR[36-39].
                 I f A is a number w i t h g b i t s and B a number w i t h                        bits, then
       (A,B)        is a number w i t h a+b bits a n d
                            (A, B) [ (36-b) - 3 5 ]=B
             ,   (




                                                      -
                            (A,B) [ (36-a-b) ( 35-b) ]=A
               D e s t i n a t i o n names a l w a y s a p p e a r a s NAME- a n d t h e y are t h e
       o n l y names i n t h i s manua1,which a r e w r i t t e n w i t h a f i n a l to-.                 If
       a r e g i s t e r i s both a s o u r c e and a d e s t i n a t i o n , these a r e a l w a y s
       c a l l e d NAME (the s o u r c e ) a n d NAME- ( t h e d e s t i n a t i o n )             Also, some
       o p e r a t i o n s c a n be i n i t i a t e d by e i t h e r prircary o r s e c o n d a r y
       f u n c t i o n s , a n d t h e s e a r e g i v e n t h e same name i n F1 and F2,                When
     . a f i e l d i n t h e m i c r o i n s t r u c t i o n is u s e d t o address a memory M,
       t h e f i e l d i s c a l l e d MA ( @ . g o ,                 LA,    RA,          SA).       Sources ,
       d e s t i n a t i o n s , and functions p e r t a i n i n g t o t h e disk c o n t r o l
                              *




c.     s e c t i o n of t h e . m i c r o p r o c e s s o r h a v e names b e g i n n i n g w i t h llKflr
                                  . ..
              The word 4 1 i l l e g a 1 1 f      means llmust be avoided by the programmer,
       s i n c e t h e . r e s u l t i s n o t w e l l - d e f i n e d by t h e i m p l e m e n t a t i o n of t h e
       processor.Ic             T h e - h a r d w a r e does not check f o r i l l e g a l o p e r a t i o n s .
                        MAP




      II
       1




6
W36

 L.        IMAWTEN- I
         MICROPROCESSOR / Lampson, e t al.                                                            MAXC 8 . 1 / Page 6
         Xerox Palo A l t o R e s e a r c h C e n t e r                                               March 2, 1972


c-       2.0 Control

                 The c o n t r o l s e c t i o n of t h e p r o c e s s o r c o n s i s t s o f a n 1 1 - b i t
         program counter (NPC) , a n 1 1 - b i t x 1 2 - l e v e l s u b r o u t i n e s t a c k ,
         gating            t o produce an i n s t r u c t i o n memory address, a n d t h e
         i n s t r u c t i o n memory.
                 T h e p r o c e s s o r h a s s i n g l e i n s t r u c t i o n lookahead, L e o, t h e
         f e t c h of a n i n s t r u c t i o n o c c u r s d u r i n g t h e e x e c u t i o n of t h e
         previous instruction.                    A l l i n s t r u c t i o n s r e q u i r e one c y c l e f o r
         execution.          A n i d l e c y c l e ( d u r i n g which a n i n s t r u c t i o n is f e t c h e d
         from t h e c o n t r o l memory, b u t no i n s t r u c t i o n is e x e c u t e d ) o c c u r s
         o n l y a f t e r a read or w r i t e of t h e i n s t r u c t i o n memory.             Execution
         of a n i n s t r u c t i o n c a n be d e l a y e d one o r more c y c l e s by t h e memory
         i n,-e r f a c e ; see s e c t i o n 5,
             t
                       Three           fields     of         t h e m i c r o i n s t r u c t i o n a r e used for c o n t r o l .
         T h e s e a r e a n e l e v e n - b i t b r a n c h address f i e l d    (BA) , a f i v e - b i t
         f i e l d (BC) w h i c h s p e c i f i e s o n e of 3 2 c o n d i t i o n s t o be t e s t e d t o
         d e t e r m i n e w h e t h e r a b r a n c h is t o be d o n e , and a t m - b i t f i e l d
         which s p e c i f i e s t h e t y p e of branch                  (BT).     The BT f i e l d i s
         i n t e r p r e t e d as follows:

                               -
                               TYPE
                                0
                                                                 EFFECT
                                                                 GOTO (BA f i e l d ) I F ( c o n d i t i o n )
                                   1                             CALL (BA f i e l d ) I F ( c o n d i t i o n )
                                   2                             RETURN I F ( c o n d i t i o n )
                                   3                             DGOTO (BA f i e l d ) I F ( c o n d i t i o n )
              c--^--
                   I_    . . ..
                         .     .

         I f BT = DGOTO a n d t h e b r a n c h c o n d i t i o n i s t r u e , n o i n t e r r u p t c a n
         o c c u r a f t e r t h i s i n s t r u c t i o n (see s e c t i o n 2.1) ,
                                            -- - - - -   -       -                  -

             The c o n d i t i o n selected b y BC (see Appendix B) i s tested, a n d
         if  t r u e , t h e b r a n c h s p e c i f i e d by BT occurs,                  The     branch
      c o n d i t i o n s which t e s t t h e v a l u e s of t h e ALU c u t p u t and t h e b u s
      r e f e r t o t h e v a l u e s computed b y t h e p r e v i o u s i n s t r u c t i o n ( u n l e s s
     c
     -F1 = FRZBALUEC and INT=8 i n t h a t i n s t r u c t i o n , i n which case t h e y
        --
      have t h e same r e s u l t t h a t t h e y would have had                              in     that
     ;instruction).             T h o s e which test b i t s i n r e g i s t e r s r e f e r t o t h e
      value a t - t h e b e g i n n i n g of t h e c u r r e n t i n s t r u c t i c n , Note t h a t t h e
      complement of e v e r y b r a n c h c o n d i t i o n i s a l s o a branch c o n d i t i o n .
                                                  .._._"_ -
                                                        ,        .   - -   .-   -       .   .   -.-


              T a b l e l a s p e c i f i e s how t h e n e x t i n s t r u c t i o n a n d t h e n e x t
      program c o u n t e r . (NPC) v a l u e a r e d e t e r m i n e d b y t h e c u r r e n t
                                                             '



      i n s t r u c t i o n and t h e i n t e r r u p t system.              Note t h a t a deferred
      b r a n c h .(DGi)TD) " a X l B w s t h e n e x t i n s t r u c t i o n i n s e q E n c e t o be
     < e x e c u t e d before s e n d i n g c o n t r o l t o t h e l o c a t i o n s p e c i f i e d by BA.
      The e f f e c t o f a DGOTO can t h e r e f o r e be c a n c e l l e d ' b y a GOTQ o r
      RETURN            i n t h e n e x t i n s t r u c t i o n , a n d a CALL i n t h e n e x t
      i n s t r u c t i o n w i l l push the address s u p p l i e d by t h e DGOTO,                     The
      e f f e c t of DGOTO B[25-353             is p r o v i d e d by F2=LOADPC.
         MICROPXOCESSOR d Lampson, e t al,                                                 MAXC 8 1 / Page 7
                                                                                                 .
         X e r o x P a l o A l t o Research C e n t e r                                    March 2 , 1972


                 The         12-level      s u b r o u t i n e stack h o l d s r e t u r n links for
         s u b r o u t i n e c a l l s and i n t e r r u p t s , The ways i n which the stack can
         be a f f e c t e d by t h e c u r r e n t i n s t r u c t i o n are s p e c i f i e d i n Table l b .
         T h e STACK- d e s t i n a t i o n p u s h e s two 11-bit f i e l d s from t h e b u s o n t o
         t h e - s t a c k ; normally t h i s i s combined w i t h F2=LOADPC t o provide a
         3-level d i s p a t c h .         I t is i l l e g a l t o do a RETURN i n                           the
         i n s t r u c t i o n following o n e w h i c h does STACK-.                    N o e x p l i c i t PUSH
         o p e r a t i m i s provided, s i n c e t h e same e f f e c t c a n be obtained b y
                            LOADPC, I3[25-35 ]-argument                    t o be p u s h e d ;
                            CALL .+l;

         The       s t a c k c a n be r e a d o n t o t h e b u s , (right j u s t i f i e d ) ; it i s
         i l l e g a l t o do t h i s i n a n i n s t r u c t i o n w h i c h h a s a CALL o r PUSH o f
         t h e stack,



                                          . .




'   c.
         - .
         ....


         . .-
         .
         --.a     - .                                                         .   *



         . . .--.- .
         ..       .     .




                                                .     .
                                                    . . -
                                                          _
                                                              .
                                                                  . . --
                                                                  -
                                                                      -




         ,- . .
             MICROPROCESSOR / Lampson, e t a l .                                          MAxC 8 1
                                                                                                .        / Page   a
             Xerox Palo A l t o Research Center                                           March 2, 1972

                                                                                                                      (r

             A c t i o n of C u r r e n t            Address o f Next                   Next V a l u e
             Instruction                             I n s t r u c t i o n (XMA)        of NPC
             No b r a n c h                                   N PC                      NPC + 1
             N o branch G i n t e r r u p t                   INTADR                    NPC
             GOTO                                             BA                        BA + 1
             GOTO G i n t e r r u p t                         INTADR                     BA
             DGOTO                                             NPC                       BA
             DGOTO G i n t e r r u p t       --not     a l l o w e d ; i n t e r r u F t is delayed-
             CALL                                              BA                        EA + 1
             CALL E I n t e r r u p t                          INTADR                    BA
             RETURN                                            STACK                     STACK + 1
             RETURN & i n t e r r u p t             I NTADR          STACK
             Note :   F2=LOADPC makes the n e x t v a l u e of NPC                                   be B[25-35]
                      r e g a r d l e s s of w-hat is s a i d above,

                                        T a b l e la. , I n s t r u c t i o n S e q u e n c i n g



         *   A c t i c n of C u r r e n t            Effect on S t a c k
             1n s t r u c ti on

lC           CALL                                             PUSH NPC
L    .       RETURN                                           POP
             F1 O r F2=POP*                          POP
             BD=STACK-.*                                      PUSH B[1-111, t h e n             .
                                                              P U S H BC13-23 J

                   * I l l e g a l i t h e same i n s t r u c t i o n w i t h CALL o r RETURN
                                    n

                                              Table lb:          S t a c k Actions
             MICROPROCESSOR / Lampson, e t al,                                         mxc     8.1      Page 9
             Xerox P a l o Alto Research C e n t e r                                   March 2, 1 9 7 2



             21
              .      Interrupts
                     An i n t e r r u p t system i s p r o v i d e d t o allow h i g h s p e e d d e v i c e s
             s u c h a s t h e d i s k s t o be s e r v i c e d , The e l e m e n t s of t h e i n t e r r u p t
             s y s t e m are:
                   1..      A   flag,  INT, which d e t e r m i n e s w h e t h e r t h e processor i s
                            i n normal mode o r i n i n t e r r u p t mode,
                   2.       D u p l i c a t e copies of some p r o c e s s o r registers; see below
                            for d e t a i l s .
                   3,       A    1 6 - b i t A R M r e g i s t e r , o n e b i t per i n t e r r u p t c h a n n e l ,
                            T h i s r e g i s t e r may be a b u s d a t a s i n k o r s o u r c e (selected
                            by f u n c t i o n s ) ,    An     i n t e r r u p t r e q u e s t for which t h e             '


                            c o r r e s p n d i n g ARM b i t i s 0 i s i g n o r e d .
         .         4,   -   A
                            2.2)    -
                                   single      interkupt          e n a b l e f l a g i n t h e F r e g i s t e r (see

             The       f i r s t 1 6 m i c r o i n s t r u c t i o n s a r e reserved for a n i n t e r r u p t
             t r a n s f e r vector, When a n i n t e r r u p t o c c u r s , the i n s t r u c t i o n i n
             l o c a t i o n n ( 0 5 n L 17B) is e x e c u t e d and XNT i s s e t , T h e
             i n t e r r u p t i n s t r u c t i o n i s s i m p l y sandwiched i n t o t h e n o r m a l flow                 .
.
    c,
    --       of c o n t r o l , so t h a t when i t is i n e x e c u t i o n , NPC c o n t a i n s t h e
             a d d r e s s of t h e i n s t r u c t i o n w h i c h t h e program would have e x e c u t e d
             d u r i n g t h a t c y c l e i f t h e i n t e r r u p t had n o t o c c u r r e d . The
             i n t 2 r r u p t i n s t r u c t i o n must c o n t a i n an u n c G n d i t i o n a 1 CALL t o s a v e
             NPC on t h e s t a c k and s e n d c o n t r o l t o t h e s t a r t of the i n t e r r u p t
             r o u t i n e , T h e last i n s t r u c t i o n of t h e i n t e r r u p t r o u t i n e s h o u l d b e
             a RETUZN w h i c h i n c l u d e s t h e I R E T f u n c t i o n , T h i s f u n c t i o n c l e a r s
             INT and r e s t o r e s t h e s t a t e t o i t s p r e - i n t e r r u p t v a l u e , - See below
             for a d e s c r i p t i o n of t h e t i m i n g .
                     The scheme just described w o r k s o n l y i f e v e r y t h i n g c u r r e n t l y
             known a b o u t t h e s e q u e n c i n g of t h e main program i s c o n t a i n e d i n
             the NPC v a l u e , S i n c e t h i s i s n o t the case immediately a f t e r t h e
             e x e c u t i o n of an i n s t r u c t i o n w h i c h loads NPC w i t h a n y t h i n g e x c e p t
             IMA + 1, a n i n t e r r u p t i s n o t permitted t o o c c u r a f t e r s u c h a n
             i n s t r u c t i o n , b u t must w a i t f o r a more o p p o r t u n e moment.            Only
             i n s t r u c t i o n s c o n t a i n i n g F2=LOADPC o r a s u c c e s s f u l DGOTO have t f i s
             problem, and t h e processor a u t o m a t i c a l l y i n h i b i t s an i n t e r r u p t
             from o c c u r r i n g in t h e c y c l e a f t e r t h e s e i n s t r u c t i o n s ,
                   It       is     t h e programmer's r e s p o n s i b i l i t y t o i n h i b i t i n t e r r u p t s
             i n other cases where t h a t           is n e c e s s a r y by s e t t i n g F2=INHI,NT,
             T h i s must be done                      - .-   I                       . ..
                   1.       If BD = RMW-                  or F1 = RMWREF o r RMWTiEFDXK, s i n c e a n
    c                       i n t e r r u p t c a n n o t be a l l o w e d d u r i n g t h e Wl p h a s e of a Raw
                            memory r e f e r e n c e , T h e p r o c e s s o r a u t o m a t i c a l l y i n h i b i t s
                            i n t e r r u p t s a f t e r e v e r y i n s t r u c t i o n of t h e RM p h a s e
      MICROPROCESSOR / Lampson,              et al,                                                           MAXC 8.1 / Page 10
      Xerox P a l o A l t o R e s e a r c h C e n t e r                                                       March 2 , 1972


                                   except  the f i r s t , so t h e programmer need p r o v i d e F2 =
                                   INHINT  o n l y on the i n s t r u c t i o n which starts       the
                                   reference.
                    2..            If BD = WRITE- o r 5'1 = WREF or WREFDXK and MDR does n o t
                                   y e t c o n t a i n t h e d a t a which i s t o be w r i t t e n (see s e c t i o n
                                   5).         If      a n o t h e r i n s t r u c t i o n i s e x e c u t e d before MDR i s
                                   loaded, t h e programmer must have F2 = INHINT on t h a t
                                   instruction also,                       I t i s n o t n e c e s s a r y t o INHINT on a n
                                   i n s t r u c t i o n c o n t a i n i n g WRESTART, b u t i f - b y t h e e n d o f
                                   t h e i n s t r u c t i o n a f t e r t h e WRESTART, MDX i s n o t loaded,
                                   t h e n t h a t i n s t r u c t i o n must I N H I N T ,        I t i s not     necessary
                                   t o have F2 = I N H I N L ' on t h e i n s t r u c t i o n which l o a d s MDR,
                                   s i n c e a n i n t e r r u p t . a f t e r t h a t i n s t r u c t i o n c a u s e s no
                                   t.ro ubl e
                                   -   ~   ..
                                                  .
                                                  ~.



      When i n t e r r u p t s a r e i n b i b i t e d , a n y pending i n t e r r u p t i s s i m p l y
      delayed.           N o - p e n d i n g i n t e r r u p t r e q u e s t i s l o s t . Note t h a t a l o o p
      c o n s i s t i n g e n t i r e l y of i n s t r u c t i o n s w i t h s u c c e s s f u l D G O T W s ,
      LOADPC's or I N H I N T ' S w i l l lock . o u t i n t e r r u p t s i n d e f i n i t e l y .
      -:.o
      t
              Because m i c r & i n t e r r u p t r o u t i n e s a r e used for data t r a n s f e r s
            and from t h e d i s k packs,                  it i s i m p o r t a n t t o avoid t i m e -
      consuming s t a t e s a v i n g and r e s t o r i n g b y m i c r o - i n t e r r u p t r o u t i n e s . .
      W i t h ' a s i n g l e d i s k u n i t i n o p e r a t i o n , e a c h a d d i t i o n a l micro-
      i n s t r u c t i o n i n t h e i n t e r r u p t r o u t i n e r e d u c e s t h r o u g h p u t by 1%.
      C o n s e q u e n t l y , c o n s i d e r a b l e extra hardware h a s been p u t i n t o
      a u t o m a t e s t a t e s a v i n g and r e s t o r i n g d u r i n g i n t e r r u p t s .
      ..   ..
      :.     . During        non- i n t e r r u p t             instruction    execution,     duplicate
      XegLsters for P,. X , . Y and BALUBC a r e loaded whenever t h e p r i m a r y
      registers a r e loaded., . During an i n t e r r u ~ t , however,                               these
      d u p l i c a t e r e g i s t e r s remain f r o z e n a t t h e i r former v a l u e s .           The
      p r i m a r y X, Y , and BALUBC r e g i s t e r s a r e lo a d e d from t h e d u p l i c a t e s
      by t h e - IRET f u n c t i o n , . The- f i r s t - i n s t r u c t i o n - . of t h e i n t e r r u p t
      l m u t h e i s " e x p e c t e d t o - s a v e NPC on t h e s t a c k b y c a l l i n g t h e
      i n t e r r u p t r o u t i n e , . and t o . s a v e Q i n o n e of t h e r e g i s t e r banks,
     -say:-*.-at,r-    SAVEDQ;: : t h i s -:
                                     .             is why- d u p l i c a t e s f o r Q and NPC a r e n o t
      provid2d.. The f i n a l - i n s t r u c t i o n of t h e i n t e r r u p t r o u t i n e must,
      t o cestore a l l t h e s e t h i n g s , i n c l u d e :
      _. . -
     --.---I
        _.I.. 9 ~         -- -I..I_~  - - . . .. - . . . _ _--.
                                        .
                                                            -                .
                                                                            ..   _-              1       >-   f




      ': :-
      I    ,
           :   ..            *I IRET., RETURN, Q-SAVEDQ,                 PlP1
      -.             - .
                     I     I



                           -
                               .



                               .                _ .    .-"   "   -   .   -   I   ~   ..
                                                                                      .   .
                                                                                           . .
      A- ' d u p l i c a t e - r e g i s t e r - f o r K U N I T i s a l s c F r o v i d e d , b u t this is
      handled . i n a d i f f e r e n t way, d i s c u s s e d i n s e c t i o n 7 ,             A l s o note
      t h a t - .AC and- F- axe.-n o t - d u p l i c a t e d . (because i n t e - r r u p t r o u t i n e s o n l y
      c h a n g e F i n t e n t i o n a l l y and d o n ' t u s e AC)                                .
             The i n t e r r u p t -system a c c e p t s 1 6 l e v e l s c a l l e d i n t e r r u p t
c.    r e q u e s t s : ( I R E Q i , 1 = 0 t o 15). I n t e r r u p t i w i l l Occur a f t e r the
      e x e c u t i o n of the c u r r e n t i n s t r u c t i o n i f :
       MICROPROCESSOR / Lampson, e t a l .                                            EAXC 8 1 / Page 1 1
                                                                                            .
       Xerox Palo A l t o Research C e n t e r                                        March 2, 1972



c-                   1 -
                      .    INT      = 0 (i.e. , n o i n t e r r u p t is i n p r o g r e s s ) or FI =
                           PREIRET i n t h e p r e v i o u s i n s t r u c t i o n . Note t h a t t h i s
                           i m p l i e s t h a t if P R U R E T is n o t used, a t l e a s t one non-
                            i n t e r r u p t i n s t r u c t i o n i s e x e c u t e d a f t e r each i n t e r r u p t
                           . r o u t i n e i s done, before t h e next one is s t a r t e d . To
                            a v o i d t h i s , t h e n e x t t o l a s t i n s t r u c t i o n of                 the
                            i n t e r r u p t r o u t i n e s h o u l d s p e c i f y F1 = PREIRET.      The n e x t
                            i n s t r u c t i o n a f t e r one which h a s PREIRET must h a v e I R E T .
                     2.    IENABLE       ( a f l a g r e g i s t e r bit) = 1. When t h e i n t e r r u p t
                           s y s t e m is d i s a b l e d a l l i n t e r r u p t s h a v e t o w a i t .
                     3.    N o READ-MODIFY-WRITE is i n i t s RM p h a s e (i.e.,                                  has
                           s t a r t e d t o read b u t n o t s t a r t e d t o write).
                     4 -
                      .    The c u r r e n t i n s t r u c t i o n does          n o t have F2 = INHZNT o r
                           LOADPC or a s u c c e s s f u l DGOTO,

                     5..   i i s t h e l a r g e s t number for w h i c h A R M i AND I R E Q i = 1.

       Changes i n t h e v a l u e of A R M or IENABLE do n o t a f f e c t t h e
     . i n t e r r u p t system u n t i l t h e s e c o n d f o l l o w i n g i n s t r u c t i o n . Thus i f
       i n s t r u c t i o n i clears IENABLE, a n i n t e r r u p t may o c c u r ( i f t h e
       o t h e r c o n d i t i o n s a r e s a t i s f i e d ) a f t e r i o r a f t e r i+l, b u t will

c      n o t occur a f t e r i + 2 .




       .,   *-   .




c,     . .
,   e




            MICROPROCESSOR / Lampson, e t a l .                                       MAXC 8,1 / Page 12
            Xerox P a l o Alto Research C e n t e r                                   March 2 , 1 9 7 2


        c   2a2     F l a q Reqister
                    The 3 6 - b i t f l a g r e g i s t e r F s e r v e s as a r e F o s i t a r y f o r v a r i o u s
            f l a g s i n t h e p r o c e s s o r and p r o v i d e s a number of g e n e r a l - p u r p o s e
            s i n g l e b i t f l a g s w h i c h c a n be c o n v e n i e n t l y nianipulated, Some
            b i t s of F a r e s e t ox cleared by assorted e v e n t s i n t h e p r o c e s s o r ;
            t h e s e a r e mentioned i n c o n n e c t i o n w i t h t h e d e s c r i p t i o n of t h e
            r e l e v a n t e v e n t a n d summarized in Appendix E. I n a d d i t i o n , there
            a r e o p e r a t i o n s which wowk on a l l t h e b i t s of F;
                  NOT F                              reads NOT F o n t o t h e b u s
                  SETF [ s ]                         sets t h e b i t s of F which a r e 1 i n S [ s ]
                  SETFCC s , cond ]                  does SETFCs] i f t h e b r a n c h c o n d i t i o n i s
                                                     t r u e . _ ( t h e r e w i l l a l s o be a b r a n c h i f t h e
                                                     condition is t r u e )
                  CLEARns J                          c l e a r s t h e b i t s of F which a r e 1 i n S[sJ
                  CLEARFCC s, cond J                 dQes CLEARFCs] i f t h e branch c o n d i t i o n i s
                                                     t r u e (there w i l l also be a b r a n c h i f t h s
                                                     condition is true)
                              ,
                  SETFBC s cond]                     does S E T F l s ] i f t h e b r a n c h c o n d i t i o n i s
                                                     t r u e , CLEARF [ s ] i f i t i s f a l s e ( t h e r e
                                                     w i l l a l s o be a b r a n c h i f t h e c o n d i t i o n is
                                                     true).
                  SETSF[ s 3                         sets b i t s of F selected by S[ s J[ 32-35 J
        c                                             ( i a e a , K,. J, H , a n d G ) i f (F A N D S[sJ AND
                                                     -20B) # 0 ,
            A l l of these a r e s p e c i f i e d by f u n c t i o n s e x c e p t NOT F, w h i c h i s a
            bus s o u r c e , SETSF[ s3 is also a F2, G , w # J and K are b i t s of F
            w h i c h c a n be t e s t e d by branch c o n d i t i o n s ; t h e y c a n also be set i n
            a v a r i e t y of ways (see Appendix E .     )
                  For i =        ,6,    1,   . . ., 35,      if
                                       the i n s t r u c t i o n c o n t a i n s SETF, CLEARF o r SETFB, o r
                                       i f i t c o n t a i n s SETFC o r CLEARFC and t h e b r a n c h
                                       c o n d i t i o n i s t r u e , or i 2 32 a n d i t c o n t a i n s SETSF;
                                       b i t i of t h e word r e a d from S i s 1            ;
                                       bit      i of t h e f l a g r e g i s t e r ( F [ i J) is b e i n g s e t or
                                       cleared i n d e p e n d e n t l y by some o t h e r p a r t of t h e
                                       processor ;.
            then t h e new v a l u e of F[ i ] i s t h e OR of t h e v a l u e i t would hav2
            gotten from (1) a n d ( 2 ) above, a n d t h e v a l u e i t would h a v e g o t t e n
            from ( 3 ) above. .




             -               -                   :                -              I
        MICROPROCESSOR / Lampson, e t a l .                                                                  MAXC 8 1 1 Page 13
                                                                                                                   .
        Xerox P a l o Alto Research C e n t e r                                                              March 2, 1972



c       3.0 Arithrnetic/Loqic Section

               The a r i t h m e t i c / l o g i c s e c t i o n of t h e p r o c e s s o r is shown i n the
        u p p e r l e f t q u a r t e r of F i g u r e 1.         I t c o n s i s t s of two r e g i s t e r .
        b s n k s L and R w i t h 32 r e g i s t e r s p e r bank, two working r e g i s t e r s P
        and Q,       m u l t i p l e x i n g for i n p u t s t o P a n d Q, and a 36-bit
        arithme t i c / l o g i c     unit     (ALU)                       .
        3.1     R e q i s t e r Banks
                The t w o r e g i s t e r banks a r e addressable from t w o f i v e - b i t
        f i e l d s L A and RA i n t h e m i c r o i n s t r u c t i o n , or from t h e low o r d e r
        f i v e b i t s of the 8 - b i t X.- r e g i s t e r , or from t h e 4 - b i t AC register.
        T h e s a u r c e of a r e g i s t e r bank address is determined by t h e
        a p p r o p r i a t e A f i e l d as f o l l o w s :
                                                         %         *




    .         1)       A = 0 or 1   : t a k e t h e address frcm X
              2)       A = 2 o r 3:       ta&e t h e address frcm AC
              3)       A = 4:    a d d r e s s _ r e g i s t e r 4 , b u t n e v e r write i n t o i t
                               - (see be$ow)
              4)       A > 4:    address, register A
                The above r u l e s imply t h a t r e g i s t e r s 0 - 3 c a n only be
        r e f e r e n c e d frcm X o r AC, (and r e g i s t e r 4 can be s t o r e d i n t o o n l y
'C      when addressed v i a X or A .                @      Far t h e l e f t bank, i f LA = 1 (3) a n d
        X [32-35]=0(AC=B) , t h e i n s t r u c t i o n w i l l read t h e v a l u e                     0
        r e g a r d l e s s of t h e c o n t e n t s of t h e r e g i s t e r addressed a n d w i l l n o t
        write i n t o t h e r e g i s t e r bank. T h i s k l u d g e i s p o v i d e d so t h a t
        PDP-18            indexing         and         self-instructions          can    be   emulated
        conveniently.            R A = 1 , 5 3 ) . 1 is t h e same a s RA=0 ( 2 ) .

              The X r e g i s t e r c a n , be: goaded from            f       1   -




                   B 28-35']
                    [
                   B 14--17.3 ( P D P 4 0 ,Lindex-.f i e l d ) ':-- -
                    [                                                        .       -__- - .

                   B[ 6-11"] j+dJPpI?-.lOl y t e ; - p o l n t.e r s i. z .e. . f i e l d )
                             . -. .
                               CL
                                                  b               .      .
                                                              - .
                           c c : - .c: 1 -.:r. 1.
                                             :      ....     -&
                                                              .-         -             "




              The AC reg:ist eg Fan, be . & a d e d - h b m l .                 .__ ..~
                                                                                   -
                   Bc 32-35.)             -'  5;:          ,,
                                                         . .
                                                                -
                                                                  . . . ...
                                                             '.-,:.I
                                                                           -                I   ~   .^
                                                                                               - ._._ .
                                                                                                    .-
                                                                                                    a    (
                                                                                                         .
                                                                                                             -
                                                                                                                 r
                                                                                                                     "



                   B[ 9-12;]..      2.-(P?P-,0. AC. f i e l d )   . . .           .. . . . . ,
                                                                                       I-
                                                                                                  . ._.                  ~




                              - --C)P P : 5 ;- 2."
                                               ,,
                                                     '

                Both r e g i , s t e r s may be incremented and . decremented w i t h
        f u n c t i o n s a n d may: be read-onto: t h e b u s ( r i g h t j u s t i f i e d )               X[ 30-
        35.1-   may a l s o - b e r e a d , o n t o - t h e b u s - l e f t - j u s t i f i e d ( L e o, into B [ P
        $3; t h i s p u t s i t i n t h e PDP-10 b y t e p o i n t e r p s i t i o n f i e l d . Two
        b r a n c h c o n d i t i o n s e x i s t t o t e s t t h e s i g n of X.                  The value of X
         ( b u t n o t .AC) i s p r e s e r v e d across an i n t e r r u F t .
                I n each i n s t r u c t i o n i t i s p o s s i b l e t o a r e a d from or w r i t e into
c.       ( b u t n o t b o t h ) t h e l e f t r e g i s t e r bank, and i n d e p e n d e n t l y t o do
        t h e same w i t h t h e r i g h t r e g i s t e r bank.             T h e d e c i s i o n on whether
        t o read or w r i t e is made a s follows. I f t h e r e g i s t e r bank is
     MICROPROCESSOR / Lampson, e t a l ,                                     MAXC 8 - 1 / Page 1 4
     Xerox Palo A l t o Research C e n t e r                                 March 2, 1972


     addressed by PS o r QS, i t i s read, ` O t h e r w i s e , it i s w r i t t e n
     u n l e s s t h e microinstruction a d d r e s s e s r e g i s t e r 4 , i n which case
     n o t h i n g i s done, Note t h a t L A = 1 or 3 may o v e r r i d e t h i s for t h e
     l e f t bank i f r e g i s t e r 0 i s addressed by X[32-351 o r AC.
     3.2        and Q     Rcqisters; C y c l e a n d Mask

           The        m u l t i p l e x e r s on t h e i n p u t s t o P and Q are under c o n t r o l
     of t w o f i e l d s PS a n d QS i n t h e m i c r o i n s t r u c t i o n ,   The p o s s i b l e
     i n p u t s f o r t h e working registers selected b y t h e s e f i e l d s a r e
     g i v e n i n T a b l e s 2 and 3.          P a n d Q a r e always l o a d e d w i t h t h e d a t a
     s p e c i f i e d by these t a b l e s w i t h two e x c e p t i o n s ; P i s n o t l o a d e d
     if F 1 = LDPALUH AND [email protected]=H; P 0 is n o t l o a d e d i f FZ=ASHOVF.
                                                    . .~
              When P is l o a d e d from a n y t h i n g e x c e F t B, P1 o r ALU RSH 1 it
                                                                        -
                                                                                                                 '


     is possible t o mask t h e i n p u t with 2**n                   1, L e a , keep the
     r i g h t m o s t n b i t s of i n p u t a n d z e r o t h e rest. This a c t i o n is
     selected by one of four f u n c t i o n s :
                   Function                        -
                                                   N
                   SAMASK                          SA
                   BMIASK                          BAS
                   AMASK                           AF ( l i m i t s N to     <    40B)
                   XMASR                           X register
     where t h e mask l e n g t h n = M A X ( 3 6 , N mod 64).                   If F l is n o t o n e of
     t h e s e f o u r , no masking takes p l a c e ,
            Note that t h e mask a n d PS f e a t u r e s allow a n a r b i t r a r y f i e l d
     t o be e x t r a c t e d from P (or Q, u s i n g RCYQQ or NOTALU, u s i n g
     RCYNOTALUQ) and p u t i n t o P r i g h t j u s t i f i e d . The f i e l d c a n be
     s p e c i f i e d either by t h e i n s t r u c t i c n ( u s i n g one of SA, BA and AF)
     o r b y t h e X(3length) and Y ( r i g h t c y c l e r e q u i r e d ) r e g i s t e r s .
           F2=ASHOVF,           i n a d d i t i o n t o i n h i b i t i n g t h e l o a d i n g of PB, s e t s
     t h e f l a g r e g i s t e r b i t OVF t o 1 i f P0#P1; t h e i n t e n d e d u s e is t o
     s e t O V F i f a l e f t s h i f t would have changed t h e s i g n of P. T h e r e
     are branch c o n d i t i o n s (QODD, QEVEN) t o t e s t t h e bottom bit of Q
     a t t h e `start of t h e i n s t r u c t i o n .                   They a r e i l l e g a l i f Q w a s
     loaded from a slow s o u r c e i n t h e l a s t h s t r u c t i . c n .
             I n normal mode (INT = 0 ) I b o t h P a n d P1 are loaded when
     l o a d i n g of P is specified by t h e i n s t r u c t i o n . I n t h e i n t e r r u p t
     r o u t i n e s (INT=1) , t h e l o a d i n g of P1 is i n h i b i t e d . P1 t h u s
     preserves t h e c o n t e n t s of P a c r o s s t h e i n t e r r u F t r o u t i n e .    The
     h s t i n s t r u c t i o n of t h e i n t e r r u p t r o u t i n e s h o u l d therefore h a v e
     PS = P 1 a s w e l l as IRET.

c.
          MICROP'ROCESSOR / Lampson, e t a l e                                                                                                                                  MAXC 8,l / Page 15
          Xerox P a l o Alto Research C e n t e r                                                                                                                               March 2, 1 9 7 2


          3.3                      _ A r i t h m e t i c and Loqic O p e r a t i o n s

           -      The A L U c a n compute a l l 1 6 Boolean f u n c t i o n s of P and Q as
          w e l l as a number of a r i t h m e t i c f u n c t i o n s ,           Its operation i s
          c o n t r o l l e d by a 5 - b i t f i e l d i n t h e i n s t r u c t i o n c a l l e d AF. The
          v a l u e s of A F which produce t h e v a r i o u s ALU f u n c t i o n s a r e
          s p e c i f i e d i n Table 4.                                                                     '




                 The a r i t h m e t i c f u n c t i o n s ( A F 1 2 0 ) are a f f e c t e d by t h e v a l u e
                                                                  :
         of C A R R Y I N , which i s 0 unless one of t h e f u n c t i o n f i e l d s s e l e c t s
         1.( F l o r F2=CARRYl) or 3 ( F l = S E T J C B C A R R Y J )                                                                                                      .
                                                                                                                                                                            )



                 I n a d d i t i o n t o t h e 3 6 - b i t r e s u l t s p e c i f i e d by T a b l e 4 , the
         ALU p r o v i d e s three a d d i t i o n a l b i t s f o r the a r i t h m e t i c f u n c t i o n s
         s t a r r e d i n T a b l e 4-,
                       ALWCB                                    is t h e c a r r y o u t of b i t 0 from t h e twos-complement
                                                                add s p e c i f i e d i n p a r e n t h e s e s i n Table 4,
                           ALUCl                                is t h e c a r r y o u t of b i t 1
                           OVERFLOW                             i s ALUCiEl # ALUC1,                  I t is 0 i f t h e 3 6 - b i t twos-
                                             :-..               complement            result        correctly         represents       the
                                           -                    s p e c i f i e d f u n c t i o n , 1 i f t h e r e s u l t is wrong by
                                             , -
                                                                -
                                                                +235
                                                                                           ..                                         L_


                                                                                                                                      _ .

               The f k n c t i o n SETOVPC01 se-t.s Elag r e g i s t e r b i t s PC0 a n d PCl t o
         t h e values of ALUC0 a n d ALUCLkeS..pectively and crs OVERFLOW i n t o
     .   flag r e g i s t e r b i t OVF,         T h e " - ' f u n c t i o n SETJCBCLAXRYJ sets J t o
         ALUCB,    The. f u n. c t. i o n SETHQVF- sets H to ALUCB#ALUCl,
                                                                    . _ ,          ..       .

                 The v a l u e of                                                          t o 0 i s stored i n
                                                                          t h e 3 6 - b i t ALU o u t p u t r e l a t i v e
          BALURC and may be t e s t e d , b y a branch c o n d i t i o n i n t h e n e x t
         ,instruction.            AL'U8 (for PDP-lf3 f l o a t i n g p o i n t n o r m a l i z a t i o n ) and
         30-axe a l s o s t o r e d - i n EVUUBC -and. may'be tested.                        T h i s information                              ~"



         3's a u t o m a t i c a l l y p r e s e r v e d a c r o s s i n t e r r u p t s ,
                                                                              - -                 If   INT=ET a n d
         :Fl=FRZBALUBC-, B A L - U B C ' h -'frozen a t i t s p r e v i o u s v a l u e r a t h e r than
         b e i n g u p d a t e d to k e f i e c ' t '-the're'sults o t h e c u r r e n t i n s t r u c t i o n ,
         .      . , _- -
                r
               * - -
                       *

                       .
                         - . -
                           r
                               .

                               .
                                    __ .. r.,," , - _ . _.- - - . . . . ..". . -. .. . . .
                                    _  -
                                                         .
                                                                    - _
                                                                         ..
                                                                                    -.
                                                                                        -   -   I




                                                                                                ,
                                                                                                ?
                                                                                                        <


                                                                                                        . .-
                                                                                                         %
                                                                                                             .   .   P



                                                                                                                         I.   r
                                                                                                                                  . ' -
                                                                                                                                          ~
                                                                                                                                              ~




                                                                                                                                                  _ -       I       .




         3.4
         . -
         h..                   Communication w i t h thkl.3us                                                                                      -    .
                                                                              ?\..?-. - .-
                   ~




                       ) - . -.        '..          ,   -   .   r     .   ~                         I
                                                                                                                                          .       _     .       .       .

             ./  The a r i t h m e t k c j . l o g i c . . sec-ti"on- communicates w i t h t h e r e s t of
                                                     -      .
         -&he p r o c e s s o r v i a a 't h e - bus- .- (aside from f l a g b i t s and b r a n c h
         conditions),               A s mentioned above, X and AC c a n be l o a d e d from or
          read- o n t o t h e b u s , a n d , P -- or, Q can be l o a d e d fvom t h e bus.
          L o a d h g o f - P-and Q - i s . ' c o n t . r e l l e d -
                                                                    by PS and Q S a s described above
         knd does n o t r e q u i r e . t h e ' d - e s t i n a t i o n f i e l d . Note t h a t P and Q
                                                            :is,
         s$&L.>klwa s : ..- Zoa3e.d. ." o. -i&-the programmer' s r e s p o n s i b i l i t y t o
                          y                   s,
          ! ~ v L e s e l e c t P and Q S Q when he does n o t wish t h e v a l u e s t o
                   PS
          @ha_nge-. ' I n a d d i t i o n , ' Q and the ALU r e s u l t nay be read o n t o the
          b u s by- s p e c i f y i n g them a s s o u r c e s , and. t h e r e is a f u n c t i o n
          READALU t o or t h e ALU r e s u l t w i t h the b u s v a l u e s p e c i f i e d by the
(J        s o u r c e f i e l d , . Note t h a t t h e ALU is a slow bus source,
    t




             MICROPROCESSOR / Lampson, et al.                         MAXC 8.1 / Page 16
             Xerox Palo Alto Research C e n t e r                     Earch 2, 1972




             PS ( o c t a l )     P Input                            Notes (see n e x t page)
             0-4 6                PQ RCYC 0-461                      1
             47                   B                                  cannot be masked
             50                   P1                                 cannot be masked
             51                   ALU
             52                   A L U ARSHC 1 (P0-ALUCB)           c a n n o t be masked
             53                   L [email protected]                            4
             54                   L LsH[2]                           4.
             55                   L LSHCl]                           4
             56                   L
             57                   L RSH[ 13
             6&1                  L RSH[ 2 3
         .   61                   L RSH[3]
             62                   R LSH[ 3 7
             63                   R LsH[2]
             64                   R LSH[ 1 J
             65                   R
             66                   R RSHCl]
             67                   R RSHC2 J
             70                   R RSH[ 3 )
*       c,   71
             72
             73
                                  PQ LCY[3]
                                  PQ LCY[ 2 1
                                  PQ I J W ~ J
             74                   unused
             75                   PQ RCYCYJ                          1,2: I l l e g a l i f Y w a s
                                                                     loaded from a slow
                                                                     s o u r c e o n the pre-
                                                                     v i o u s instruction.
                                  PQ RCYC 44-Y]                      1,2,3:       Illegal i f Y
                                                                     was loaded on              the
                                                                     previous i n s t r u c t i o n ,
                                                                     or if I N T = = l . BEWARE.
                                  unused



                                  Table 2:     P Input S e l e c t i o n
                                  ."I_ -




--
-                                                                I                  I
    MICROPROCESSOR / Lampson, e t a l .                                      MAXC 8 . 1 / Page 1 7
    Xerox P a l 0 Alto Research Center                                       March 2 , 1 9 7 2


    Notes:
    1.. PQ        is    a    7 2 - b i t number which can have one of the following
    va h e s :

          Condition                      L e f t 36 b i t s      R i q h t 36 b i t s
          F l o r F2=RCYQQ               Q                       Q
          F1 or F2=RCYfiQ                0                       Q
          Fl=RCYNC>TALUQ                         NOT A L U               Q      (must have AF<20B)
          othe rwi s e                           P                       Q
    The  resulting           P i n p u t i s the leftmost 3 6 b i t s of t h e c y c l e d 72-
    bit number.
    2..       A l s o s e t s H to (Y I 44B)                  If Y > 4 4 B , then l e t C = (Y IF PS
    = PQ RCY Y ELSE 44B-Y MOD 100B IF PS = PQ RCY44E-Y)                                    The P i n p u t
    w i l l be PQ LCY (1,2,3) foy C = 77E,76E,75E.                             It w i l l be something
    w e l l - d e f i n e d b u t u s e l e s s o t h e r w i s e , L e e , if H is s e t t o 1 t h e
    r e s u l t i s probably wrong.
    3.     Note t h a t RCY44B-Y i s not the same as LCY Y , s i n c e i t is a l s o
    n e c e s s a r y to exchange P and Q.
    4..   Zeros a r e s h i f t e d into the v a c a t e d b i t p s i t i o n s .


                        Table 2:.       P Input Selection (continu.ed)




I
I
                                                                     I                  I
                                                                                                             .
                                                                                                             -
                                                                                                             -
**



     MICROPROCESSOR / Lampson, et al.                        MAXC 8 1
                                                                   .    1 Page 18
     Xerox Palo A l t o Research C e n t e r                 March 2,   1972            \




     es      *           p Input          Notes
         0               L
         1               R
         2               ALU

         3               B
         4               Q                Q       is a slow sink i f t h e n e x t
                                          i n s t r u c t i o n has BC=Q9DD o r QEVEN
                                      #


         5               R RSH 1          Q0iALU35 IF PS=ALU RSHl ELSE
                                          R35 IF * ELSE 0

         6               Q RSH 1          Q0qP35 IF F2=ASHOVE ELSE
                                          Q35 IF * ELSE 0

         7               Q LSH 1          Q35- (ALUB#G) IF FlzQ3SALUG ELSE
                                          [email protected] 1.F * ELSE 0


     *       F1 = RCYQQ or F 2 = RCYQQ or F1 = RCYNOTALUQ



                               Table 3: Q I n p u t Selection
?




    MICROPROCESSOR / Lampson, et a l .                                           MAXC 8 1 / Page 1 9
                                                                                       .
    Xerox P a l o Alto R e s e a r c h Center                                    March 2, 1 9 7 2




    AF              Result                                AF                R e s 



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