Service Manuals, User Guides, Schematic Diagrams or docs for : xerox alto memos_1976 BitBLT_Jan76

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                  Inter-Office Memorandum


    To             Alto Users                                                   Date           Jan uary 28,
    1976


    From           Dan Ingalls & Diana Merry                                    Location        Palo Alto


    Subject        Bit Boundary Block Transfer                                  Organization    LRG
                   (Bit BLT)



XEROX
    THE "Bit BLT" package is a group of subroutines that implements various operations on display
    windows. Among their several uses are displaying characters, scrolling, and "painting" with brushes.
    The operations which can be performed using Bit BLT are not necessarily confined to uses with the
    Alto display, but the description below focuses on simple cases with the display in mind. More
    complicated possibilities are left to the reader to invent as necessity and curiousity dictate. The
    binary files required are available as BitBLTB.Br, BitBLTA.Br, BitBLT.Mb in  BitBLT.Dm
    The sources, BitBLTB.Bcpl, BitBLTA.Asm, BitBLT.M u are available on .


    Dcfinitions

    A bit map is a region of memory that describes a collection of scan-lines which have a base core
    address (bea) and bit map width (bmw). the later being a word value. Our discussion will assume
    that scan-lines run horizontally from left to right (bits are addressed from x = 0 to x = bmw*16-1).
    Scan-lines appear consecutively in a bit map. Thus the point (0,0) is at the upper left of the
    display bit map -- y ;: 0 is the first scan-line, the next y = 1, etc. The core address of the first
    word of scan-line 0 is bea. The number of scan-lines is not relevant, for the purposes of Bit BLT.
    (Note that these conventions are similar to those for the Alto display bit map; if ben and bmw are
    both even. the bit map may be displayed using standard Alto facilities.)

    A block is a rectangle within a bit map. It.has four corners which need not fall on word
    boundaries. Any given block is described by a block descriptor whose contents are:

                  Bit map's base core address (bca)
                  Bit map's width in words (bmw)
                  Block's Lcftx ("x offset'~)
                  Block's Topy ("y offset")
                  Block's \Vidth
                  Block's Hcight

    Thus the block is defined by:
                Leftx S. x S. Leftx + Width-1
                Topy S. y S. Topy + Height - 1.

    It is left to the caller to insure that ranges of x and y indeed fall within the bit map.
                                                                                                         2


The Bcpt routine contains a structure declaration for managing this descriptor:

            structure BITRECfANGLE :
                          [
                          bca
                          bmw
                          Leftx

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