xerox BitBLT Jan76 free download

File name BitBLT_Jan76.pdf

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