Atari ST series Technical Reference Manual page 161

BitBlt takes a bit image from one memory area and com­
bines it with a bit image in another memory area, according
to the logic operation selected. The source and destination
image rectangles are the same size. The width of this block
(in pixels) is stored in B__WD, and its height is stored in B__
HT, while the number of bit planes is passed in PLANE__CT
(this value may be changed by the BitBlt call). The starting
address of the source image block goes into S__FORM, while
the starting address of the destination image block is placed
in D__FORM. These image blocks must start on word boun­
daries.
The x and y offsets of the source image are passed in S__
XMIN and S__YMIN, while the corresponding destination
offsets are stored in D__XMIN and D__YMIN. These images
may overlap one another without harm, but bear in mind
that there is no clipping or boundary checking performed;
it's up to you to make sure that the blit stays within the im­
age memory block.
Two other values concerning the image block must also
be passed. The S__NXWD and D__NXWD variables should
contain the number of bytes between data words that belong
to the same bit plane. For image blocks in ST screen mem­
ory, this value is 2 for the high resolution screen, 4 for me­
dium resolution, and 8 for low resolution. S__NXLN and D__
NXLN should contain the width of each line in the image
block in bytes.
If the entire screen is used, this width is 80 bytes for the
monochrome screen and 160 bytes for each of the color
screens. S__NXPL and D__NXPL should contain byte offsets
from a word in one bit plane to a word in the next bit plane.
Because of the way the ST screen memory is interleaved, this
value is 2 for all ST screen resolution modes.
The way source and definition blocks are combined is
determined by the raster operation code (opcode) selected.
There are 16 possible logic operations. These are shown in
Table 7-7.
Table 7-7. The 16 Possible Logic Operations for Combining Source
and Definition Blocks
Opcode Logic Operation*
0 D1 = 0
1 D1 = S AND D
2 D1 = S AND (NOT D)
Line A Routines
Clear destination block
Description
153