Atari ST series Technical Reference Manual page 162

Table 7-7. The 16 Possible Logic Operations for Combining Source
and Definition Blocks (continued)
Opcode Logic Operation*
3 D1 = S
4 D1 = (NOT S) AND D
D1 = D
5
6 D1 = S XOR D
D1 = S OR D
7
8 D1 = NOT (S OR D)
9 D1 = NOT (S XOR D)
D1 = NOT D
10
11 D1 = S OR (NOT D)
12 D1 = NOT S
13 D1 = (NOT S) OR D
14 D1 = NOR (S AND D)
15 D1 = 1
S is the source image, D is the destination
after the operation.
The BitBlt function allows you to choose a separate logic
operation for each bit plane. Up to four opcodes, one byte
each in length, may be stored in the OP__TAB variable. The
opcode to select for a particular plane is determined by the
bit value of FG__COL and BG__COL for that plane. BG__
COL holds bit 0 of the opcode selection, and FG__COL holds
bit 1. Let's say, for example, you've selected a FG__COL of 5
and a BG__COL of 6. The binary equivalents are
FG = 0101
BG = 0110
In this example, plane 0 will use the opcode stored in
byte 2 of O P_TAB, since bit 0 of FG is 1 and bit 0 of BG is 0
(10 binary = 2). Plane 1 will use the opcode stored in byte 1
of OP__TAB, plane 2 will use the opcode stored in byte 3,
and plane 3 will use the opcode stored in byte 0 of
OP__TAB.
Logic operations can be confusing, particularly when
they involve multiple bit planes. If you wish to transfer a sin­
gle-plane source image to a multiplane destination, set
S__NXPL to 0, so the same source will be moved to all desti­
nations. To use FG__COL and BG__COL to represent actual
foreground and background colors (replace mode), set OP__
TAB to $F740. If you want the destination to use FG__COL
154
CHAPTER 7
Description
Replace mode
Erase mode
Destination unchanged
XOR mode
Transparent mode
Reverse transparent mode
Fill destination block
image, and D1 is the destination image