Atari 800

Display Instruction Format: Each instruction consist~ of ei t ner an
opcode only, or of an opcode followed by two (2) bytes of operand.
lopcodel ------Single Byte Display Instruction
lopcodel
!operandi ----Triple Byte Display Instruction
I operandi
The opcode is always fetched first and placed in the Instruction
Register. This opcode defines the type of instruction (1 or 3 bytes)
and will cause two more bytes to be fetched if needed. If fetched,
these next two (2) bytes will be placed in the Memory Scan Counter,
or in the Display List Counter (if the instruction is a Jump).
Display Instruction Register (IR): This register is loaded with the
opcode of the current display list instruction. It cannot be accessed
directly by the programmer. There are three basic types of display list
instructions: blank, jump, and display.
Blank
( 1-byte)
ID7ID6ID5ID41 Ol Ol Ol OJ
This instruction is used to create 1 to 8 blank lines on the
display (blackground color).
Jump
(3-bytes)
D7
D6
- D4
D3 -DO
1 = display list instruction interrupt
0-7 = 1-8 blank lines
0 = blank
ID7ID61 XI XI Ol Ol Ol 1!
This instruction is used to reload the Display List Counter.
The next two bytes specify the address to be loaded (LSB first).
D7
1
display list instruction interrupt
D6 0
jump
(creates one blank line on display)
1
jump and
wait
until end of next vertical blank
D5-D4
X
don't
care
D3-DO 1 jump
Display
(1 or 3 bytes) ID7ID6ID5ID4ID3ID2ID1!DOI
This instruction specifies the type of display for the next
display block.
D7
1
D6 0
1
DS 1
D4
1
D3-DO
2-F
display list instruction interrupt
1 byte instruction
3 byte instruction (reload Memory Scan Counter
using address in next two bytes, LSB first).
vertical scroll enable
horizontal scroll enable
display mode (memory or character map -
see following pages).
II.8