Commodore Amiga A500 Technical Reference Manual page 212

*NOTE BFD
=
Blitter finished disable. When this bit is true, the
Blitter Finished flag will have no effect on the Cop-
per. When this bit is zero, the Blitter Finished flag
must be true (in addition t o the rest of the bit com-
parisons) before the Copper can exit from its wait
state, or skip over an instruction. Note that the V7
comparison cannot be masked.
The Copper is basically a 2-cycle machine that requests the bus only
during odd memory cycles (4 memory cycles per in). This prevents
collisions with Display, Audio, Disk, Refresh, and Sprites, all of which
use only even cycles. It therefore needs (and has) priority over only
the Blitter and Micro.
There are only three types of instructions: MOVE immediate, WAIT
until, and SKIP
if.
All instructions (except for WAIT) require 2 bus cy-
cles (and two instruction words). Since only the odd bus cycles are
requested,
4
memory cycle times are required per instruction (mem-
ory cycles are 2 8 0 ns).
There are two indirectjump registers, COP1 L C and COPZLC. These
are 18-bit pointer registers whose contents are used t o modify the
program counter for initialization orjumps. They are transferred t o
the program counter whenever strobe addresses COPJMPl o r
COPJMP2 are written. In addition, COP1 LC is automatically used a t
the beginning of each vertical blank time.
It is important that one of the jump registers be initialized and its
jump strobe address hit, after power up but before Copper DMA is
initialized. This insures a determined startup address and state.
DIWSTRT
Display window start (upper left vertical-hori-
zontal position)
DIWSTOP Display window stop (lower right vertical-
horizontal position)
These registers control the Display Window size and position, by lo-
cating the upper left and lower right corners.
BIT# 15.1 4.1 3,12,1 1 , l
0,09.08,07,06,05,04,03,02,01,00
USE
v 7 v 6 v 5 v 4 v 3 v 2 v l v O h 7 h 6 h 5 h 4 h 3 h Z h l h O
DIWSTRT is vertically restricted t o the upper
2/5
of the display
(v8
=
O), and horizontally restricted t o the left
3 / 4
of the display
(h8
=
0).
DIWSTOP is vertically restricted t o the lower
l / 2
of the display
(v8
=
/
=
v7), and horizontally restricted t o the right
l / 4
of the display
(h8=
1).
DDFSTRT Display data fetch start (horiz.position)
DDFSTOP Display data fetch stop (horiz.position)