Commodore Amiga A2000 - Figure 3-7 Memory Organization for a Basic Bit-Plane

_____________ _____________
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Mem. Location N Mem. location N+38
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Mem. Location N+40 | Mem. location N+78
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_____________ \|/ _____________
| | | | | | | | ___________V_________\ | | | | | | | |
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Mem. Location N+7960 Mem. location N+7998
Figure 3-7: Memory Organization for a Basic Bit-Plane
Access to bit-planes in memory is provided by two address registers, BPLxPTH and
BPLxPTL, for each bit-plane (12 registers in all). The "x" position in the name holds the
bit-plane number; for example BPL1PTH and BPL1PTL hold the starting address of PLANE
1. Pairs of registers with names ending in PTH and PTL contain 19-bit addresses. 68000
programmers may treat these as one 32-bit address and write to them as one long word.
You write to the high-order word, which is the register whose name ends in "PTH."
The example below shows how to set the bit-plane pointers. Assuming two
bit-planes, one at $21000 and the other at $25000, the processor sets
BPL1PT to $21000 and BPL2PT to $25000. Note that this is usually the
Copper's task.
;
; Since the bit plane pointer registers are mapped as a full 680x0 long-
; word data, we can store the addresses with a 32-bit move...
;
LEA CUSTOM,a0 ; Get base address of custom hardware...
MOVE.L $21000,BPL1PTH(a0) ; Write bit-plane 1 pointer
MOVE.L $25000,BPL2PTH(a0) ; Write bit-plane 2 pointer
Note that the memory requirements given here are for the playfield only. You may need
to allocate additional memory for other parts of the display, sprites, audio, animation and
for your application programs. Memory allocation for other parts of the display is
discussed in the chapters describing those topics.
- 48 Playfield Hardware -