Commodore Amiga A1000, Hardware Reference Manual

Page: 55 / 380

ALLOCATING MEMORY FOR BIT-PLANES
After you set the number of bit-planes and specify resolution modes, you are ready to
allocate memory. A bit-plane consists of an end-to-end sequence of words at consecutive
memory locations. When operating under the Amiga operating system, use a system call
such as AllocMem() to remove a block of memory from the free list and make it available
to the program. If the machine has been taken over, simply reserve an area of memory
for the bit-planes. Next, set the bit plane pointer registers (BPLxPTH/BPLxPTL) to point to
the starting memory address of each bitplane you are using. The starting address is the
memory word that contains the bits of the upper left-hand corner of the bit-plane.
Table 3-6 shows how much memory is needed for basic playfields. You may need to
balance your color and resolution requirements against the amount of available memory
you have.
Table 3-7: Playfield Memory Requirements, NTSC
Number of Bytes
Picture Size Modes per Bit-Plane
320 X 200 Low-resolution, 8,000
non-interlaced
320 X 400 Low-resolution, 16,000
interlaced
640 X 200 High-resolution, 16,000
non-interlaced
640 X 400 High-resolution, 32,000
interlaced
- 46 Playfield Hardware -