Commodore Amiga A500 Technical Reference Manual page 201

Bitplane Addressing
Some computer bitmap displays are organized so that the bitplanes
for each pixel are all located within the same address. This is called
pixel addressing.
If the entire data word of one address is used for a
single pixel with
8
bit planes, the data word will look like this. (num-
bers are bitplanes):
The data compression can be improved by packing more than one
pixel into a single address like this:
or like this, if there are only
4
bitplanes:
The IC device uses a bitmap technique called
Bitplane Addressing.
This separates the bitplanes in memory. To create a
4
plan (1 6 color)
image, the bitplane display DMA channels fetch from
4
separate
areas of memory like this:
These are held in buffer registers and are used together as pixels,
one bit a t a time, by the display (left to right).
This technique allows reduced odd numbers of bitplanes (such as
3
or 5) while maintaining packing efficiency and speed. It also allows
grouping bitplanes into two separate images, each with independent
hardware high speed image manipulation, line draw, and area fill.
DMA Channel Functions Each channel has an 1 8 bit RAM address pointer that is placed on the
MA memory address bus and is used to select the location of the
DMA data transfer from anywhere in
256K
words (51 2K bytes) of
RAM.
An eight bit destination address is simultaneously placed on the reg-
ister address bus (RGA), sending the data to the corresponding reg-
ister.