Z80 Datasheet, PDF (59/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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Bus Control
Most DMACs do not control the system bus in the same way that a CPU
controls it. For example, many DMACs do not have a straightforward
interface to the system data bus but rather multiplex a portion of the
memory address onto the data bus, from which it must be latched by
external logic. Nor do most DMACs generate all of the bus control
signals that the CPU generates, and therefore they lack some degree of
bus control when they operate.
The Z80 DMA is unique among 8-bit DMACs because it generates
exactly the same bus control signals for read and write cycles that the Z80
CPU does, and also because it has exactly the same logical and electrical
interface to the data and address buses as the CPU. This means the other
system components cannot discern the difference between the Z80 DMA
and CPU; control by these devices is totally interchangeable. In the
sequential DMA transfer method (a read cycle followed by a write cycle),
it also means that the Z80 DMA pins can be tied directly to the corre-
sponding Z80 CPU pins without any of the external interfacing logic that
some DMACs require. This property considerably simplifies design and
lowers part counts.
Programmability
How a DMAC starts, transfers data, and stops is determined by control
information written to the DMAC by the CPU prior to the transfer. Status
registers, which can be read by the CPU to determine the transfer condition
after the DMAC stops transferring, are also typically provided.
The degree of programmability is directly related to the DMACs flexibility
in handling a variety of transfer tasks. Most DMACs are limited in their
programmability. The Z80 DMA, by contrast, has over 140 bits of control
information used to tailor the device (and retailor it between operations) for
a wide variety of tasks and environments.
UM008101-0601
Direct Memory Access