Z80 Datasheet, PDF (163/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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Bus Characteristics
Similar to the Z80, the 8080 and 8085 have 8-bit data paths and 16-bit
addresses. The DMA is matched well to these numbers; it can search whole
data words and directly address any byte in the memory.
The 8086 and the Z8000 CPUs have 16-bit data paths and larger address
spaces, thus making it somewhat harder to use the Z80 DMA. Searching
can be done for match bytes in either half of the data word, but not for a
whole unique word. Often this is not a problem because byte matches
suffice, for example, in detecting special ASCII characters in a data block.
The problem of larger address spaces can be resolved by using an external
segment or page register, latched to the appropriate high-order addresses
before the DMA becomes bus master, or by other schemes such as
indexing. This requires some external hardware.
To conserve pins, the 8085, 8086, Z8001, and Z8002 multiplex addresses
and data. Strobes allowing demultiplexing then become part of the bus
structure and must be accounted for in DMA interface. In such cases, the
DMA should be connected to the demultiplexed address and data lines
rather than closer to the processor. Figure 56 is a simplified diagram of
this concept.
UM008101-0601
Direct Memory Access