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Z80 Datasheet(PDF) 163 Page - Zilog, Inc. |
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Z80 Datasheet(HTML) 163 Page - Zilog, Inc. |
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163 / 330 page  < %27 2GTKRJGTCNU 7UGT /CPWCN UM008101-0601 Direct Memory Access 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. |
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