Z80 Datasheet, PDF (114/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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Write Register 1 Group
Bits 7, 2, 1, and 0, as Figure 41 illustrates, select the base register byte for
this group. The group is used only when Port A is used, for example, do not
program it for a search only, simultaneous transfer, or simultaneous transfer/
search with Port B as the source. It specifies the following characteristics:
Device Type (Port A)
Bit 3 identifies Port A as either memory or I/O. This specification causes
the proper control line MREQ or IORQ to come active for cycles involving
that port.
Variable/Fixed Addressing (Port A)
Bits 4 and 5 specify whether the Port A address increments, decrements, or
remains fixed for each byte of data transferred or searched. The first byte of
data in an operation uses the starting address entered for Port A in WR0.
Incrementing or decrementing begins on the second byte of the operation.
Variable Cycle (Port A)
If bit 6 is set to 0, the DMA’s variable-cycle timing feature is not used;
instead, standard Z80 timing for read and write cycles is used, which is
described in the “Timing” chapter. If bit 6 is set to 1, the next byte written
to the DMA after the WR1 base register byte is the Port A variable-timing
byte. This allows the length of the port’s read and write cycles to be
shortened. The choices for overall cycle timing of the DMA, including
activation of the IORQ, MREQ, RD, and WR lines, are specified in bits 1
and 0 as follows:
4 clock cycles
3 clock cycles
2 clock cycles
UM008101-0601
Direct Memory Access