Z80 Datasheet, PDF (110/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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Write Registers
Control bytes must be written to all relevant registers in the DMA at power-
up initialization. This section describes and illustrates each of the write
registers, WR0 through WR6, to which control bytes can be written. The
convention of calling the control bytes written to WR6 “commands” is
often used, because they are commonly used within CPU interrupt service
routines and at other times during system operation in addition to their use
at power-up initialization of the DMA.
“Internal Structure” on page 71 gives an organizational overview of the
write registers (Figure 30) and describes the access method. Control bytes
are written to one or more of the write register groups (WR6-WR0) by first
writing a byte to the “base register” in that group. All groups have base
registers and most groups have additional associated registers. The asso-
ciated registers in a group are sequentially accessed by first writing a byte
to the base register. The base register byte contains both control bits for
DMA function control, and pointer bits (1s) to one or more of the asso-
ciated registers in the base register’s group.
Figure 39 for WR0 illustrates this. In this figure, the sequence in which
associated registers within a group can be written to is shown by the
vertical position of the associated registers. For example, if a byte written to
the DMA contains the bits that identify WR0 (bits D0, D1, and D7), and
also contains 1s in the bit positions that point to associated registers 2 and 4,
then the next two bytes written to the DMA after the base register byte is
stored in these two associated registers, in that order.
Figure 40 through Figure 46 illustrate and describe each of seven base
registers and their associated registers. These figures, unlike Figure 30, do
not include the 16-bit counters associated with the starting-address and
block-length registers.
UM008101-0601
Direct Memory Access