Z80 Datasheet, PDF (95/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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Table 12. Contents of Counters After DMA Stops Due to Byte Match (Search or Transfer/
Search Operations)
Class
Mode
Match Occurs Bytes Transferred At Stop
On This Byte If Transferring
Byte Counter
Continuous M
M**
M-1**
Source Port
Destination Port
Address Counter* Address Counter
As±(M**
***
Notes:
* Address can increment (+) or decrement (-) from the programmed starting address (As), which is the first
address for transfer or search.
** Occurs only when the Ready line is still inactive just prior to the beginning of the last possible cycle in the
operation. For example, Ready is sampled inactive on the rising edge of CLK in the last cycle of the last read
operation.
*** Search only has no destination. Simultaneous transfer/search cannot have both ports variable. This class of
operation is programmed as a DMA search only operation, with variable addresses assigned to the
programmed source port. What the DMA senses as the source port may be either the real source or destination,
as determined by external hardware. See the 'Applications' chapter.
Bus Control
The DMA transfers and searches data by controlling the system buses in the
same way that the Z80 CPU controls them to perform read and write cycles.
Specifically, the DMA controls the following lines:
• Address Bus (16 bits)
• Data Bus (8 bits)
• IORQ
• MREQ
• RD
• WR
In addition, the DMA can also be programmed to watch a WAIT line
through its dual-purpose CE/WAIT pin.
UM008101-0601
Direct Memory Access