Zilog Z08470 - Manual (Page 142)

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UM008101-0601 Direct Memory Access
processed until the bus is released). Second, to enable the DMA, the
ENABLE AFTER RETI command must be used in the service routine after
an Interrupt on Ready. The typical purpose of interrupting when the Ready
line comes active is to allow the CPU time to determine where a transfer
should go, which it does in the service routine. This often occurs in systems
with dynamic memory allocation and it improves the efficiency with which
memory can be allocated. For example, the CPU might write and load new
starting addresses for a memory destination to the DMA in the service
routine. Only at the end of the service routine is the DMA enabled to
request the bus. The
ENABLE AFTER RETI command, which must precede
the ENABLE DMA command, resets a latch that is set when the Interrupt on
Ready first occurred.
For non-Z80 CPU environments, the DISABLE INTERRUPTS , ENABLE
INTERRUPTS , and RESET AND DISABLE INTERRUPTS commands are
available. They can simulate the Z80 CPU’s interrupt-acknowledge cycle
and return-from-interrupt instruction, both of which the DMA needs to
perform and return from interrupts.
Byte Matching (Searches)
In stopping, or stopping and interrupting on match (WR3, WR4), to
perform additional operations with the DMA, the following sequence of
commands are written:
1. LOAD or CONTINUE.
2. REINITIALIZE STATUS BYTE.
3. ENABLE DMA.
Another command (with the exception of ENABLE DMA ) must precede the
REINITIALIZE STATUS BYTE command. Table 11 on page 76 describes
the contents of various counters when stopping on byte match.