Zilog Z08470 Manual de usuario descargar pdf (Pagina 142)

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

,andRESET 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:

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.

1 2 ... 137 138 139 140 141 142 143 144 145 146 147 ... 329 330

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Zilog Z08470 Manual de usuario Pagina 142