Z80 Datasheet, PDF (128/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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end of the service routine, the CPU writes a RESET AND DISABLE
INTERRUPTS command, then an ENABLE INTERRUPTS command,
and then an ENABLE DMA command before executing its return-from-
interrupt instruction.
This command, when followed by an ENABLE INTERRUPTS command,
takes the place of the Z80 RETI instruction. It is not needed in a Z80 envi-
ronment. Because RESET AND DISABLE INTERRUPTS unforces a
FORCED-READY condition, the RESET AND DISABLE INTERRUPTS
must precede a FORCE READY command when the latter is used.
Enable After RETI (B7)
This command is used only when the DMA is operated in the Interrupt On
Ready mode (programmed in WR4). It enables the DMA to request the bus
again after returning from an interrupt. This command is always used in
Z80 CPU environments to get further bus requesting after an Interrupt on
Ready. It is sometimes used in other environments, such as the 8080.
An Interrupt on Ready (IOR) latch is set during such an interrupt. This latch
prevents the DMA from requesting the bus from the time the Ready line
goes active until the time the latch is reset by the ENABLE AFTER RETI
command (in Z80 and some other environments, there is an overlap in bus-
request prevention by the IOR and the IUS latches).
In a Z80 CPU interrupt service routine, the order of DMA commands and
CPU instructions MUST be:
1. •
2. •
3. •
4. ENABLE AFTER RETI command
5. ENABLE DMA command
UM008101-0601
Direct Memory Access