Z80 Datasheet, PDF (129/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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RETI instruction
Read Status Byte (BF)
This command causes the next CPU read of the DMA to access the status
byte, which is illustrated in “Read Registers” on page 114.
If other read registers are being read, the sequence of reading (as defined by
the read mask) must be completed before issuing this command.
Reinitialize Status Byte (8B)
This command reinitializes bits 4 and 5 of the status byte. After reinitial-
ization, the status byte looks like this:
Table 14. Reinitialize Status Byte
Bit
Value
Meaning
0
I/O
DMA operation has/has not occurred
1
I/O
Ready line active/inactive
2
X
Undefined bit
3
0/1
Interrupt pending/not pending
4
1
Match not found
5
1
Not end-of-block
6
X
Undefined bit
7
X
Undefined bit
The DISABLE DMA or any other command must be used before the
REINITIALIZE STATUS BYTE command after having stopped on end-of-
block or byte match. Due to a potential hardware race condition internal to
UM008101-0601
Direct Memory Access