Z80 Datasheet, PDF (25/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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Interrupt Control Logic
The Interrupt Control Logic insures that the CTC acts in accordance with
Z80 system interrupt protocol for Nested Priority Interrupting and Return
From Interrupt. The priority of any system device is determined by its
physical location in a daisy-chain configuration. Two signal lines, CIEI and
IEO, are provided in CTC devices to form this system daisy-chain. The
device closest to the CPU has the highest priority. Within the CTC,
interrupt priority is predetermined by channel number, with Channel 0
having highest and Channel 3 the lowest priority. See Table 3. The purpose
of a CTC-generated interrupt, as with any peripheral device, is to force the
CPU to execute an interrupt service routine. According to Z80 system
interrupt protocol, lower priority devices or channels may not interrupt
higher priority devices or channels that have not had their interrupt service
routines completed. However, high priority devices or channels may
interrupt the servicing of lower priority devices or channels.
Table 3. Interrupt Vector Register
7
6
5
4
3
Supplied by User
R/W
2
1
Channel Identifier
R/W
0
Word
R/W
Bit Number
7–3
2–1
0
Field
Reserved
Channel Identifier
(Automatically
inserted by CTC)
Word
R/W
R/W
R/W
R/W
Value Description
Supplied by User
11 Channel 3
10 Channel 2
01 Channel 1
00 Channel 0
1 Control
0 Interrupt Vector
UM008101-0601
Counter/Timer Channels