Nonmaskable interrupt response, Figure 9, Interrupt request/acknowledge cycle – Zilog Z08470 User Manual
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UM008007-0715
Nonmaskable Interrupt Response
Z80 CPU
User Manual
13
Nonmaskable Interrupt Response
Figure 10 shows the request/acknowledge cycle for the nonmaskable interrupt. This signal
is sampled at the same time as the interrupt line, but this line takes priority over the normal
interrupt and it cannot be disabled under software control. Its usual function is to provide
immediate response to important signals such as an impending power failure. The CPU
response to a nonmaskable interrupt is similar to a normal memory read operation. The
only difference is that the contents of the data bus are ignored while the processor auto-
matically stores the Program Counter in the external stack and jumps to address
0066h
.
The service routine for the nonmaskable interrupt must begin at this location if this inter-
rupt is used.
Figure 9. Interrupt Request/Acknowledge Cycle
In
Refresh
PC
M1
Last M Cycle of Instruction
CLK
INT
MREQ
RD
WAIT
M1
IORQ
Last T State
A15 –A0
D7–D0
TW*
TW*
T1
T2
T3