Sharp MZ-80B Owner's Manual page 115

106
Figure 5.0-1 is a summary of the effect of different instructions on the two enable flip flops .
Action IFF
1
IFF 2
CPU Reset 0 0
DI 0 0
EI
LDA,I
•
•
IFF 2 -+ Parity flag
LDA,R
•
•
IFF 2 -+Parity flag
AcceptNMI
0
•
RETN
IFF 2
•
IFF2-+ IFF
I
"•" indicates no change
FIGURE 5.0-1
INTERRUPT ENABLE/DISABLE FLIP FLOPS
CPU RESPONSE
Non Maskable
A nonmaskable interrupt will be accepted at all times by the CPU. When this occurs, the CPU ignores the next
instruction that it fetches and instead does a restart to location 0066H . Thus, it behaves exactly as if it had received a
restart instruction but, it is to a location that is not one of the 8 software restart locations. A restart is merely a call to
a specific address in page 0 of memory.
Maskable
The CPU can be progr:tmmed to respond to the maskable interrupt in any one of three possible modes.
ModeO
This mode is identical to the 8080A interrupt response mode. With this mode , the interrupting device can place
any instruction on the data bus and the CPU will execute it. Thus, the interrupting device provides the next instruction
to be executed instead of the memory. Often this will be a restart instruction since the interrupting device only need
supply a single byte instruction. Alternatively, any other instruction such as a 3 byte call to any location in memory
could be executed.
The number of clock cycles necessary to execute this instruction is 2 more than the normal number for the instruc-
tion . This occurs since the CPU automatically adds 2 wait states to an interrupt response cycle to allow sufficient time
to implement an external daisy chain for priority control. Section 3.0 illustrates the detailed timing for an interrupt
response . After the application of RESET the CPU will automatically enter interrupt Mode 0.
Mode 1
When this mode has been selected by the programmer, the CPU will respond to an interrupt by executing a restart
to location 0038H . Thus the response is identical
to
that for a non maskable interrupt except that the call location is
0338H instead of 0066H. Another difference is that the number of cycles required to complete the restart instruction
is 2 more than normal due to the two added wait states.