KS57C2308/P2308/C2316/P2316 ADDRESS SPACES
2-17
INTERRUPT STATUS FLAGS (IS0, IS1)
PSW bits IS0 and IS1 contain the current interrupt execution status values. You can manipulate IS0 and IS1
flags directly using 1-bit RAM control instructions
By manipulating interrupt status flags in conjunction with the interrupt priority register (IPR), you can process
multiple interrupts by anticipating the next interrupt in an execution sequence. The interrupt priority control circuit
determines the IS0 and IS1 settings in order to control multiple interrupt processing. When both interrupt status
flags are set to "0", all interrupts are allowed. The priority with which interrupts are processed is then determined
by the IPR.
When an interrupt occurs, IS0 and IS1 are pushed to the stack as part of the PSW and are automatically
incremented to the next higher priority level. Then, when the interrupt service routine ends with an IRET
instruction, IS0 and IS1 values are restored to the PSW. Table 2-6 shows the effects of IS0 and IS1 flag settings.
Table 2-6. Interrupt Status Flag Bit Settings
IS1
Value IS0
Value Status of Currently
Executing Process Effect of IS0 and IS1 Settings
on Interrupt Request Control
0 0 0 All interrupt requests are serviced
0 1 1 Only high-priority interrupt(s) as determined in the
interrupt priority register (IPR) are serviced
1 0 2 No more interrupt requests are serviced
1 1 – Not applicable; these bit settings are undefined
Since interrupt status flags can be addressed by write instructions, programs can exert direct control over
interrupt processing status. Before interrupt status flags can be addressed, however, you must first execute a DI
instruction to inhibit additional interrupt routines. When the bit manipulation has been completed, execute an EI
instruction to re-enable interrupt processing.
++ PROGRAMMING TIP — Setting ISx Flags for Interrupt Processing
The following instruction sequence shows how to use the IS0 and IS1 flags to control interrupt processing:
INTB DI ;Disable interrupt
BITR IS1 ;IS1 ← 0
BITS IS0 ;Allow interrupts according to IPR priority level
EI ;Enable interrupt