CHAPTER 19 INTERRUPT FUNCTIONS
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19.4.2
Software interrupt request acknowledgment
A software interrupt request is acknowledged by BRK instruction execution. Software interrupts cannot be
disabled.
If a software interrupt request is acknowledged, the contents are saved into the stacks in the order of the
program status word (PSW), then program counter (PC), the IE flag is reset (0), and the contents of the vector
table (0007EH, 0007FH) are loaded into the PC and branched.
Restoring from a software interrupt is possible by using the RETB instruction.
Caution
Can not use the RETI instruction for restoring from the software interrupt.
19.4.3
Multiple interrupt servicing
Multiple interrupt servicing occurs when another interrupt request is acknowledged during execution of an
interrupt.
Multiple interrupt servicing does not occur unless the interrupt request acknowledgment enabled state is selected
(IE = 1). When an interrupt request is acknowledged, interrupt request acknowledgment becomes disabled (IE =
0). Therefore, to enable multiple interrupt servicing, it is necessary to set (1) the IE flag with the EI instruction
during interrupt servicing to enable interrupt acknowledgment.
Moreover, even if interrupts are enabled, multiple interrupt servicing may not be enabled, this being subject to
interrupt priority control. Two types of priority control are available: default priority control and programmable
priority control. Programmable priority control is used for multiple interrupt servicing.
In the interrupt enabled state, if an interrupt request with a priority equal to or higher than that of the interrupt
currently being serviced is generated, it is acknowledged for multiple interrupt servicing. If an interrupt with a
priority equal to or lower than that of the interrupt currently being serviced is generated during interrupt servicing,
it is not acknowledged for multiple interrupt servicing. However, when setting the IE flag to 1 during the
interruption at level 0, other level 0 interruptions can be allowed.
Interrupt requests that are not enabled because interrupts are in the interrupt disabled state or because they
have a lower priority are held pending. When servicing of the current interrupt ends, the pending interrupt request
is acknowledged following execution of at least one main processing instruction execution.
Table 19 - 8 shows Relationship Between Interrupt Requests Enabled for Multiple Interrupt Servicing During
Interrupt Servicing and Figures 19 - 13 and 19 - 14 show multiple interrupt servicing examples.
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