System Integration Module (SIM)
Exception Control
MC68HC908AB32 — Rev. 1.0 Technical Data
MOTOROLA System Integration Module (SIM) 121
If more than one interrupt is pending at the end of an instruction
execution, the highest priority interrupt is serviced first. Figure 8-11
demonstrates what happens when two interrupts are pending. If an
interrupt is pending upon exit from the original interrupt service routine,
the pending interrupt is serviced before the LDA instruction is executed.
Figure 8-11. Interrupt Recognition Example
The LDA opcode is prefetched by both the INT1 and INT2 RTI
instructions. However, in the case of the INT1 RTI prefetch, this is a
redundant operation.
NOTE:
To maintain compatibility with the M6805 Family, the H register is not
pushed on the stack during interrupt entry. If the interrupt service routine
modifies the H register or uses the indexed addressing mode, software
should save the H register and then restore it prior to exiting the routine.
8.6.1.2 SWI Instruction
The SWI instruction is a non-maskable instruction that causes an
interrupt regardless of the state of the interrupt mask (I-bit) in the
condition code register.
NOTE:
A software interrupt pushes PC onto the stack. A software interrupt does
not push PC – 1, as a hardware interrupt does.
CLI
LDA
INT1
PULH
RTI
INT2
BACKGROUND
#$FF
PSHH
INT1 INTERRUPT SERVICE ROUTINE
PULH
RTI
PSHH
INT2 INTERRUPT SERVICE ROUTINE
ROUTINE
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