Mitsubishi microcomputers
M16C / 61 Group
SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER
Interrupt
The operation of saving registers carried out in the interrupt sequence is dependent on whether the
content of the stack pointer, at the time of acceptance of an interrupt request, is even or odd. If the
content of the stack pointer (Note) is even, the content of the flag register (FLG) and the content of the
program counter (PC) are saved, 16 bits at a time. If odd, their contents are saved in two steps, 8 bits at
a time. Figure 1.13.7 shows the operation of the saving registers.
Note: Stack pointer indicated by U flag.
(1) Stack pointer (SP) contains even number
Sequence in which order
registers are saved
Address
Stack area
[SP] – 5 (Odd)
[SP] – 4 (Even)
[SP] – 3(Odd)
[SP] – 2 (Even)
[SP] – 1(Odd)
Program counter (PC )
L
(2) Saved simultaneously,
all 16 bits
Program counter (PC
Flag register (FLG
M
)
L
)
(1) Saved simultaneously,
all 16 bits
Flag register
(FLG
Program
counter (PC )
H
)
H
[SP]
(Even)
Finished saving registers
in two operations.
(2) Stack pointer (SP) contains odd number
Address
Stack area
Sequence in which order
registers are saved
[SP] – 5 (Even)
[SP] – 4(Odd)
[SP] – 3 (Even)
[SP] – 2(Odd)
[SP] – 1 (Even)
Program counter (PC )
L
(3)
(4)
Program counter (PC
Flag register (FLG
M
)
Saved simultaneously,
all 8 bits
L
)
(1)
(2)
Program
counter (PC )
Flag register
(FLG
H
H
)
[SP]
(Odd)
Finished saving registers
in four operations.
Note: [SP] denotes the initial value of the stack pointer (SP) when interrupt request is acknowledged.
After registers are saved, the SP content is [SP] minus 4.
Figure 1.13.7. Operation of saving registers
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MITSUBISHI [ Mitsubishi Group ]
