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Overview of Interrupts
7.1.2.1 Interrupt Service Fetch Packet (ISFP)
Figure 7–2. Interrupt Service Fetch Packet
7-6
An ISFP is a fetch packet used to service an interrupt. Figure 7–2 shows an
ISFP that contains an interrupt service routine small enough to fit in a single
fetch packet (FP). To branch back to the main program, the FP contains a
branch to the interrupt return pointer instruction (B IRP). This is followed by a
NOP 5 instruction to allow the branch target to reach the execution stage of
the pipeline.
Note:
If the NOP 5 was not in the routine, the CPU would execute the next five
execute packets that are associated with the next ISFP.
ISFP for INT6
0C0h
0C4h
0C8h
0CCh
0D0h
0D4h
0D8h
0DCh
The interrupt service
routine for INT6 is short
enough to be contained
in a single fetch packet.
If the interrupt service routine for an interrupt is too large to fit in a single FP,
a branch to the location of additional interrupt service routine code is required.
Figure 7–3 shows that the interrupt service routine for INT4 was too large for
a single FP, and a branch to memory location 1234h is required to complete
the interrupt service routine.
000h
020h
040h
060h
Instr1
080h
Instr2
0A0h
Instr3
0C0h
Instr4
0E0h
Instr5
100h
Instr6
120h
B IRP
140h
NOP 5
160h
180h
1A0h
1C0h
1E0h
Interrupt service table
(IST)
RESET ISFP
NMI ISFP
Reserved
Reserved
INT4 ISFP
INT5 ISFP
INT6 ISFP
INT7 ISFP
INT8 ISFP
INT9 ISFP
INT10 ISFP
INT11 ISFP
INT12 ISFP
INT13 ISFP
INT14 ISFP
INT15 ISFP
Program memory
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