PROGRAMMER'S MODEL
S3C4510B
Exception Entry/Exit Summary
Table 2-2 summarizes the PC value preserved in the relevant R14 on exception entry, and the recommended
instruction for exiting the exception handler.
Table 2-2. Exception Entry/Exit
Return Instruction
Previous State
ARM R14_x
Notes
THUMB R14_x
PC + 2
PC + 2
PC + 2
PC + 4
PC + 4
PC + 4
PC + 8
–
BL
MOV PC, R14
PC + 4
PC + 4
PC + 4
PC + 4
PC + 4
PC + 4
PC + 8
–
1
1
1
2
2
1
3
4
SWI
MOVS PC, R14_svc
MOVS PC, R14_und
SUBS PC, R14_fiq, #4
SUBS PC, R14_irq, #4
SUBS PC, R14_abt, #4
SUBS PC, R14_abt, #8
NA
UDEF
FIQ
IRQ
PABT
DABT
RESET
NOTES:
1. Where PC is the address of the BL/SWI/Undefined Instruction fetch which had the prefetch abort.
2. Where PC is the address of the instruction which did not get executed since the FIQ or IRQ took priority.
3. Where PC is the address of the Load or Store instruction which generated the data abort.
4. The value saved in R14_svc upon reset is unpredictable.
FIQ
The FIQ (Fast Interrupt Request) exception is designed to support a data transfer or channel process, and in
ARM state has sufficient private registers to remove the need for register saving (thus minimizing the overhead
of context switching).
FIQ is externally generated by taking the nFIQ input LOW. This input can except either synchronous or
asynchronous transitions, depending on the state of the ISYNC input signal. When ISYNC is LOW, nFIQ and
nIRQ are considered asynchronous, and a cycle delay for synchronization is incurred before the interrupt can
affect the processor flow.
Irrespective of whether the exception was entered from ARM or Thumb state, a FIQ handler should leave the
interrupt by executing
SUBS
PC,R14_fiq,#4
FIQ may be disabled by setting the CPSR's F flag (but note that this is not possible from User mode). If the F flag
is clear, ARM7TDMI checks for a LOW level on the output of the FIQ synchroniser at the end of each instruction.
2-12
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