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S3C2410A PROGRAMMER'S MODEL
2-11
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 Notes
ARM R14_x THUMB R14_x
BL MOV PC, R14 PC + 4 PC + 2 1
SWI MOVS PC, R14_svc PC + 4 PC + 2 1
UDEF MOVS PC, R14_und PC + 4 PC + 2 1
FIQ SUBS PC, R14_fiq, #4 PC + 4 PC + 4 2
IRQ SUBS PC, R14_irq, #4 PC + 4 PC + 4 2
PABT SUBS PC, R14_abt, #4 PC + 4 PC + 4 1
DABT SUBS PC, R14_abt, #8 PC + 8 PC + 8 3
RESET NA – – 4
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, ARM920T checks for a LOW level on the output of the FIQ synchronizer at the end of each instruction.