PROGRAMMER'S MODEL S3C2440A RISC MICROPROCESSOR
2-12
IRQ
The IRQ (Interrupt Request) exception is a normal interrupt caused by a LOW level on the nIRQ input. IRQ has a
lower priority than FIQ and is masked out when a FIQ sequence is entered. It may be disabled at any time by
setting I bit in the CPSR, though this can only be done from a privileged (non-User) mode.
Irrespective of whether the exception was entered from ARM or Thumb state, an IRQ handler should return f rom
the interrupt by executing
SUBS PC,R14_irq,#4
Abort
An abort indicates that the current memory access cannot be completed. It can be signaled by the external
ABORT input. ARM920T checks for the abort exception during memory access cycles.
There are two types of abort:
• Prefetch Abort: occurs during an instruction prefetch.
• Data Abort: occurs during a data access.
If a prefetch abort occurs, the prefetched instruction is marked as invalid, but the exception will not be taken until
the instruction reaches the head of the pipeline. If the instruction is not executed – the abort doesn’t take place
because a branch occurs while it is in the pipeline -.
If a data abort occurs, the action taken depends on the instruction type:
• Single data transfer instructions (LDR, STR) write back modified base registers: the Abort handler must be
aware of this.
• The swap instruction (SWP) is aborted as though it had not been executed.
• Block data transfer instructions (LDM, STM) complete. If write-back is set, the base is updated. If the
instruction would have overwritten the base with data (ie it has the base in the transfer list), the overwriting is
prevented. All register overwriting is prevented after an abort is indicated, which means in particular that R15
(always the last register to be transferred) is preserved in an aborted LDM instruction.
The abort mechanism allows the implementation of a demand paged virtual m emory system. In such a system the
processor is allowed to generate arbitrary addresses. When the data at an address is unavailable, the Memory
Management Unit (MMU) signals an abort. The abort handler must then work out the cause of the abort, mak e the
requested data available, and retry the aborted instruction. The application program needs no knowledge of the
amount of memory available to it, nor is its state in any way affected by the abort.
After fixing the reason for the abort, the handler should execute the following irrespective of the state (ARM or
Thumb):
SUBS PC,R14_abt,#4 ; for a prefetch abort, or
SUBS PC,R14_abt,#8 ; for a data abort
This restores both the PC and the CPSR, and retries the aborted instruction.