ARM R4 Debug events, Precise and imprecise aborts, Fault status information, Abort exceptions

Level One Memory System

Debug events

The debug logic in the processor can be configured to generate breakpoints or vector capture events on instruction fetches, and watchpoints on data accesses. If the processor is software-configured for monitor-mode debugging, an abort is taken when one of these events occurs, or when a BKPT instruction is executed. For more details, see Chapter 11 Debug.

Precise and imprecise aborts

See Aborts on page 2-22for more information about the differences between precise and imprecise aborts.

8.3.2Fault status information

When an abort occurs, information about the cause of the fault is recorded in a number of registers, depending on the type of abort:

•Abort exceptions

•Precise abort exceptions on page 8-10

•Imprecise abort exceptions on page 8-10.

Abort exceptions

The following registers are updated when any abort exception is taken:

Link Register

The r14_abt register is updated to provide information about the address of the instruction that the exception was taken on, in a similar way to other types of exception. See Exceptions on page 2-16for more details. This information can be used to resume program execution after the abort has been handled.

Note

When a prefetch abort has occurred, ARM recommends that you do not use the link register value for determining the aborting address, because 32-bit Thumb instructions do not have to be word aligned and can cause an abort on either halfword. This applies even if all of the code in the system does not use the extra 32-bit Thumb instructions introduced in ARMv6T2, because the earlier BL and BLX instructions are both 32 bits long. Use the Fault Address Register instead, as described in this section.

Saved Program Status Register

The SPSR_abt register is updated to record the state and mode of the processor when the exception was taken, in a similar way to other types of exception. See Exceptions on page 2-16for more details.

Fault Status Register

There are two fault status registers, one for prefetch aborts (IFSR) and one for data aborts (DFSR). These record the type of abort that occurred, and whether it occurred on a read or a write. In particular, this enables the abort handler to distinguish between precise aborts, imprecise aborts, and debug events. For details of the format of this register and the encodings used, see Fault Status and Address Registers on page 4-45.

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