ARM R4F, r1p3 manual Imprecise aborts, Aborts in Strongly Ordered and Device memory

Programmer’s Model

Imprecise aborts

An imprecise abort, also known as an asynchronous abort, is one for which the exception is taken on a later instruction to the instruction that generated the aborting memory access. The abort handler cannot determine which instruction generated the abort, or the state of the processor when the abort occurred. Therefore, imprecise aborts are normally fatal.

Imprecise aborts can be generated by store instructions to normal-type or device-type memory. When the store instruction is committed, the data is normally written into a buffer that holds the data until the memory system has sufficient bandwidth to perform the write access. This gives read accesses higher priority. The write data can be held in the buffer for a long period, during which many other instructions can complete. If an error occurs when the write is finally performed, this generates an imprecise abort.

Imprecise abort masking

The nature of imprecise aborts means that they can occur while the processor is handling a different abort. If an imprecise abort generates a new exception in such a situation, the r14_abt and SPSR_abt values are overwritten. If this occurs before the data is pushed to the stack in memory, the state information about the first abort is lost. To prevent this from happening, the CPSR contains a mask bit to indicate that an imprecise abort cannot be accepted, the A-bit. When the A-bit is set, any imprecise abort that occurs is held pending by the processor until the A-bit is cleared, when the exception is actually taken. The A-bit is automatically set when abort, IRQ or FIQ exceptions are taken, and on reset. You must only clear the A-bit in an abort handler after the state information has either been stacked to memory, or is no longer required.

Only one pending imprecise abort of each imprecise abort type is supported. The processor supports the following pending imprecise aborts:

•Imprecise external abort

If a subsequent imprecise external abort is signaled while another one is pending, the later one is ignored and only one abort is taken.

•One TCM write external error for each TCM port.

•Cache write parity or ECC error.

If a subsequent cache parity or ECC error is signaled while another one is pending, the later one is normally ignored and only one abort is taken. However, if the pending error was correctable, and the later one is not correctable, the pending error is ignored, and one abort is taken for the error that cannot be corrected.

Memory barriers

When a store instruction, or series of instructions has been executed to normal-type or device-type memory, it is sometimes necessary to determine whether any errors occurred because of these instructions. Because most of these errors are reported imprecisely, they might not generate an abort exception until some time after the instructions are executed. To ensure that all possible errors have been reported, you must execute a DSB instruction. Abort exceptions are only taken because of these errors if they are not masked, that is, the CPSR A-bit is clear. If the A-bit is set, the aborts are held pending.

Aborts in Strongly Ordered and Device memory

When a memory access generates an abort, the instruction generating that access is abandoned, even if it has not completed all its memory accesses, and the abort exception is taken. The abort handler can then do one of the following:

•fix the error and return to the instruction that was abandoned, to re-execute it

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