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ARM INSTRUCTION SET S3C2410A
3-44
INCLUSION OF THE BASE IN THE REGISTER LIST
When write-back is specified, the base is written back at the end of the second cycle of the instruction. During a
STM, the first register is written out at the start of the second cycle. A STM which includes storing the base, with the
base as the first register to be stored, will therefore store the unchanged value, whereas with the base second or
later in the transfer order, will store the modified value. A LDM will always overwrite the updated base if the base is in
the list.
DATA ABORTS
Some legal addresses may be unacceptable to a memory management system, and the memory manager can
indicate a problem with an address by taking the ABORT signal HIGH. This can happen on any transfer during a
multiple register load or store, and must be recoverable if ARM920T is to be used in a virtual memory system.
Abort during STM Instructions
If the abort occurs during a store multiple instruction, ARM920T takes little action until the instruction completes,
whereupon it enters the data abort trap. The memory manager is responsible for preventing erroneous writes to the
memory. The only change to the internal state of the processor will be the modification of the base register if write-
back was specified, and this must be reversed by software (and the cause of the abort resolved) before the
instruction may be retried.
Aborts during LDM Instructions
When ARM920T detects a data abort during a load multiple instruction, it modifies the operation of the instruction to
ensure that recovery is possible.
• Overwriting of registers stops when the abort happens. The aborting load will not take place but earlier ones may
have overwritten registers. The PC is always the last register to be written and so will always be preserved.
• The base register is restored, to its modified value if write-back was requested. This ensures recoverability in the
case where the base register is also in the transfer list, and may have been overwritten before the abort occurred.
The data abort trap is taken when the load multiple has completed, and the system software must undo any base
modification (and resolve the cause of the abort) before restarting the instruction.
INSTRUCTION CYCLE TIMES
Normal LDM instructions take nS + 1N + 1I and LDM PC takes (n+1)S + 2N + 1I incremental cycles, where S,N and
I are defined as sequential (S-cycle), non-sequential (N-cycle), and internal (I-cycle), respectively. STM instructions
take (n-1)S + 2N incremental cycles to execute, where n is the number of words transferred.