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S3C2410A ARM INSTRUCTION SET
3-31
USE OF R15
Write-back must not be specified if R15 is specified as the base register (Rn). When using R15 as the base register
you must remember it contains an address 8 bytes on from the address of the current instruction.
R15 must not be specified as the register offset (Rm).
When R15 is the source register (Rd) of a register store (STR) instruction, the stored value will be address of the
instruction plus 12.
Restriction on the use of base register
When configured for late aborts, the following example code is difficult to unwind as the base register, Rn, gets
updated before the abort handler starts. Sometimes it may be impossible to calculate the initial value.
After an abort, the following example code is difficult to unwind as the base register, Rn, gets updated before the
abort handler starts. Sometimes it may be impossible to calculate the initial value.
EXAMPLE:
LDR R0,[R1],R1
Therefore a post-indexed LDR or STR where Rm is the same register as Rn should not be used.
DATA ABORTS
A transfer to or from a legal address may cause problems for a memory management system. For instance, in a
system which uses virtual memory the required data may be absent from main memory. The memory manager can
signal a problem by taking the processor ABORT input HIGH whereupon the Data Abort trap will be taken. It is up to
the system software to resolve the cause of the problem, then the instruction can be restarted and the original
program continued.
INSTRUCTION CYCLE TIMES
Normal LDR instructions take 1S + 1N + 1I and LDR PC take 2S + 2N +1I incremental cycles, where S,N and I are
defined as sequential (S-cycle), non-sequential (N-cycle), and internal (I-cycle), respectively. STR instructions take
2N incremental cycles to execute.