ARM ARM7TDMI User Manual Page 65

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Programmer’s Model
ARM DDI 0234B Copyright © 2001 ARM Limited. All rights reserved. 2-23
The abort mechanism enables the implementation of a demand-paged virtual memory
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, make the requested
data available, and retry the aborted instruction. The application program does not have
to know 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 must execute the following return
instruction irrespective of the processor operating state at the point of entry:
SUBS PC,R14_abt,#8
This action restores both the PC, and the CPSR, and retriesthe aborted instruction.
2.9.7 Software interrupt instruction
The Software Interrupt (SWI) is used to enter Supervisor mode, usually to request a
particular supervisor function. A SWI handler returns by executing the following
irrespective of the processor operating state:
MOVS PC, R14_svc
This action restores the PC and CPSR, and returns to the instructionfollowing the SWI.
The SWI handler reads the opcode to extract the SWI function number.
2.9.8 Undefined instruction
When the ARM7TDMI-S processor encounters an instruction that neither it nor any
coprocessor in the system can handle, the ARM7TDMI-S core takes the undefined
instruction trap. Software can use this mechanism to extend the ARM instruction set by
emulating undefined coprocessor instructions.
Note
The ARM7TDMI-S processor is fully compliant with the ARM architecture v4T, and
traps all instruction bit patterns that are classified as undefined.
After emulating the failed instruction, the trap handler executes the following
irrespective of the processor operating state:
MOVS PC,R14_und
This action restores the CPSR and returns to the next instructionafter the undefined
instruction.
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