ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

The System Level Programmers’ Model
If SPSR.J and SPSR.T are not both 0, indicating that the exception occurred in Thumb state or
ThumbEE state, use an exception return instruction with a subtraction of 2.
For more information, see Exception return on page B1-38.
Note
Undefined Instruction exceptions cannot occur in Jazelle state
If handling the Undefined Instruction exception requires instruction emulation, followed by return to
the next instruction after the instruction that caused the exception, the instruction emulator must use
the instruction length to calculate the correct return address, and to calculate the updated values of
the IT bits if necessary.
Conditional execution of undefined instructions
The conditional execution rules described in Conditional execution on page A8-8 apply to all instructions.
This includes UNDEFINED instructions and other instructions that would cause entry to the Undefined
Instruction exception.
If such an instruction fails its condition check, the behavior depends on the architecture profile and the
potential cause of entry to the Undefined Instruction exception, as follows:
In the ARMv7-A profile:
— If the potential cause is the execution of the instruction itself and depends on data values the
instruction reads, the instruction executes as a NOP and does not cause an Undefined
Instruction exception.
— If the potential cause is the execution of an earlier coprocessor instruction, or the execution of
the instruction itself but does not depend on data values the instruction reads, it is
IMPLEMENTATION DEFINED whether the instruction executes as a NOP or causes an Undefined
Instruction exception.
An implementation must handle all such cases in the same way.
In the ARMv7-R profile, the instruction executes as a NOP and does not cause an Undefined
Instruction exception.
Note
Before ARMv7, all implementations executed any instruction that failed its condition check as a NOP, even
if it would otherwise have caused an Undefined Instruction exception. Undefined Instruction handlers
written for these implementations might assume without checking that the undefined instruction passed its
condition check. Such Undefined Instruction handlers are likely to need rewriting, to check the condition is
passed, before they function correctly on all ARMv7-A implementations.
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