S3C4510B
INSTRUCTION SET
PSR TRANSFER (MRS, MSR)
The instruction is only executed if the condition is true. The various conditions are defined in Table 3-2.
The MRS and MSR instructions are formed from a subset of the data processing operations and are implemented
using the TEQ, TST, CMN and CMP instructions without the S flag set. The encoding is shown in Figure 3-11.
These instructions allow access to the CPSR and SPSR registers. The MRS instruction allows the contents of the
CPSR or SPSR_<mode> to be moved to a general register. The MSR instruction allows the contents of a general
register to be moved to the CPSR or SPSR_<mode> register.
The MSR instruction also allows an immediate value or register contents to be transferred to the condition code
flags (N,Z,C and V) of CPSR or SPSR_<mode> without affecting the control bits. In this case, the top four bits of
the specified register contents or 32 bit immediate value are written to the top four bits of the relevant PSR.
OPERAND RESTRICTIONS
— In user mode, the control bits of the CPSR are protected from change, so only the condition code flags of the
CPSR can be changed. In other (privileged) modes the entire CPSR can be changed.
— Note that the software must never change the state of the T bit in the CPSR. If this happens, the processor
will enter an unpredictable state.
— The SPSR register which is accessed depends on the mode at the time of execution. For example, only
SPSR_fiq is accessible when the processor is in FIQ mode.
— You must not specify R15 as the source or destination register.
— Also, do not attempt to access an SPSR in User mode, since no such register exists.
3-19
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