msr

Move Immediate to Special Register (Thumb)

MSR <spec_reg>, #<imm>

Writes an immediate to a status register (Thumb).

Pseudocode Operation

imm_value ← ZeroExtend(imm8, 32)
if spec_reg == CPSR then
  CPSR[31:24] ← imm_value[31:24]
else if spec_reg == SPSR then
  SPSR[31:24] ← imm_value[31:24]

Example

MSR nzcv, #16

Encoding

Binary Layout
11110011100
31:21
R
20
Rn
19:16
10
15:14
0
13
0
12
mask
11:8
0
7
0
6
0
5
0
4
0
3
0
2
0
1
0
0
 
Format Thumb System
Opcode 0xF3808000
Extension T32 (System)

Operands

  • spec_reg
    CPSR/SPSR
  • imm
    Value

Related

Other forms of msr

  • msr Move to System Register
  • msr Move to Special Register (Banked)
  • msr Move Immediate to Special Register (A32)
  • msr Move Register to Special Register (Thumb)
  • msr Move Register to Status Register

More in T32 (System)

Reference

View in Arm A64 ISA Reference ↗ Arm AArch32 ISA

Instruction Forms

Encoding Instruction ISA Bit pattern
0x0120F200 MSR{<c>}{<q>} <banked_reg>, <Rn> A32 cond | 00010 | R | 1 | 0 | M1 | 1111 | 0 | 0 | 1 | M | 0000 | Rn
0xF3808020 MSR{<c>}{<q>} <banked_reg>, <Rn> T32 11110011100 | R | Rn | 10 | 0 | 0 | M1 | 0 | 0 | 1 | M | 0 | 0 | 0 | 0
0x0320F000 MSR{<c>}{<q>} <spec_reg>, #<imm> A32 cond | 00110 | R | 10 | mask | 1 | 1 | 1 | 1 | imm12
0x0120F000 MSR{<c>}{<q>} <spec_reg>, <Rn> A32 cond | 00010 | R | 1 | 0 | mask | 1111 | 0 | 0 | 0 | 0 | 0000 | Rn
0xF3808000 MSR{<c>}{<q>} <spec_reg>, <Rn> T32 11110011100 | R | Rn | 10 | 0 | 0 | mask | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0

Description

Move general-purpose register to Special register moves selected bits of a general-purpose register to the APSR, CPSR or SPSR_<current_mode>. Because of the Do-Not-Modify nature of its reserved bits, a read-modify-write sequence is normally required when the MSR instruction is being used at Application level and its destination is not APSR_nzcvq (CPSR_f). If an MSR (register) moves selected bits of an immediate value to the CPSR, the PE checks whether the value being written to PSTATE.M is legal. See Illegal changes to PSTATE.M. An MSR (register) executed in User mode: An MSR (register) executed in System mode is unpredictable if it attempts to update the SPSR. The CPSR.E bit is writable from any mode using an MSR instruction. Arm deprecates using this to change its value.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    if write_spsr then
        if PSTATE.M IN {M32_User,M32_System} then
            UNPREDICTABLE;
        else
            SPSRWriteByInstr(R[n], mask);
    else
        // Attempts to change to an illegal mode will invoke the Illegal Execution state mechanism
        CPSRWriteByInstr(R[n], mask);