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CMP (immediate)

Compare (immediate) subtracts an immediate value from a register value. It updates the condition flags based on the result, and discards the result.

It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 and T2 ) .

A1

313029282726252423222120191817161514131211109876543210
!= 111100110101Rn(0)(0)(0)(0)imm12
cond

A1

CMP{<c>}{<q>} <Rn>, #<const>

n = UInt(Rn);  imm32 = A32ExpandImm(imm12);

T1

1514131211109876543210
00101Rnimm8

T1

CMP{<c>}{<q>} <Rn>, #<imm8>

n = UInt(Rn);  imm32 = ZeroExtend(imm8, 32);

T2

15141312111098765432101514131211109876543210
11110i011011Rn0imm31111imm8

T2

CMP{<c>}.W <Rn>, #<const> // (<Rd>, <const> can be represented in T1)

CMP{<c>}{<q>} <Rn>, #<const>

n = UInt(Rn);  imm32 = T32ExpandImm(i:imm3:imm8);
if n == 15 then UNPREDICTABLE;

For more information about the constrained unpredictable behavior, see Architectural Constraints on UNPREDICTABLE behaviors.

Assembler Symbols

<c>

See Standard assembler syntax fields.

<q>

See Standard assembler syntax fields.

<Rn>

For encoding A1: is the general-purpose source register, encoded in the "Rn" field. The PC can be used, but this is deprecated.

For encoding T1: is a general-purpose source register, encoded in the "Rn" field.

For encoding T2: is the general-purpose source register, encoded in the "Rn" field.

<imm8>

Is a 8-bit unsigned immediate, in the range 0 to 255, encoded in the "imm8" field.

<const>

For encoding A1: an immediate value. See Modified immediate constants in A32 instructions for the range of values.

For encoding T2: an immediate value. See Modified immediate constants in T32 instructions for the range of values.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    (result, nzcv) = AddWithCarry(R[n], NOT(imm32), '1');
    PSTATE.<N,Z,C,V> = nzcv;

Operational information

If CPSR.DIT is 1, this instruction has passed its condition execution check, and does not use R15 as either its source or destination:

  • The execution time of this instruction is independent of:
    • The values of the data supplied in any of its registers.
    • The values of the NZCV flags.
  • The response of this instruction to asynchronous exceptions does not vary based on:
    • The values of the data supplied in any of its registers.
    • The values of the NZCV flags.
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