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CMP (register)

Compare (register) subtracts an optionally-shifted register 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 , T2 and T3 ) .

A1

313029282726252423222120191817161514131211109876543210
!= 111100010101Rn(0)(0)(0)(0)imm5stype0Rm
cond

Rotate right with extend (imm5 == 00000 && stype == 11)

CMP{<c>}{<q>} <Rn>, <Rm>, RRX

Shift or rotate by value (!(imm5 == 00000 && stype == 11))

CMP{<c>}{<q>} <Rn>, <Rm> {, <shift> #<amount>}

n = UInt(Rn);  m = UInt(Rm);
(shift_t, shift_n) = DecodeImmShift(stype, imm5);

T1

1514131211109876543210
0100001010RmRn

T1

CMP{<c>}{<q>} <Rn>, <Rm> // (<Rn> and <Rm> both from R0-R7)

n = UInt(Rn);  m = UInt(Rm);
(shift_t, shift_n) = (SRType_LSL, 0);

T2

1514131211109876543210
01000101NRmRn

T2

CMP{<c>}{<q>} <Rn>, <Rm> // (<Rn> and <Rm> not both from R0-R7)

n = UInt(N:Rn);  m = UInt(Rm);
(shift_t, shift_n) = (SRType_LSL, 0);
if n < 8 && m < 8 then UNPREDICTABLE;
if n == 15 || m == 15 then UNPREDICTABLE;

CONSTRAINED UNPREDICTABLE behavior

If n < 8 && m < 8, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The instruction executes as described, with no change to its behavior and no additional side effects.
  • The condition flags become unknown.

T3

15141312111098765432101514131211109876543210
111010111011Rn(0)imm31111imm2stypeRm

Rotate right with extend (imm3 == 000 && imm2 == 00 && stype == 11)

CMP{<c>}{<q>} <Rn>, <Rm>, RRX

Shift or rotate by value (!(imm3 == 000 && imm2 == 00 && stype == 11))

CMP{<c>}.W <Rn>, <Rm> // (<Rn>, <Rm> can be represented in T1 or T2)

CMP{<c>}{<q>} <Rn>, <Rm>, <shift> #<amount>

n = UInt(Rn);  m = UInt(Rm);
(shift_t, shift_n) = DecodeImmShift(stype, imm3:imm2);
if n == 15 || m == 15 then UNPREDICTABLE; // Armv8-A removes UNPREDICTABLE for R13

For more information about the constrained unpredictable behavior of this instruction, 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 first general-purpose source register, encoded in the "Rn" field. The PC can be used, but this is deprecated.

For encoding T1 and T3: is the first general-purpose source register, encoded in the "Rn" field.

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

<Rm>

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

For encoding T1, T2 and T3: is the second general-purpose source register, encoded in the "Rm" field.

<shift> Is the type of shift to be applied to the second source register, encoded in stype:
stype <shift>
00 LSL
01 LSR
10 ASR
11 ROR
<amount>

For encoding A1: is the shift amount, in the range 1 to 31 (when <shift> = LSL or ROR) or 1 to 32 (when <shift> = LSR or ASR) encoded in the "imm5" field as <amount> modulo 32.

For encoding T3: is the shift amount, in the range 1 to 31 (when <shift> = LSL or ROR) or 1 to 32 (when <shift> = LSR or ASR), encoded in the "imm3:imm2" field as <amount> modulo 32.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    shifted = Shift(R[m], shift_t, shift_n, PSTATE.C);
    (result, nzcv) = AddWithCarry(R[n], NOT(shifted), '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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