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CMP (extended register)

Compare (extended register) subtracts a sign or zero-extended register value, followed by an optional left shift amount, from a register value. The argument that is extended from the <Rm> register can be a byte, halfword, word, or doubleword. It updates the condition flags based on the result, and discards the result.

This is an alias of SUBS (extended register). This means:

313029282726252423222120191817161514131211109876543210
sf1101011001Rmoptionimm3Rn11111
opSRd

32-bit (sf == 0)

CMP <Wn|WSP>, <Wm>{, <extend> {#<amount>}}

is equivalent to

SUBS WZR, <Wn|WSP>, <Wm>{, <extend> {#<amount>}}

and is always the preferred disassembly.

64-bit (sf == 1)

CMP <Xn|SP>, <R><m>{, <extend> {#<amount>}}

is equivalent to

SUBS XZR, <Xn|SP>, <R><m>{, <extend> {#<amount>}}

and is always the preferred disassembly.

Assembler Symbols

<Wn|WSP>

Is the 32-bit name of the first source general-purpose register or stack pointer, encoded in the "Rn" field.

<Wm>

Is the 32-bit name of the second general-purpose source register, encoded in the "Rm" field.

<Xn|SP>

Is the 64-bit name of the first source general-purpose register or stack pointer, encoded in the "Rn" field.

<R> Is a width specifier, encoded in option:
option <R>
00x W
010 W
x11 X
10x W
110 W
<m>

Is the number [0-30] of the second general-purpose source register or the name ZR (31), encoded in the "Rm" field.

<extend> For the 32-bit variant: is the extension to be applied to the second source operand, encoded in option:
option <extend>
000 UXTB
001 UXTH
010 LSL|UXTW
011 UXTX
100 SXTB
101 SXTH
110 SXTW
111 SXTX
If "Rn" is '11111' (WSP) and "option" is '010' then LSL is preferred, but may be omitted when "imm3" is '000'. In all other cases <extend> is required and must be UXTW when "option" is '010'.
For the 64-bit variant: is the extension to be applied to the second source operand, encoded in option:
option <extend>
000 UXTB
001 UXTH
010 UXTW
011 LSL|UXTX
100 SXTB
101 SXTH
110 SXTW
111 SXTX
If "Rn" is '11111' (SP) and "option" is '011' then LSL is preferred, but may be omitted when "imm3" is '000'. In all other cases <extend> is required and must be UXTX when "option" is '011'.
<amount>

Is the left shift amount to be applied after extension in the range 0 to 4, defaulting to 0, encoded in the "imm3" field. It must be absent when <extend> is absent, is required when <extend> is LSL, and is optional when <extend> is present but not LSL.

Operation

The description of SUBS (extended register) gives the operational pseudocode for this instruction.

Operational information

If PSTATE.DIT is 1:

  • 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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