CMP (extended register) -- A64

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.

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.

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'.

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.

<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.

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.

Internal version only: aarchmrs v2023-12_rel, pseudocode v2023-12_rel, sve v2023-12_rel ; Build timestamp: 2023-12-15T16:46

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
sf	1	1	0	1	0	1	1	0	0	1	Rm					option			imm3			Rn					1	1	1	1	1
	op	S						opt																			Rd