SUB, SUBS (immediate) -- AArch32

Subtract (immediate) subtracts an immediate value from a register value, and writes the result to the destination register.

If the destination register is not the PC, the SUBS variant of the instruction updates the condition flags based on the result.

The field descriptions for <Rd> identify the encodings where the PC is permitted as the destination register. If the destination register is the PC:

It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 , T2 , T3 , T4 and T5 ) .

if Rn == '1111' && S == '0' then SEE "ADR"; if Rn == '1101' then SEE "SUB (SP minus immediate)"; constant d = UInt(Rd); constant n = UInt(Rn); constant setflags = (S == '1'); constant imm32 = A32ExpandImm(imm12);

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0	0	0	1	1	1	1	imm3	Rn	Rd

T1

SUB <c>{<q>} <Rd>, <Rn>, #<imm3> // (Inside IT block)

SUBS{<q>} <Rd>, <Rn>, #<imm3> // (Outside IT block)

constant d = UInt(Rd); constant n = UInt(Rn); constant setflags = !InITBlock(); constant imm32 = ZeroExtend(imm3, 32);

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0	0	1	1	1	Rdn	imm8

T2

SUB <c>{<q>} <Rdn>, #<imm8> // (Inside IT block, and <Rdn>, <imm8> can be represented in T1)

SUB<c>{<q>} {<Rdn>,} <Rdn>, #<imm8> // (Inside IT block, and <Rdn>, <imm8> cannot be represented in T1)

SUBS{<q>} <Rdn>, #<imm8> // (Outside IT block, and <Rdn>, <imm8> can be represented in T1)

SUBS{<q>} {<Rdn>,} <Rdn>, #<imm8> // (Outside IT block, and <Rdn>, <imm8> cannot be represented in T1)

constant d = UInt(Rdn); constant n = UInt(Rdn); constant setflags = !InITBlock(); constant imm32 = ZeroExtend(imm8, 32);

SUB (S == 0)

SUB <c>.W {<Rd>,} <Rn>, #<const> // (Inside IT block, and <Rd>, <Rn>, <const> can be represented in T1 or T2)

SUB{<c>}{<q>} {<Rd>,} <Rn>, #<const>

SUBS (S == 1 && Rd != 1111)

SUBS.W {<Rd>,} <Rn>, #<const> // (Outside IT block, and <Rd>, <Rn>, <const> can be represented in T1 or T2)

SUBS{<c>}{<q>} {<Rd>,} <Rn>, #<const>

if Rd == '1111' && S == '1' then SEE "CMP (immediate)"; if Rn == '1101' then SEE "SUB (SP minus immediate)"; constant d = UInt(Rd); constant n = UInt(Rn); constant setflags = (S == '1'); constant imm32 = T32ExpandImm(i:imm3:imm8); // Armv8-A removes UNPREDICTABLE for R13 if (d == 15 && !setflags) || n == 15 then UNPREDICTABLE;

T4

SUB{<c>}{<q>} {<Rd>,} <Rn>, #<imm12> // (<imm12> cannot be represented in T1, T2, or T3)

SUBW{<c>}{<q>} {<Rd>,} <Rn>, #<imm12> // (<imm12> can be represented in T1, T2, or T3)

if Rn == '1111' then SEE "ADR"; if Rn == '1101' then SEE "SUB (SP minus immediate)"; constant d = UInt(Rd); constant n = UInt(Rn); constant setflags = FALSE; constant imm32 = ZeroExtend(i:imm3:imm8, 32); // Armv8-A removes UNPREDICTABLE for R13 if d == 15 then UNPREDICTABLE;

if Rn == '1110' && IsZero(imm8) then SEE "ERET"; constant d = 15; constant n = UInt(Rn); constant setflags = TRUE; constant imm32 = ZeroExtend(imm8, 32); if n != 14 then UNPREDICTABLE; if InITBlock() && !LastInITBlock() then UNPREDICTABLE;

Assembler Symbols

<c>	See Standard assembler syntax fields.

<q>	See Standard assembler syntax fields.

<Rdn>

Is the general-purpose source and destination register, encoded in the "Rdn" field.

<imm8>	For encoding T2: is a 8-bit unsigned immediate, in the range 0 to 255, encoded in the "imm8" field.
	For encoding T5: is a 8-bit unsigned immediate, in the range 0 to 255, encoded in the "imm8" field. If <Rn> is the LR, and zero is used, see ERET.

<Rd>

For encoding A1: is the general-purpose destination register, encoded in the "Rd" field. If omitted, this register is the same as <Rn>. If the PC is used:

For the SUB variant, the instruction is a branch to the address calculated by the operation. This is an interworking branch, see Pseudocode description of operations on the AArch32 general-purpose registers and the PC.
For the SUBS variant, the instruction performs an exception return, that restores PSTATE from SPSR_<current_mode>. Arm deprecates use of this instruction unless <Rn> is the LR.

For encoding T1, T3 and T4: is the general-purpose destination register, encoded in the "Rd" field. If omitted, this register is the same as <Rn>.

<Rn>	For encoding A1 and T4: is the general-purpose source register, encoded in the "Rn" field. If the SP is used, see SUB (SP minus immediate). If the PC is used, see ADR.
	For encoding T1: is the general-purpose source register, encoded in the "Rn" field.
	For encoding T3: is the general-purpose source register, encoded in the "Rn" field. If the SP is used, see SUB (SP minus immediate).

<imm3>

Is a 3-bit unsigned immediate, in the range 0 to 7, encoded in the "imm3" field.

<imm12>

Is a 12-bit unsigned immediate, in the range 0 to 4095, encoded in the "i:imm3:imm8" field.

<const>	For encoding A1: an immediate value. See Modified immediate constants in A32 instructions for the range of values.
	For encoding T3: an immediate value. See Modified immediate constants in T32 instructions for the range of values.

Operation

if ConditionPassed() then EncodingSpecificOperations(); constant (result, nzcv) = AddWithCarry(R[n], NOT(imm32), '1'); if d == 15 then if setflags then ALUExceptionReturn(result); else ALUWritePC(result); else R[d] = result; if setflags then PSTATE.<N,Z,C,V> = nzcv;

Operational information

If CPSR.DIT is 1 and this instruction does not use R15 as either its source or destination:

Internal version only: isa v01_31, pseudocode v2024-03_rel ; Build timestamp: 2024-03-25T10:05