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Assembler User Guide

Preface Overview of the Assembler Overview of the ARM Architecture Structure of Assembly Language Modules Writing ARM Assembly Language Condition Codes Using the Assembler Symbols, Literals, Expressions, and Operators VFP Programming Assembler Command-line Options ARM and Thumb Instructions ARM and Thumb instruction summary Instruction width specifiers Flexible second operand (Operand2) Syntax of Operand2 as a constant Syntax of Operand2 as a register with optional shi Shift operations Saturating instructions Condition code suffixes ADC ADD ADR (PC-relative) ADR (register-relative) ADRL pseudo-instruction AND ASR B BFC BFI BIC BKPT BL BLX BX BXJ CBZ and CBNZ CDP and CDP2 CLREX CLZ CMP and CMN CPS CPY pseudo-instruction DBG DMB DSB EOR ERET HVC ISB IT LDC and LDC2 LDM LDR (immediate offset) LDR (PC-relative) LDR (register offset) LDR (register-relative) LDR pseudo-instruction LDR, unprivileged LDREX LSL LSR MCR and MCR2 MCRR and MCRR2 MLA MLS MOV MOV32 pseudo-instruction MOVT MRC and MRC2 MRRC and MRRC2 MRS (PSR to general-purpose register) MRS (system coprocessor register to ARM register) MSR (ARM register to system coprocessor register) MSR (general-purpose register to PSR) MUL MVN NEG pseudo-instruction NOP ORN (Thumb only) ORR PKHBT and PKHTB PLD and PLI POP PUSH QADD QADD8 QADD16 QASX QDADD QDSUB QSAX QSUB QSUB8 QSUB16 RBIT REV REV16 REVSH RFE ROR RRX RSB RSC SADD8 SADD16 SASX SBC SBFX SDIV SEL SETEND SEV SHADD8 SHADD16 SHASX SHSAX SHSUB8 SHSUB16 SMC SMLAxy SMLAD SMLAL SMLALD SMLALxy SMLAWy SMLSD SMLSLD SMMLA SMMLS SMMUL SMUAD SMULxy SMULL SMULWy SMUSD SRS SSAT SSAT16 SSAX SSUB8 SSUB16 STC and STC2 STM STR (immediate offset) STR (register offset) STR, unprivileged STREX SUB SUBS pc, lr SVC SWP and SWPB SXTAB SXTAB16 SXTAH SXTB SXTB16 SXTH SYS TBB and TBH TEQ TST UADD8 UADD16 UASX UBFX UDIV UHADD8 UHADD16 UHASX UHSAX UHSUB8 UHSUB16 UMAAL UMLAL UMULL UND pseudo-instruction UQADD8 UQADD16 UQASX UQSAX UQSUB8 UQSUB16 USAD8 USADA8 USAT USAT16 USAX USUB8 USUB16 UXTAB UXTAB16 UXTAH UXTB UXTB16 UXTH WFE WFI YIELD VFP Instructions Directives Reference Via File Syntax

SUBS pc, lr

10.138 SUBS pc, lr

Exception return, without popping anything from the stack.

Syntax

SUBS{cond} pc, lr, #imm ; ARM and Thumb code
MOVS{cond} pc, lr ; ARM and Thumb code
op1S{cond} pc, Rn, #imm ; ARM code only and is deprecated
op1S{cond} pc, Rn, Rm {, shift} ; ARM code only and is deprecated
op2S{cond} pc, #imm ; ARM code only and is deprecated
op2S{cond} pc, Rm {, shift} ; ARM code only and is deprecated
where:
op1
is one of ADC, ADD, AND, BIC, EOR, ORN, ORR, RSB, RSC, SBC, and SUB.
op2
is one of MOV and MVN.
cond
is an optional condition code.
imm
is an immediate value. In Thumb code, it is limited to the range 0-255. In ARM code, it is a flexible second operand.
Rn
is the first operand register. ARM deprecates the use of any register except LR.
Rm
is the optionally shifted second or only operand register.
shift
is an optional condition code.

Usage

SUBS pc, lr, #imm subtracts a value from the link register and loads the PC with the result, then copies the SPSR to the CPSR.
You can use SUBS pc, lr, #imm to return from an exception if there is no return state on the stack. The value of #imm depends on the exception to return from.

Notes

SUBS pc, lr, #imm writes an address to the PC. The alignment of this address must be correct for the instruction set in use after the exception return:
  • For a return to ARM, the address written to the PC must be word-aligned.
  • For a return to Thumb, the address written to the PC must be halfword-aligned.
  • For a return to Jazelle, there are no alignment restrictions on the address written to the PC.
No special precautions are required in software to follow these rules, if you use the instruction to return after a valid exception entry mechanism.
In Thumb, only SUBS{cond} pc, lr, #imm is a valid instruction. MOVS pc, lr is a synonym of SUBS pc, lr, #0. Other instructions are undefined.
In ARM, only SUBS{cond} pc, lr, #imm and MOVS{cond} pc, lr are valid instructions. Other instructions are deprecated in ARMv6T2 and above.

Caution

Do not use these instructions in User mode or System mode. The assembler cannot warn you about this.

Architectures

This ARM instruction is available in all versions of the ARM architecture.
This 32-bit Thumb instruction is available in ARMv6T2 and above, except the ARMv7-M architecture.
There is no 16-bit version of this instruction in Thumb.
Non-ConfidentialPDF file icon PDF versionARM DUI0379H
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