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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

STREX

10.136 STREX

Store Register Exclusive.

Syntax

STREX{cond} Rd, Rt, [Rn {, #offset}]
STREXB{cond} Rd, Rt, [Rn]
STREXH{cond} Rd, Rt, [Rn]
STREXD{cond} Rd, Rt, Rt2, [Rn]
where:
cond
is an optional condition code.
Rd
is the destination register for the returned status.
Rt
is the register to store.
Rt2
is the second register for doubleword stores.
Rn
is the register on which the memory address is based.
offset
is an optional offset applied to the value in Rn. offset is permitted only in Thumb instructions. If offset is omitted, an offset of 0 is assumed.

Operation

STREX performs a conditional store to memory. The conditions are as follows:
  • If the physical address does not have the Shared TLB attribute, and the executing processor has an outstanding tagged physical address, the store takes place, the tag is cleared, and the value 0 is returned in Rd.
  • If the physical address does not have the Shared TLB attribute, and the executing processor does not have an outstanding tagged physical address, the store does not take place, and the value 1 is returned in Rd.
  • If the physical address has the Shared TLB attribute, and the physical address is tagged as exclusive access for the executing processor, the store takes place, the tag is cleared, and the value 0 is returned in Rd.
  • If the physical address has the Shared TLB attribute, and the physical address is not tagged as exclusive access for the executing processor, the store does not take place, and the value 1 is returned in Rd.

Restrictions

PC must not be used for any of Rd, Rt, Rt2, or Rn.
For STREX, Rd must not be the same register as Rt, Rt2, or Rn.
For ARM instructions:
  • SP can be used but use of SP for any of Rd, Rt, or Rt2 is deprecated in ARMv6T2 and above.
  • For STREXD, Rt must be an even numbered register, and not LR.
  • Rt2 must be R(t+1).
  • offset is not permitted.
For Thumb instructions:
  • SP can be used for Rn, but must not be used for any of Rd, Rt, or Rt2.
  • The value of offset can be any multiple of four in the range 0-1020.

Usage

Use LDREX and STREX to implement interprocess communication in multiple-processor and shared-memory systems.
For reasons of performance, keep the number of instructions between corresponding LDREX and STREX instructions to a minimum.

Note

The address used in a STREX instruction must be the same as the address in the most recently executed LDREX instruction.

Architectures

ARM STREX is available in ARMv6 and above.
ARM STREXB, STREXD, and STREXH are available in ARMv6K and above.
All these 32-bit Thumb instructions are available in ARMv6T2 and above, except that STREXD is not available in the ARMv7-M architecture.
There are no 16-bit versions of these instructions.

Examples

    MOV r1, #0x1                ; load the ‘lock taken’ value
try
    LDREX r0, [LockAddr]        ; load the lock value
    CMP r0, #0                  ; is the lock free?
    STREXEQ r0, r1, [LockAddr]  ; try and claim the lock
    CMPEQ r0, #0                ; did this succeed?
    BNE try                     ; no – try again
    ....                        ; yes – we have the lock
Related concepts
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