strexb

Store Register Exclusive Byte (A32)

STREXB<c> <Rd>, <Rt>, [<Rn>]

Stores a byte if address is still exclusive.

Details

Conditionally stores a byte to memory if the address is still marked as exclusive, returning a status in Rd (0=success, 1=failure). No condition flags are modified by the store result; success/failure is reported only in Rd. This is an A32-only instruction that atomically releases the exclusive lock and performs the write only if the lock is still held from a prior LDREXB.

Pseudocode Operation

if ConditionPassed(cond) then
  address ← Rn
  byte_value ← Rt[7:0]
  if ExclusiveMonitorSet(address, 1) then
    [address] ← byte_value
    Rd ← 0
    ClearExclusiveMonitor(address, 1)
  else
    Rd ← 1

Example

STREXB r0, r3, [r1]

Encoding

Binary Layout
cond
00011
10
0
Rn
Rd
1
1
1
1
1001
Rt
 
Format Load/Store Excl
Opcode 0x01C00F90
Extension A32 (Atomic)

Operands

  • Rd
    Status
  • Rt
    Transfer general-purpose register (load/store)
  • Rn
    First source / base general-purpose register

Reference (Arm AArch32 ISA)

Instruction Forms

Encoding Instruction ISA Bit pattern
0x01C00F90 STREXB{<c>}{<q>} <Rd>, <Rt>, [<Rn>] A32 cond | 00011 | 10 | 0 | Rn | Rd | 1 | 1 | 1 | 1 | 1001 | Rt
0xE8C00F40 STREXB{<c>}{<q>} <Rd>, <Rt>, [<Rn>] T32 11101000110 | 0 | Rn | Rt | 1111 | 01 | 00 | Rd

Description

Store Register Exclusive Byte derives an address from a base register value, stores a byte from a register to the derived address if the executing PE has exclusive access to the memory at that address, and returns a status value of 0 if the store was successful, or of 1 if no store was performed. For more information about support for shared memory see Synchronization and semaphores. For information about memory accesses see Memory accesses.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    address = R[n];
    if AArch32.ExclusiveMonitorsPass(address,1) then
        MemA[address,1] = R[t]<7:0>;
        R[d] = ZeroExtend('0', 32);
    else
        R[d] = ZeroExtend('1', 32);