LDREX 和 STREX

最新推荐文章于 2023-02-28 21:56:51 发布

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本文详细介绍了ARMv6架构中的LDREX和STREX指令，这两种指令用于实现内存操作的原子性。文章解释了如何使用这些指令进行独占访问，并给出了一段示例代码，展示了如何使用LDREX和STREX来实现简单的锁机制。

LDREX and STREX

Load and Store Register Exclusive.

LDREX{cond} Rt, [Rn {, #offset}]

STREX{cond} Rd, Rt, [Rn {, #offset}]

where:

cond

is an optional condition code.

Rd

is the destination register for the returned status.

Rt

is the register to load or store.

Rt2

is the second register for doubleword loads or 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 32-bit Thumb instructions. If offset is omitted, an offset of 0 is assumed.

LDREX loads data from memory.

If the physical address has the Shared TLB attribute, LDREX tags the physical address as exclusive access for the current processor, and clears any exclusive access tag for this processor for any other physical address.
Otherwise, it tags the fact that the executing processor has an outstanding tagged physical address.

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.

Note

The address used in a STREX instruction must be the same as the address in the most recently executed LDREXinstruction. The result of executing a STREX instruction to a different address is unpredictable.

ARM LDREX and STREX are available in ARMv6 and above.

    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