multithreading - InterlockedExchange and memory alignment

Question

I am confused that Microsoft says memory alignment is required for InterlockedExchange however, Intel documentation says that memory alignment is not required for LOCK. Am i missing something, or whatever? thanks

from Microsoft MSDN Library

Platform SDK: DLLs, Processes, and Threads InterlockedExchange

The variable pointed to by the Target parameter must be aligned on a 32-bit boundary; otherwise, this function will behave unpredictably on multiprocessor x86 systems and any non-x86 systems.

from Intel Software Developer’s Manual;

• LOCK instruction Causes the processor’s LOCK# signal to be asserted during execution of the accompanying instruction (turns the instruction into an atomic instruction). In a multiprocessor environment, the LOCK# signal insures that the processor has exclusive use of any shared memory while the signal is asserted.

  The integrity of the LOCK prefix is not affected by the alignment of the memory field. Memory locking is observed for arbitrarily misaligned fields.

• Memory Ordering in P6 and More Recent Processor Families

  Locked instructions have a total order.

• Software Controlled Bus Locking

  The integrity of a bus lock is not affected by the alignment of the memory field. The LOCK semantics are followed for as many bus cycles as necessary to update the entire operand. However, it is recommend that locked accesses be aligned on their natural boundaries for better system performance: ?Any boundary for an 8-bit access (locked or otherwise). ?16-bit boundary for locked word accesses. ?32-bit boundary for locked doubleword accesses. ?64-bit boundary for locked quadword accesses.

Answer 1

Once upon a time, Microsoft supported WindowsNT on processors other than x86, such as MIPS, PowerPC, and Alpha. These processors all require alignment for their interlocked instructions, so Microsoft put the requirement in their spec to ensure that these primitives would be portable to different architectures.