Clarification on Atomic Boundaries (P16) 2017-08-18 - Stafford Horne

Section 7.3 Atomicity explains that a upon load a reservation is made at the address of the load memory location and that subsequent stores to the same memory location will cancel the reservation. It is not clear whether stores of byte or half-word size overlapping the memory location cause reservation cancellation.

In current implementations stores anywhere within the 32-bit boundary will invalidate the memory reservation. This should be clarified.

Changes

Change the sentence in 7.3 Atomicity from:

The reservation for a subsequent l.swa is cancelled if another store to the same memory location occur, another master writes the same memory location (snoop hit), another l.swa (to any memory location) is executed, another l.lwa is executed or a context switch (exception) occur.

to

The reservation for a subsequent l.swa is cancelled if another store overlapping the same memory location occurs, another master writes overlapping the same memory location (snoop hit), another l.swa (to any memory location) is executed, another l.lwa is executed or a context switch (exception) occur. The overlapping writes may be byte or half-word size.