atomic reads and writes

[Ricardo Hassan]

Can anyone tell me, on an ARM7 are simple assignments atomic? For
example if in C I write "a = 4;", is that atomic. Also, if I do if (a
== 4), is that atomic?

Thanks,

Ric

[Boudewijn Dijkstra]

Op Wed, 08 Jun 2011 02:05:34 +0200 schreef Ricardo Hassan

Post by Ricardo Hassan
Can anyone tell me, on an ARM7 are simple assignments atomic? For
example if in C I write "a = 4;", is that atomic. Also, if I do if (a
== 4), is that atomic?

They could be, depending on the guarantees that your compiler makes.

[Sprow]

Post by Ricardo Hassan
Can anyone tell me, on an ARM7 are simple assignments
atomic?  For example if in C I write "a = 4;", is that
atomic.  Also, if I do if (a
== 4), is that atomic?

If the variable has natural alignment I would expect the compiler to
boil the assignment into an STRB, STRH, or STR. These are atomic.

For a 64 bit variable, either an STM sequence or STRD will be produced
(depending on architecture) and these can usually be configured to be
either atomic or interruptable. For example if the store crossed a
page boundary into a page that doesn't exist then you'd have to wait
for the data abort handler to fix up the missing page.

If the variable has arbitrary alignment then anything bigger than an 8
bit variable will probably be cut up into smaller stores which wont be
atomic,
Sprow.