.Code%
MVN       R0, #0              \ Set new state of flag - no access allowed.
ADR       R2, Flag%           \ Get address of flag.
.Loop%
SWP       R1, R0, [R2]        \ Swap old and new states of flag.
TEQS      R1, #0              \ Is access allowed?
BNE       Loop%               \ If not, keep waiting.
.DoSomething%
\                             \ ... Do something ...
.Exit%
MOV       R0, #0              \ Set new state of flag - access allowed.
ADR       R2, Flag%           \ Get address of flag.
STR       R0, [R2]            \ Store new state of flag.
MOV       R15, R14            \ Return to caller.
.Flag%
EQUD      0                   \ Reserve memory for flag.