Is the operation below an atomic operation in a multitasking environment like RTX-51? If it is not, then what is supposed to do to make it an atomic one except using locking mechanisms?
void func() { int locVal = globalVal++; ... ... }
Most processors don't have any hardware support for atomic operations, so the compiler would have to disable interrupts.
Unfortunately that only moves the problem elsewhere, but doesn't solve it. The trouble being that in general you have to save the previous state of the interrupt-enable flag before you disable interrupts, so you can clean up after yourself. In order for that saved state to be valid, these two operations have to be combined into an atomic step.
I.e. you need an atomic instruction to work around the lack of an atomic instruction. Chicken/Egg, here we come!
Only if you do things like disabling interrupts in an ISR without re-enabling them before leaving the ISR, or your processor has an interrupt system where there's a window after disabling interrupts in which interrupts can still occur.
And even the venerable 8051 has an atomic "jump if bit is set and clear bit" instruction.
"The trouble being that in general you have to save the previous state of the interrupt-enable flag before you disable interrupts [...]".
From my second post in this thread "This is similar to disabling interrupts, but with an automatic enable, and the program doesn't need to know if interrupts was enabled or disabled when processing the lock instruction or processing instructions with lock flags."
But as noted by Christoph Franck, you seldom have a problem with the atomicity of caching the interrupt flag state before turning off interrupts. The problem is more often the number of instructions needed.
For Keil C51 specifically, see:
#pragma disable
http://www.keil.com/support/man/docs/c51/c51_disable.htm