Hi.
I just started learning assembly and there's lots of thing that I must know.
I have a question about AX, BX, CX, DX registers.
Are they unique for every program or every segment? Or they are just one for all programs?

Thanks

registers are part of processor, physical cells to store bytes/words. they are not unique for program, but there is a stack mechanism where program may store values from registers before, say, call subroutine ("another program"), and then restore these values after subroutine returns.

They (or rather their contents) are "unique" for every context in multitasking systems.

For every core (including hyperthreading and SMP systems) of each CPU, there is one set of registers shared by all processes and applications. For a multi-CPU/core/processor system there is one set of registers for each logical CPU.

What about other running programs, when we change their contents ?
Do they (other programs) use them (registers) in this time (when we are changing them) ?

Thanks

Each individual CPU can only run one task at a time. Multi-tasking is merely an illusion created by the OS by switching between tasks quickly.

To elaborate on what revolution just said: In order to make it appear to each program as if it has all the registers all to itself, the OS stores the values in the registers to RAM when it switches to a different task, and puts in the values of the stored registers of the task it's switching to.

In other words, as an application programmer, you don't have to worry about the fact that the registers are shared between your program and others. The OS does some stuff behind the scenes to allow you to use all registers as if your program is the only one running.

Thanks