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Asked: 2026-05-19T22:15:59+00:00

Sorry, simple question but it is something that is always confused me when speaking

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Sorry, simple question but it is something that is always confused me when speaking about Operating Systems theory.

My book claims that reentrancy is made possible by using “the stack” – does this mean that there is one ‘stack’ and it is, somehow, shared among all routines, or does “the stack” refer to the abstract idea of a stack, and a stack is created and stored in main memory every time some subroutine is called ? (Assuming it is needed).

And what about the Stack Pointer in the CPU in this case ? How does that function, in either of these cases ?

Apologies for the vagueness of my description, the whole idea is just kind of shrouded in conflicting ideas for me !

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  1. Editorial Team

    This is platform specific (depends on used hardware and software).

    Windows on x86/x64 has a memory for system stack ready for each thread. It is reused anytime you do a system call in that thread.

    System is re-entrant when multiple independent processes or threads can call system routines at once. In this case each of those threads uses its own system stack. System stack (technically ring 0 stack) is independent from user mode (ring 3) stack.

    I can also provide more information, just ask if you wish to know something more specific.

    Stack pointer:

    • During normal execution, each thread has its own normal stack and its own stack pointer. It moves down on each call etc.
    • When one particular thread calls operating system routine, current stack pointer value is stored and stack pointer is set to always the same address – system (ring 0) stack.
    • Each thread has its own system (ring 0) stack.
    • When system call ends, original value of stack pointer is restored. So here we actually abandon system stack and go back to normal stack.
    • Size of “normal” stack is set in compiler/linker. It can grow to megabytes. But system stack has a limited static size and you can never change its location or size. It is created exactly so big so any system call can fit there.
    • If one system routine internally calls another system routine, this is detected by hardware and it is called directly, i.e. in that case no special arrangement of stack pointers is made.
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