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I have a question about the register file. As I know, the stack pointer is one of the special registers in the register file. Why it is determined specifically? I mean compiler can define any register as a stack pointer and it can use it because it knows which is defined as the stack pointer during the compilation. So, no need for any special register for it. Thank you.

Peter Cordes
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Ömer GÜZEL
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    For application programming, it doesn't matter too much, except that instructions that are hardcoded to use the stack pointer can have shorter encodings. But for system programming, you typically need the CPU to help you switch stacks when handling interrupts and exceptions, so the hardware has to know which register is the stack pointer. And the simplest way is for the architecture to declare that it's a particular fixed one. – Nate Eldredge Apr 07 '22 at 05:33
  • Are you talking about the actual storage where the register-file is kept? That's correct, it doesn't have to be special in the way it's stored. (But it can be, e.g. with a [stack-engine on modern x86](https://stackoverflow.com/questions/36631576/what-is-the-stack-engine-in-the-sandybridge-microarchitecture) to shortcut data dependencies on the stack pointer for instructions like push/pop, tracking RSP outside of the register file). – Peter Cordes Apr 07 '22 at 05:37
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    By contrast, on ISAs like MIPS (and maybe RISC-V?), there aren't any instructions use any register as a stack pointer, and not even interrupt handling does. So [`$sp` being `$29`](https://www.doc.ic.ac.uk/lab/secondyear/spim/node10.html) is purely a software convention that everyone chooses to follow. Many other ISAs *do* have some implicit uses of a specific register as a stack pointer, especially for interrupt handling. But this isn't about the register *file* (the actual physical storage used for register values, or that architectural registers are renamed onto in a CPU with reg renaming) – Peter Cordes Apr 07 '22 at 05:40
  • Additionally, thank you @PeterCordes I got your point. I am not talking about "the actual storage where the register-file is kept". And you answered in the second comment. – Ömer GÜZEL Apr 07 '22 at 06:57
  • As I understand: If there is an operating system, the CPU should know which one is the stack pointer. That's why one of the registers is reserved as the stack pointer. If there is not an operating system it is again a better way to use commonly to understand easily. – Ömer GÜZEL Apr 07 '22 at 06:57
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    Agreeing on which registers to use for what is part of a calling convention, or more generally an ABI (https://en.wikipedia.org/wiki/Application_binary_interface). That's what lets compilers (and humans) make machine code that can call each other's functions. But yes, the OS setting up a register to point to the stack memory for a user-space process is part of an ABI. – Peter Cordes Apr 07 '22 at 07:28
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    @PeterCordes, While the *base* RISC V instruction set would leave the choice of stack pointer register up to software/ABI, the compressed instruction set (C extension), has 16-bit loads and stores that implicitly source x29 for access to stack-based local variables, so while there is still no dedicated push & pop, the compressed loads & stores might be of lessor use were a register other than x29 used for a stack pointer. – Erik Eidt Apr 07 '22 at 12:58

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