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assemblymipsinstruction-set

Heap pointer register?

Many ISAs, such as MIPS, has a register called "stack pointer" to indicate the top of the stack. Why isn't a "heap pointer" register? Does the top of the heap get stored in memory?

Solution

  • Why isn't a "heap pointer" register?

    There is not a "heap pointer" register, because there is no malloc CPU instruction.

    The reason there is a "stack pointer" register is because there are CPU instructions which directly manipulate the stack, such as call, push, and pop on x86. These instructions generally mandate the existence of an architectural stack pointer register.

    Note: AFAIK there are no such instructions on MIPS; you must adjust the stack pointer yourself to push/pop values, and a "call" saves the return address in the Link Register. Still, the concepts of a call stack and local variables are so central to most all languages, that a register is used to track the stack location, because it is much faster than a memory access.

    Does the top of the heap get stored in memory?

    There are lots of different implementations of malloc, but they all only take a size parameter, there is no option but for malloc to keep state in global variables.