A stack register is a register on a computer’s core processor that keeps track of a call stack. This may be a dedicated register on an accumulator-based architecture machine, such as SP on an Intel x86 system. 

On a general register machine, such as the PDP-11 or RISC machines, it could be a register that is reserved by convention. Like the Data General Eclipse, some designs did not have a dedicated register for this function instead of using a reserved hardware memory address.

Compilers that enabled recursion were not available on machines before the late 1960s, such as the PDP-8 and HP 2100. Their subroutine instructions would normally save the current location in the jump address before moving the program counter to the next address. 

While this is less work than maintaining a stack because each subroutine code section only has one return point, recursion is impossible without a lot of effort on the programmer’s behalf. The main stack register of the 8086 is known as the stack pointer – SP. 

The stack segment register (SS) is typically used to record information about the memory segment that stores the current program’s call stack. SP denotes the top of the current stack. Newer values are inserted at lower memory addresses. The PUSH instruction is used to push a value to the stack. The POP instruction is used to pop a value from the stack.

The stack is mostly utilized during a function call. Still, it can also be used to temporarily hold processor register data or other variables, depending on the language and programming level. Furthermore, when using recursive functions that store partial data on the stack and call themselves again, the stack can be used for short-term large-scale data storage.

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