What Is a Stack Register?

The stack pointer register is used in stack operations. The PUSH and POP instructions obtain the intra-segment offset of the current stack segment from the SP register, so the SP register is called the stack pointer, and the SP always points to the top of the stack.

Stack pointer register

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The stack pointer register is used in stack operations. The PUSH and POP instructions obtain the intra-segment offset of the current stack segment from the SP register, so the SP register is called the stack pointer, and the SP always points to the top of the stack.

Stack is a widely used technology in computers. Based on the data in and out LIFO characteristics of the stack, it is often used to save interrupt breakpoints, save subroutine call return points, save CPU field data, etc., and also used to pass parameters between programs.

In ARM processors, register R13 is usually used as the stack pointer (SP). The ARM processor has six stack pointers (SPs) for different modes. The user mode and system mode share one SP. Each exception mode has its own dedicated R13 register (SP). They usually point to a dedicated stack for each mode, that is, the ARM processor allows the user program to have six different stack spaces. These stack pointers are R13, R13_svc, R13_abt, R13_und, R13_irq, R13_fiq.

In order to describe the stack more accurately, the stack is divided into 'increment stack' (SP changes toward large values) and 'decrement stack' (SP moves toward small values) according to the direction of stack pointer increase and decrease during the "push stack" operation ; According to whether the storage unit pointed to by the SP pointer contains stack data, the stack is divided into 'full stack' (the SP pointing unit contains valid stack data) and 'empty stack' (the SP pointing unit does not contain valid stack data).
In this way, there are four stacking modes in two pairs: full increment, empty increment, full decrement, and empty decrement.
ARM processors have very flexible stack operations and support for all four types of stacks.
R13 in the ARM processor is used as the SP. When the stack is not used, R13 can also be used as a general-purpose data register.