ARM stack study notes

1. Register R13 is often used as a stack pointer in ARM instructions

2. For the R13 register, it corresponds to 6 different physical registers, one of which is shared by the user mode and the system mode, and the other 5 physical registers correspond to the other 5 different operating modes. The following symbols are used to distinguish different physical registers:

R13_

Among them, mode is one of the following modes: usr, fiq, irq, svc, abt, und.

3. Register R13 is often used as a stack pointer in ARM instructions, but this is just a customary usage. Users can also use other registers as stack pointers. In the Thumb instruction set, some instructions require the use of R13 as a stack pointer. Since each operating mode of the processor has its own independent physical register R13, in the initialization part of the user application, it is generally necessary to initialize R13 in each mode to point to the stack space of the operating mode. In this way, when the program enters the exception mode, the registers that need to be protected can be placed in the stack pointed to by R13, and when the program returns from the exception mode, it is restored from the corresponding stack. This method can ensure the normal execution of the program after the exception occurs.

4. There are four types of stacks: A stack is a data structure that works in a First In Last Out (FILO) manner, using a special register called the stack pointer to indicate the current operation position. The stack pointer always points to the top of the stack.

When the stack pointer points to the last data pushed into the stack, it is called a full stack, and when the stack pointer points to the next empty location where data will be placed, it is called an empty stack.

At the same time, according to the way the stack is generated, it can be divided into ascending stack and descending stack. When the stack is generated from low address to high address, it is called ascending stack, and when the stack is generated from high address to low address, it is called descending stack. In this way, there are four types of stack working modes, and ARM microprocessors support these four types of stack working modes, namely:

◎ Full descending full descending stack

The stack header is a high address, and the stack grows toward lower addresses. The stack pointer always points to the last element of the stack (the last element is the last data pushed in).

The ARM-Thumb procedure call standard and the ARM and Thumb C/C++ compilers always use a Full descending type stack.

◎ Full ascending full increment stack

The stack header is a low address, and the stack grows toward higher addresses. The stack pointer always points to the last element of the stack (the last element is the last data pushed in).

◎ Empty descending empty descending stack

The stack starts at a low address and grows toward higher addresses. The stack pointer always points to the next empty location where data will be placed.

◎ Empty ascending empty ascending stack

The stack head is a high address, and the stack grows toward lower addresses. The stack pointer always points to the next empty location where data will be placed.

5. Assembly instructions for operating stacks Stack type Push instructions Pop instructions

Full descending STMFD (STMDB) LDMFD (LDMIA)

Full ascending STMFA (STMIB) LDMFA (LDMDA)

Empty descending STMED (STMDA) LDMED (LDMIB)

Empty ascending STMEA (STMIA) LDMEA (LDMDB)

example:

STMFD r13!, {r0-r5} ; Push onto a Full Descending Stack

LDMFD r13!, {r0-r5} ; Pop from a Full Descending Stack.