ARM Assembly – The Software Stack

Software Stack

Chris McCarthy

FUNCTIONS IN ASM

• Not 'native' to assembly
• We need to do a lot of the management ourselves
• Argument passing:
• How do we pass arguments from one function to another
• Storing and recalling register values
• each function we call will want to use the same registers (only 13 general purpose registers!)
• How do we manage this?
• Managing the program control
• Jumping from one function to another, and then returning back!

• Not 'native' to assembly
• We need to do a lot of the management ourselves
• Argument passing:
• How do we pass arguments from one function to another
• Storing and recalling register values
• each function we call will want to use the same registers (only 13 general purpose registers!)
• How do we manage this?
• Managing the program control
• Jumping from one function to another, and then returning back!

STACKS

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PUSH, POP AND THE STACK

• ARM computers have a software stack*.
• A separate area of RAM is available for temporary values.
• A value in a register can be pushed onto the stack to preserve it for later.
• It can be popped off later (in LIFO order).
• We can get the memory location (a pointer to it) by checking the SP (R13) register.

* SOFTWARE STACK?

• A section of RAM managed by the SP (stack pointer) register.
• A sort of 32-bit (64-bit in ARM8) wide array which starts (element 0) high in RAM and grows down as values are added to it.
• The stack pointer stores the memory location of the last value added (pushed) to the stack.
• Each push decrements SP by 4 (4 bytes per word).
• A pop operation removes the last value in the stack and increments the SP by 4 (4 bytes per word)

Software stack (depth only limited by RAM) Example:

x = don’t care. [SP] points to start of stack. X

X X X X

[SP] X

X

Software stack (depth only limited by RAM) Example:

x = don’t care. 5

push 5 – SP decremented by 4. [SP] points to 5 X X X X

X

Software stack (32-bit) (depth only limited by Example: RAM)

x = don’t care. 5

push 7 – SP decremented by 4, [SP] points to 7. 7 X X X

X

Software stack (depth only limited by RAM) Example:

x = don’t care. 5

push 7 – SP decremented by 4 7 push 0 – SP decremented by 4. [SP] points to 0 X X

X

Software stack (depth only limited by RAM) Example:

x = don’t care. 5

push 7 – SP decremented by 4 7 pop 0 – SP incremented by 4. [SP] points to 7 X X X

X

Software stack (depth only limited by RAM) Example:

x = don’t care. 5

push 7 – SP decremented by 4 X pop 0 – SP incremented by 4 X pop 7 – SP incremented by 4. [SP] points to 5. X X

X

Software stack (depth only limited by RAM) Example:

x = don’t care. 5

push 7 – SP decremented by 4 2 pop 0 – SP incremented by 4 X push 2 – SP decremented by 4. [SP] points to 2. X

X

[SP] X

X

Software stack (depth only limited by RAM) Example:

x = don’t care. 5

push 7 – SP decremented by 4 X pop 0 – SP incremented by 4 X push 2 – SP decremented by 4 X pop 2 – SP incremented by 4. [SP] points to 5. X

X

Software stack (depth only limited by RAM) Example:

x = don’t care. X

push 7 – SP decremented by 4 X pop 0 – SP incremented by 4 X push 2 – SP decremented by 4 X pop 5 – SP incremented back to it’s starting value. The stack is now empty. X [SP] points to end of stack. [SP] X X

EXAMPLE SYNTAX

• Push and pop accept multiple registers if in a {,,,...} list Correct order is push {r4,r5};back them up onto the stack preserved for;use r4 and r5 for something else {lists} pop {r4,r5};restore them from the stack
• Alternatively, do one at a time (but pop in reverse order) push {r4} push {r5}; do something pop {r5} pop {r4}

RECALL THE ABI

• Application Binary Interface (ABI) sets standard way of using ARM registers.
• r0-r3 used for function arguments and return values
• r4-r12 promised not to be altered by functions
• lrand sp used for stack management
• pc is the next instruction – we can use it to exit a function call

ABI CONVENTIONS

• ABI compliant functions:
• Use r0-r3 for passing and returning values to functions
• Promise not to alter r4-r12
• … but suppose the function needs to use many registers to do calculations??
• We can use the stack to store and recall register values!

PASSING ARGUMENTS TO FUNCTIONS

• To re -use the registers we need to:
• Back up registers we need to re -use in a function
• Store arguments for the function in r0 -r3
• Call the function
• Read the return values from r0-r1 (optional)
• Restore the registers we backed up.

SUMMARY

• Software Stack:
• Dedicated RAM used to store values FILO
• Special register “sp” used to store address of start of the stack
• Stacks allow us to store and recall register values efficiently
• Stacks integral to functions:
• We need to store and recall register values so we don’t run out of registers to use!