This page contains notes about the usual stack frame layout (calling convention) on ARM processors.

Most information is taken from ARM procedure calling conventions and recursion .

The conventions are defined by ARM and are called PCSAA.

General Notes

• BLand BLX use R14 (RL) as link register
• Simple functions (i.e. leaf functions) use MOV PC, LR to return to the caller function
• R0 - R3 are used to pass arguments to the callee (can be overwritten by callee, caller saved)
• R0 contains often the result
• R4 - R10 must be saved by the called function (callee) if needed
  • They must be unchanged after return
  • Callee saved
  • Restore before return
• R13 (SP) contains the address of top of the stack (last filled poition)
• Stack is Full-Descending:
  • SP points to the last filled location
  • Stack grows downwards (from higher to lower addresses)

PUSH/POP: STMFD/LDMFD (Store multiple Full-Descent, Load multiple Full-Descent)

LDMFD SP, {R0-R3}

is equvalent to:

LDR R0, [SP]
LDR R1, [SP, #4]
LDR R2, [SP, #8]
LDR R3, [SP, #12]

To alter (update) SP:

LDMFD SP!, {R0-R3}

LR needs to be saved on stack for non-leaf functions

_func:
  STMFD SP!, {R4-R6, LR}
  ; ... code of func
  LDMFD SP!, {R4-R6, PC} ; Pushing stacked LR directly to PC => return to caller

• The order of registers in STMFD and LDMFD is always the same: lower regisers at lower addresses

• More general stack frame:

• Quick clear down of stack frame: MOV SP, FP