ARM Nested Procedures

Nested Procedures

int sumSquare(int x, int y){
    return (mult(x, x) + y);
}

sumSquare:
push {r4-r11, lr}
MOV r4, r1
MOV r1, r0
BL mult
ADD r0, r0, r4
pop {r4-r11, lr}
BX lr @ then?

main:
MOV r0, #2
MOV r1, #3
BL sumSquare
MOV r3, r0

Without pushing lr, calling another function overwrites it.

String constants in ARM

x = 0xaabbccdd;
printf("%d\n", x);

// how printf works
%int printf(const char *format, ...)

Where to store

1. String “%d\n” — in data segment
2. Value 0xaabbccdd —

String formats (unfinished)

.ascii

.asciz

.data
mystr: .asciz "%d\n"
mynum: .word 0xaabbccdd

print_fun:
push {lr}
LDR r0, =mystr 
@ location with label mystr is store in r0
LDR r1, =mynum
LDR r1, [r1]
BL printf
pop {lr}
BX lr

Alignment

.text
.align 2

mystr is placed in a location that is a multiple of 2^2 = 4.

Data objects should start at memory addresses that are divisible by the size of the data object

• short (2-byte) at address divisible by 2
• int (4-bytes) at address divisible by 4
• double (8-byte) at address divisible by 8

struct p {
    char x;
    int y;
};
struct p sp;

In this case, sizeof(p) is 8. int should be aligned.