Assembly Language for Beginners

1.9. SCANF()

35 ; restore FP and LR:
36 ldp x29, x30, [sp], 16
37 ret

In this case thexvariable is declared as global and its address is calculated using theADRP/ADDinstruction

pair (lines 21 and 25).

MIPS

Uninitialized global variable

So now thexvariable is global. Let’s compile to executable file rather than object file and load it intoIDA.

IDA displays thexvariable in the.sbssELF section (remember the “Global Pointer”?1.5.5 on page 25),

since the variable is not initialized at the start.

Listing 1.77: Optimizing GCC 4.4.5 (IDA)

.text:004006C0 main:

.text:004006C0

.text:004006C0 var_10 = -0x10

.text:004006C0 var_4 = -4

.text:004006C0

; function prologue:

.text:004006C0 lui $gp, 0x42

.text:004006C4 addiu $sp, -0x20

.text:004006C8 li $gp, 0x418940

.text:004006CC sw $ra, 0x20+var_4($sp)

.text:004006D0 sw $gp, 0x20+var_10($sp)

; call puts():

.text:004006D4 la $t9, puts

.text:004006D8 lui $a0, 0x40

.text:004006DC jalr $t9 ; puts

.text:004006E0 la $a0, aEnterX # "Enter X:" ; branch delay slot

; call scanf():

.text:004006E4 lw $gp, 0x20+var_10($sp)

.text:004006E8 lui $a0, 0x40

.text:004006EC la $t9, __isoc99_scanf

; prepare address of x:

.text:004006F0 la $a1, x

.text:004006F4 jalr $t9 ; __isoc99_scanf

.text:004006F8 la $a0, aD # "%d" ; branch delay slot

; call printf():

.text:004006FC lw $gp, 0x20+var_10($sp)

.text:00400700 lui $a0, 0x40

; get address of x:

.text:00400704 la $v0, x

.text:00400708 la $t9, printf

; load value from "x" variable and pass it to printf() in $a1:

.text:0040070C lw $a1, (x - 0x41099C)($v0)

.text:00400710 jalr $t9 ; printf

.text:00400714 la $a0, aYouEnteredD___ # "You entered %d...\n" ; branch⤦

Çdelay slot

; function epilogue:

.text:00400718 lw $ra, 0x20+var_4($sp)

.text:0040071C move $v0, $zero

.text:00400720 jr $ra

.text:00400724 addiu $sp, 0x20 ; branch delay slot

...

.sbss:0041099C # Segment type: Uninitialized

.sbss:0041099C .sbss

.sbss:0041099C .globl x

.sbss:0041099C x: .space 4

.sbss:0041099C

IDA reduces the amount of information, so we’ll also do a listing using objdump and comment it: