1.9. SCANF()
.text:004006A8 li $gp, 0x418930
.text:004006AC sw $ra, 0x20+var_4($sp)
.text:004006B0 sw $s0, 0x20+var_8($sp)
.text:004006B4 sw $gp, 0x20+var_10($sp)
.text:004006B8 la $t9, puts
.text:004006BC lui $a0, 0x40
.text:004006C0 jalr $t9 ; puts
.text:004006C4 la $a0, aEnterX # "Enter X:"
.text:004006C8 lw $gp, 0x20+var_10($sp)
; prepare high part of x address:
.text:004006CC lui $s0, 0x41
.text:004006D0 la $t9, isoc99_scanf
.text:004006D4 lui $a0, 0x40
; add low part of x address:
.text:004006D8 addiu $a1, $s0, (x - 0x410000)
; now address of x is in $a1.
.text:004006DC jalr $t9 ; __isoc99_scanf
.text:004006E0 la $a0, aD # "%d"
.text:004006E4 lw $gp, 0x20+var10($sp)
; get a word from memory:
.text:004006E8 lw $a1, x
; value of x is now in $a1.
.text:004006EC la $t9, printf
.text:004006F0 lui $a0, 0x40
.text:004006F4 jalr $t9 ; printf
.text:004006F8 la $a0, aYouEnteredD # "You entered %d...\n"
.text:004006FC lw $ra, 0x20+var_4($sp)
.text:00400700 move $v0, $zero
.text:00400704 lw $s0, 0x20+var_8($sp)
.text:00400708 jr $ra
.text:0040070C addiu $sp, 0x20
...
.data:00410920 .globl x
.data:00410920 x: .word 0xA
Why not .sdata? Perhaps that this depends on some GCC option?
Nevertheless, nowxis in .data, which is a general memory area, and we can take a look how to work with
variables there.
The variable’s address must be formed using a pair of instructions.
In our case those areLUI(“Load Upper Immediate”) andADDIU(“Add Immediate Unsigned Word”).
Here is also the objdump listing for close inspection:
Listing 1.80: Optimizing GCC 4.4.5 (objdump)004006a0
4006a0: 3c1c0042 lui gp,0x42
4006a4: 27bdffe0 addiu sp,sp,-32
4006a8: 279c8930 addiu gp,gp,-30416
4006ac: afbf001c sw ra,28(sp)
4006b0: afb00018 sw s0,24(sp)
4006b4: afbc0010 sw gp,16(sp)
4006b8: 8f998034 lw t9,-32716(gp)
4006bc: 3c040040 lui a0,0x40
4006c0: 0320f809 jalr t9
4006c4: 248408d0 addiu a0,a0,2256
4006c8: 8fbc0010 lw gp,16(sp)
; prepare high part of x address:
4006cc: 3c100041 lui s0,0x41
4006d0: 8f998038 lw t9,-32712(gp)
4006d4: 3c040040 lui a0,0x40
; add low part of x address:
4006d8: 26050920 addiu a1,s0,2336
; now address of x is in $a1.
4006dc: 0320f809 jalr t9
4006e0: 248408dc addiu a0,a0,2268
4006e4: 8fbc0010 lw gp,16(sp)