Computer Organization

How to Assembly by Tonkaew

Creating Label

int func1() {

}

func1:

movq

• movq src, dest

example.

mov $6, %rdi
; regis rdi = 6

mov $0x4000, %rdi
; regis rdi = 0x4000
mov $6, (%rdi)
; meme mem[0x4000] = 6

• Type of movq

  • 5(%rdi) = mem[%rdi + 5]
  • (%rdi, %rsi) = mem[%rdi + %rsi]
  • (%rax, %rdi, %rsi) = mem[%rax + (%rdi * %rsi)]
  • (%rax, %rdi, 4) = mem[%rax + (4 * %rdi)]
  • 5(,%rdi,%rsi) = mem[5 + (%rdi * %rsi)]

• movl (long)

• movq (words)

Arithmatics instruction

• adds src, dest ; dest = dest + src

example.

; %rdi = 5
; %rsi = 4
addq %rdi, %rsi
; %rsi = 4 + 5 = 9

• subq src, dest
• imulq src, dest
• salq: Shift Arithmatic Left
• shlq: Shift Logic Left
• sarq: Shift Arithmatic Right
• shrq: Shift Logic Left
• xorq src, dest
• andq src, dest
• orq src, dest
• incq dest ; increase, dest = dest + 1
• decq dest ; decrese, dest = dest - 1
• negq dest ; negative, dest = -1 * dest
• notq dest ; not, dest = ~dest

Convert C <=> Assembly

int fun1(int a) {
    return a;
}

fun1:
    movs %rdi, %rax

int fun2(int a, int b) {
    return a + b;
}

fun2:
  movs %rdi, %rax ; return = a
  adds %rsi, %rax ; return = a + b

• jmp

.l1:  
    ...
    jmp .L2
    ..
    ..
.l2:
    ...
.final:
    ...

Condition flags

• ZF Zero
• OF Overflow
• SF Signed
• CF Carry

testq

• testq src2, src1
• src2 & src1 == 0 ZF = 1
• src2 & src1 < 0 SF 1

example.

; %rdi = 0
testq %rdi, %rdi
; ZF 1
; SF 0

; %rdi = 1
testq %rdi, %rdi
; ZF 0
; SF 0

adds 0b1111 + 0b0001 0b0000 CF = 1 ZF = 0

cmpq

• cmpq a, b example.

cmpq a, b
je ; jump if a == b
jne ; jump if a != b
js ; jump if (b - a) is negative
jns ; jump if (b - a) is not negative
jg ; jump if b > a
jge ; jump if b >= a
jl ; jump if b < a
jle ; jump if b <= a

int func3(int a, int b) {
    if(a > b) { // b < a
        return a; // L1
    }
    else {
        return b; // L2
    }
}

func3:
    cmpq %rdi, %rsi ; compare a, b
    jl L1
L2:
    movs %rsi, %rax
    jmp finally
L1:
    movs %rdi, %rax
finally:
    ...

    movq (%rdi), %rax ; %rax = mem[%rdi]
    leaq %rax, (%rdi) ; mem[%rdi] = %rax
    ; rdi = 0x4000 0000
    movq 4(%rdi), %rax ; %rax = mem[0x4000 0004]
    laeq 4(%rdi), %rax ; %rax = 0x4000 0004

// sums n * (n + 1) / 2
int func1(int a) {
    int b = 0;
    while(a != 0) {
        a -= 1;
        b += a;
    }
    return b;
}

func1:
    movq $0, %rbx ; %rbx = 0 = b
    movq %rbx, %rax ; %rax = b = 0 ; not exists in C
loop:
    testq %rdi, %rdi ; test a
    je endLoop ; if a == 0, jump to endLoop
    subq $1, %rdi ; %rdi -= 1; a -= 1;
    addq %rdi, %rax ; %rax += %rdi; b += a;
    jmp loop
endLoop:
    ret

Stacks %rsp: stack pointer ,%rbp: base pointer

• pushq ; push into stacks
• popq ; pop out off stacks
• call; call function, push current pc + 1 intp stacks
• ret ; pop, and set pc to that data

Branch / Jump

• jmp: jump without condition
• je: jump equal or zero
• jne: jump not equal or not zero
• js: jump negative (signed)
• jns: jump not negative (signed)
• jg: jump greater
• jge: jump greater or equal
• jl: jump less
• jle: jump less or equal
• ja: jump above (~CF & ~ZF)
• jb: jump below (CF)

อื่นๆ

• ส่วนของ Computer
  • CPU
    • ALU
    • Bus
    • Cache
    • Register
  • Memory

function in C, arguments (int a, int b, ...); -> %rdi %rsi %rdx %rcx %r8 %r9

in c return x; in asm %rax

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