intarith, Assembly

intarith

; intarith_64.asm    show some simple C code and corresponding nasm code
;                    the nasm code is one sample, not unique
;
; compile:      nasm -f elf64 -l intarith_64.lst  intarith_64.asm
; link:         gcc -m64 -nostartfiles -o intarith_64  intarith_64.o
; run:          ./intarith_64
;
; the output from running intarith_64.asm and intarith.c is:
; c=5  , a=3, b=4, c=5
; c=a+b, a=3, b=4, c=7
; c=a-b, a=3, b=4, c=-1
; c=a*b, a=3, b=4, c=12
; c=c/a, a=3, b=4, c=4
;
;The file  intarith.c  is:
;  /* intarith.c */
;  #include <stdio.h>
;  int main()
;  {
;    long int a=3, b=4, c;
;    c=5;
;    printf("%s, a=%ld, b=%ld, c=%ld\n","c=5  ", a, b, c);
;    c=a+b;
;    printf("%s, a=%ld, b=%ld, c=%ld\n","c=a+b", a, b, c);
;    c=a-b;
;    printf("%s, a=%ld, b=%ld, c=%ld\n","c=a-b", a, b, c);
;    c=a*b;
;    printf("%s, a=%ld, b=%ld, c=%ld\n","c=a*b", a, b, c);
;    c=c/a;
;    printf("%s, a=%ld, b=%ld, c=%ld\n","c=c/a", a, b, c);
;    return 0;
; }
        extern printf           ; the C function to be called

print_string:
        mov     rdi, fmt4       ; first arg, format
        ;mov    rsi, str        ; second arg
        mov     rdx, [a]        ; third arg
        mov     rcx, [b]        ; fourth arg
        mov     r8, [c]         ; fifth arg
        mov     rax, 0          ; no xmm used
        call    printf          ; Call C function
        ret

section .data           ; preset constants, writeable
a:      dq      3               ; 64-bit variable a initialized to 3
b:      dq      4               ; 64-bit variable b initializes to 4
fmt4:   db "%s, a=%ld, b=%ld, c=%ld",10,0       ; format string for printf
addp:   db "c=a+b",0
subp:   db "c=a-b",0
multp:  db "c=a*b",0
divp:   db "c=c/a",0

section .bss            ; unitialized space
c:      resq    1               ; reserve a 64-bit word
str:    resq    1               ; reserve a 94-bit word

section .text           ; instructions, code segment
        global  _start          ; for gcc standard linking
_start:                         ; label
        
lit5:                           ; c=5;
        mov     rax,5           ; 5 is a literal constant
        mov     [c],rax         ; store into c

addb:                           ; c=a+b;
        mov     rax,[a]         ; load a
        add     rax,[b]         ; add b
        mov     [c],rax         ; store into c
        mov     rsi,addp
        call print_string
subb:                           ; c=a-b;
        mov     rax,[a]         ; load a
        sub     rax,[b]         ; subtract b
        mov     [c],rax         ; store into c
        mov     rsi,subp         ; invoke the print
        call print_string

mulb:                           ; c=a*b;
        mov     rax,[a]         ; load a (must be rax for multiply)
        imul    qword [b]       ; signed integer multiply by b
        mov     [c],rax         ; store bottom half of product into c
        mov     rsi,multp       ; invoke the print
        call print_string

diva:                           ; c=c/a;
        mov     rax,[c]         ; load c
        mov     rdx,0           ; load upper half of dividend with zero
        idiv    qword [a]       ; divide double register edx rax by a
        mov     [c],rax         ; store quotient into c
        mov     rsi,divp        ; invoke the print
        call print_string

mov     rax,60           ; exit code
        xor     rdi,rdi
        syscall


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