I am writing an x86 program to print, one line at a time: 9, 8, 7, ..., 1, 0. It compiles, and the value of count is getting decremented (I checked on the gdb debugger). But when I run the program on the command line with ./myprogram, I see no output.
This is the code:
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global _start
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section .text ; Start of our code
_start:
mov byte [count], 9
decr_loop:
;; Print current count value
mov rax, 4 ; The system call for write
mov rdi, 1 ; File descriptor/ handle for stdout (console)
mov rsi, count ; Address for count variable
mov rdx, 1 ; Number of bytes to write
syscall ; Call Linux kernel. Trigger the write
;; Print newline character
mov rax, 4 ; The system call for write
mov rdi, 1 ; Console
mov rsi, newline ; Address for newline variable
mov rdx, 1 ; Number of bytes to write
syscall ; Call Linux kernel. Trigger the write
;; Decrease the count
dec byte [count]
;; Compare count to zero. Jump if greater
cmp byte [count], 0
jg decr_loop
;; This code ends the program.
mov rax, 60 ; system call for exit
xor rdi, rdi ; exit code 0
syscall ; invoke operating system to exit
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section .data
newline:
db 10 ; The integer 10 is a newline char
count:
db 9 ; init count as 9, dummy variable
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