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# A useless program, which the only thing this shit does is call exit syscall
# The exit status code can be read using
# `echo $?` on shell, which will show the exit status of the last program run.
# %eax holds the system call number
# %ebx holds the return status
# Assembler directives (or pseudo-operations).
#
# Everything starting with a period, is an instruction to the assembler itself.
# They are handled by the assembler program, not actually executed by the
# computer.
# .section command breaks the program down into sections.
# .data section lists any memory storage which will be needed for data
.section .data
# .text section lists the program instructions
.section .text
# Instructs the assembler that _start is "important to remember".
# _start is a symbol, which is going to be replaced by something else either
# during assembly or linking
# Symbols are generally used to mark locations of programs or data, so you can
# refer to them by name instead of by location number
# .globl means that the assembler should not discard this symbol after assembly,
# because the linker will need it.
# _start is a special symbol that always needs to be marked with .globl, because
# it marks the location of the start of the program.
# If the program is not marked this way, when the computer loads the program, it
# won't know where to begin running the program.
.globl _start
# Defines the value of _start label. Label == a symbol followed by a colon.
# Labels defines a symbol's value.
# Labels tell the assembler to make the symbol's value be wherever the next
# instruction or data element will be.
_start:
# different from the book, I'm using x86_64 instructions and its registers, so,
# instead of using %eax and %ebx, I'm using %rax and %rbx directly. The movl
# instruction needed to be changed to movq (move quad) to adapt to the 64-bit
# registers
# mov: (move) 16-bit
# movl: (move long) 32-bit
# movq: (move quad) 64-bit
#ins $(val), (dest) == immediate mode
movq $1, %rax # Move value 1 into %eax register
# %eax will hold the syscall number
# by the linux's calling convention
movq $100, %rbx # Move value 0 into %ebx register
# %ebx will hold the exit() syscall
# argument.
int $0x80 # Interrupt 0x80 will wake up the kernel
# execute the syscall loaded into %eax
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