77 lines
4.6 KiB
ArmAsm
77 lines
4.6 KiB
ArmAsm
# gold linker has smallest binary size others probably can emit smaller
|
||
# binaries with custom linker scripts. Their default ones
|
||
# are not optimized for hello world programs
|
||
# as --64 hello-amd64.s && ld.gold -s -n -o hello a.out
|
||
.data # section declaration
|
||
msg:
|
||
.string "All your codebase is belong to us\n" # output string
|
||
|
||
len = . - msg # length of output string
|
||
|
||
.text # section declaration
|
||
# we must export the entry point to the ELF linker or
|
||
.global _start # loader. They conventionally recognize _start as their
|
||
# entry point. Use ld −e foo to override the default.
|
||
|
||
# https://stackoverflow.com/questions/3683144/linux-64-command-line-parameters-in-assembly
|
||
# https://wiki.cdot.senecacollege.ca/wiki/X86_64_Register_and_Instruction_Quick_Start
|
||
square:
|
||
pushq %rbp # Save rbp - this must be restored at end of call
|
||
movq %rsp, %rbp # Update base pointer from current stack pointer
|
||
movq %rdi,-8(%rbp) # Move 1st argument (rdi) into stack memory
|
||
# This is not strictly necessary here, but
|
||
# is done as a way to demonstrate generic handling
|
||
# of arguments
|
||
movq -8(%rbp), %rax # Move stack memory into rax for multiplication
|
||
imulq -8(%rbp), %rax # Do multiplication
|
||
# The above 3 instructions could be done with the
|
||
# following 2 instructions instead in such a simple
|
||
# case:
|
||
#movq %rdi, %rax # Move 1st argument (rdi) to rax for processing
|
||
|
||
#imulq %rax, %rax # Do multiplication
|
||
popq %rbp # Restore rbp for return (eax/rax has return val)
|
||
retq # Return
|
||
|
||
_start:
|
||
|
||
# write our string to stdout
|
||
# https://man7.org/linux/man-pages/man2/syscall.2.html
|
||
# https://filippo.io/linux-syscall-table/
|
||
# https://github.com/torvalds/linux/blob/master/arch/x86/entry/syscalls/syscall_64.tbl
|
||
movq $1,%rax # system call number (sys_write)
|
||
movq $1,%rdi # first argument: file handle (stdout)
|
||
movq $msg,%rsi # second argument: pointer to message to write
|
||
movq $len,%rdx # third argument: message length
|
||
syscall # call kernel
|
||
# argc is stored in (%rsp)
|
||
# this is an eightbyte according to table 3.9
|
||
# of the System V AMD64 psABI
|
||
# https://gitlab.com/x86-psABIs/x86-64-ABI
|
||
#
|
||
# It is a bit questionable here whether the
|
||
# upper 32 bits of rdi are cleared when moving
|
||
# into edi, but this instruction is generated
|
||
# from compilers, which leads me to "yes".
|
||
# Documentation also states this (with some exceptions):
|
||
#
|
||
# When executing MOV Reg, Sreg, the processor
|
||
# copies the content of Sreg to the 16 least
|
||
# significant bits of the general-purpose register.
|
||
# The upper bits of the destination register
|
||
# are zero for most IA-32 processors (Pentium
|
||
# Pro processors and later) and all Intel 64
|
||
# processors, with the exception that bits 31:16
|
||
# are undefined for Intel Quark X1000 processors,
|
||
# Pentium and earlier processors.
|
||
#
|
||
# Above language pulled from MOV documentation
|
||
# in Vol 2B, Chapter 4:
|
||
# https://www.intel.com/content/dam/develop/public/us/en/documents/325462-sdm-vol-1-2abcd-3abcd.pdf
|
||
#
|
||
movl (%rsp),%edi # See: https://wiki.cdot.senecacollege.ca/wiki/X86_64_Register_and_Instruction_Quick_Start
|
||
callq square # Square our argc, result in %eax
|
||
movq %rax,%rdi # mov %eax to the first syscall argument (exit code)
|
||
movq $60,%rax # system call number (sys_exit)
|
||
syscall # call kernel and exit
|