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