72 lines
3.4 KiB
ArmAsm
72 lines
3.4 KiB
ArmAsm
// ARM 7l - 32 bit arm. Used on 32 bit hardware or 64 bit hardware with 32 bit
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// linux (e.g. most raspbian as of 2022)
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//
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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 hello-arm7l.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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square:
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// Function entry
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sub sp, sp, #4 // Move stack pointer for locals
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// Function arguments
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str r0, [sp] // Store return address in stack memory
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// not specifically necessary, but good safety
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// mechanism. We could instead simply ignore
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// the str/ldr operations here and just go
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// for it
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//
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// This would remove memory access completely,
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// but this allows us to demonstrate the
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// general pattern we can use for functions
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// Function body
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ldr r0, [sp] // Load first operand with our argument
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mul r1, r0, r0 // Do the multiplication
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mov r0, r1 // Move result to return register
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// Function exit
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add sp, sp, #4 // Restore stack pointer
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bx lr // Return
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_start:
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// https://man7.org/linux/man-pages/man2/syscall.2.html
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// Syscall numbers captured from https://syscalls.w3challs.com/?arch=arm_strong
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// arm syscall table here:
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// https://github.com/torvalds/linux/blob/v4.19/arch/arm/tools/syscall.tbl
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# Hello world to stdout
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mov r7, #4 // System call (sys_write)
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mov r0, #1 // first argument: file handle (stdout)
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ldr r1, =msg // second argument: pointer to message to write
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ldr r2, =len // third argument: message length
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swi #0 // call kernel. We are using Linux's EABI -
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// the embedded application binary interface
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// which is more consistent with the way it is
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// done in other architectures
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// Square argc
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ldr r0, [sp] // argc is on the stack
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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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bl square // Square our argc, result in r0
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// r0 will also be the first argument to sys_exit
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// so no need to load
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// exit
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mov r7, #1 // system call number (sys_exit)
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swi #0 // call kernel and exit
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