A First ARMv8 Assembly Program

Notes

• Again, you know how to write assembly code.
• But
  • We will be using a different assembler (or just gcc)
  • And a different computer (Archimedes)
  • And a different assembly language
• Hello World
  • name it hello.s
    • When gcc sees a .s extension, it knows it is assembly code

  • .global main
    
    .data
    
         message: .asciz "Hello World!\n"
    
    .text
    
    main:
        sub       sp, sp, #16
        stur      x29, [sp, #0]
        stur      x30, [sp, #8]
        mov       x29, sp
    
        ldr       x0, =message
        bl        printf
    
        mov      w0, #0
    
        ldur     x29, [sp, #0]
        ldur     x30, [sp, #8]
        add      sp, sp, #16
        ret

• Not much new here
  • .global main because we are using the c compiler
  • message: .asciz "Hello World!\n"
    • z means the compiler should terminate with a 0, that is nice
    • \n as part of the string, that is nice.
  • sub sp, sp, #16
    • Constants start with a #
    • Typing I seem to be messing up constantly and putting a $, just be warned.
    • Stack grows down
    • sp ← sp - 16
  • stur x29, [sp, #0]
    • stur : store register
    • x29 is the source in this case, a change form the normal
    • [sp, #0]: sp is the base address, #0 is the offset
    • M[sp + 0= ← x29
    • This is equivalent to pushing the frame pointer
    • ARM does not have a push or pop instruction
  • Push the link register
    • In a second bl = call
    • But puts the PC in x30, so we need to save this to know where to return.
  • mov x29, sp
    • mov is an assembler pseudo-instruction
    • Make the fp be the sp
  • ldr x0, =message
    • Load x0, the first parameter with the address of the message
  • bl printf
    • Go to the subroutine printf
    • Store the PC in x30
  • ldur x29, [sp, #0]
    • x29 ← M[sp + 0];
    • Restore the frame pointer
    • Restore the link register too!
  • add sp, sp, $16
    • sp ← sp - 16
  • ret
    • pc ← x30
• The Makefile:

  OBJS = hello
  ASFLAGS = -g
  
  all: ${OBJS}
  
  clean:
      rm -f ${OBJS}

  • Note the ASFLAGS for debugging
• You can run gdb if you wish.