boot.S

// Place in the boot section
.section ".text.boot"

// Life starts here
.globl _start

// r15 -> should begin execution at 0x8000.
// r0 -> 0x00000000
// r1 -> 0x00000C42
// r2 -> 0x00000100 - start of ATAGS
// preserve these registers as argument for the kernel
_start:
  // Set up the stack.
  mov sp, #0x8000

  // Zero the bss.
  ldr r4, =__bss_start
  ldr r9, =__bss_end
  mov r5, #0
  mov r6, #0
  mov r7, #0
  mov r8, #0

1:
  // Clear four words at a time
  stmia r4!, {r5-r8}
  
  // If we are still below bss_end, loop.
  cmp r4, r9
  blo 1b
  
  // Call the kernel
  mov fp, #0
  ldr r3, =main
  blx r3
  
// Halt if the mainline returns
halt:
  wfe
  b halt