boot.S

/* boot.S - assembly startup code */
 
// To keep this in the first portion of the binary.
.section ".text.boot"
 
// Make Start global.
.globl _start
 
// Entry point for the kernel.
// r15 -> should begin execution at 0x8000.
// r0 -> 0x00000000
// r1 -> 0x00000C42
// r2 -> 0x00000100 - start of ATAGS
// preserve these registers as argument for kernel_main
_start:
	// Setup the stack.
	ldr	sp, =stack_top//#0x8000
 ldr  r2, =__bss_start__
  b  LoopFillZerobss
/* Zero fill the bss segment. */  
FillZerobss:
  movs  r3, #0
  str  r3, [r2], #4
    
LoopFillZerobss:
  ldr  r3, = __bss_end__
  cmp  r2, r3
  bcc  FillZerobss
	// Clear out bss.
	//ldr	r4, =__bss_start__
	//ldr	r9, =__bss_end__
	//mov	r5, #0
	//mov	r6, #0
	//mov	r7, #0
	//mov	r8, #0
	//b       2f
 
1:
	// store multiple at r4.
	//stmia	r4!, {r5-r8}
 
	// If we are still below bss_end, loop.
2:
	//cmp	r4, r9
	//blo	1b
 
	// Call kernel_main
	ldr	r3, =kernel_main
	blx	r3
 
	// halt
halt:
	wfe
	b	halt