// crt0.S for executables
// interrupts and stack are already configured by OS
// _start does the following tasks:
//  1) save registers (ra, t0..t6, a0..a7)
//  2) initialize BSS
//  3) call main
//  4) restore registers
//  5) return to caller (LiteOS shell)

        .global _start
_start:
        // save context
	addi sp, sp, -16*4
	sw ra,  0*4(sp)
	sw t0,  1*4(sp)
	sw t1,  2*4(sp)
	sw t2,  3*4(sp)
	sw a0,  4*4(sp)
	sw a1,  5*4(sp)
	sw a2,  6*4(sp)
	sw a3,  7*4(sp)
	sw a4,  8*4(sp)
	sw a5,  9*4(sp)
	sw a6, 10*4(sp)
	sw a7, 11*4(sp)
	sw t3, 12*4(sp)
	sw t4, 13*4(sp)
	sw t5, 14*4(sp)
	sw t6, 15*4(sp)
	

	// initialize .bss
	la t0, _fbss
	la t1, _ebss
1:	beq t0, t1, 3f
	sw zero, 0(t0)
	addi t0, t0, 4
	j 1b
3:

        call main
	
	// restore context
	lw ra,  0*4(sp)
	lw t0,  1*4(sp)
	lw t1,  2*4(sp)
	lw t2,  3*4(sp)
	lw a0,  4*4(sp)
	lw a1,  5*4(sp)
	lw a2,  6*4(sp)
	lw a3,  7*4(sp)
	lw a4,  8*4(sp)
	lw a5,  9*4(sp)
	lw a6, 10*4(sp)
	lw a7, 11*4(sp)
	lw t3, 12*4(sp)
	lw t4, 13*4(sp)
	lw t5, 14*4(sp)
	lw t6, 15*4(sp)
	addi sp, sp, 16*4
	
	ret