Initial commit

kernel.s

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+.global kernel_main, __bss, __bss_end, __stack_top
+
+.section .rodata
+
+hello_world: .asciz "\nHello world!\n"
+
+.section .text
+
+bzero:
+	la t0, __bss
+	la t1, __bss_end
+
+.Lbzero_loop:
+	bgt t0, t1, .Lbzero_end
+	sw zero, (t0)
+	addi t0, t0, 4
+
+.Lbzero_end:
+	ret
+
+# Arguments:
+# a0 - Character.
+.type putchar, @function
+putchar:
+	li a1, 0
+	li a2, 0
+	li a3, 0
+	li a4, 0
+	li a5, 0
+	li a6, 0
+	li a7, 1 # sbi_console_putchar.
+
+	ecall
+	ret
+
+.type kernel_main, @function
+kernel_main:
+	# Prologue.
+	addi sp, sp, -32
+	sw ra, 28(sp)
+	sw s0, 24(sp)
+	addi s0, sp, 32
+
+	sw s1, 20(sp)
+
+	la s1, hello_world
+
+.Lkernel_main_if:
+	lw a0, (s1)
+	beqz a0, .Lkernel_main
+
+	call putchar
+	addi s1, s1, 1
+
+	j .Lkernel_main_if
+
+.Lkernel_main:
+	j .Lkernel_main
+
+	lw s1, 20(sp)
+
+	# Epilogue.
+	lw ra, 28(sp)
+	lw s0, 24(sp)
+	add sp, sp, 32
+	ret
+
+.section .text.boot
+boot:
+	la a0, __stack_top
+	mv sp, a0 # Set the stack pointer
+	call bzero
+	j kernel_main