RockOS/kernel/prekernel.s

.code16
.text
.global _start
_start:
	jmp everything

.org 20
.global kernel_size
kernel_size:
	.long _end - _start

everything:
	mov $0x1000, %ax 
	mov %ax, %es 
	mov %ax, %ds

	# reset disk
	mov $0, %ah
	int $0x13

	# stop the cursor
	mov $1, %ah
	mov $0,%cl
	mov $0x20,%ch
	int $0x11

	# get the system memory, if this call fails then halt
	clc
	mov $0, %bx
	mov $0xe801, %ax
	int $0x15
	jc halt
	shr $10, %ax
	shr $4, %bx
	add %ax, %bx
	mov %bx, total_mem_mb-_start

	# vga text mode
	mov $0x0003, %ax 
	int $0x10
	
enable_a20:
  #enable the a20 gate
	inb $0x92, %al
	orb $2, %al
	outb %al, $0x92
prep_protected:
	cli
	lgdt gdt_init-_start
	mov %cr0, %eax
	or $0x01, %eax
	mov %eax, %cr0 # protected bit set
  	ljmpl $8, $protected

halt16:
  jmp halt16

.code32
protected:
	mov $16, %ax
  	mov %ax, %es
	mov %ax, %ds
	mov %ax, %ss
	mov $5*8, %ax
	ltr %ax
	mov $0, %ax
	mov %ax, %fs
	mov %ax, %gs
	mov $0xFFF0, %sp
	mov $0xFFF0, %bp
	ljmpl $8, $kmain

# some assembly stubs we can call from the kernel

.global idt_load
.extern idtr       
idt_load:
	lidt idtr
	ret            

.global reboot
reboot:
	cli
	lidt reboot_struct
	int $1
reboot_struct:
	.word 0
	.long 0

.global halt
halt:
	cli
	hlt
	jmp halt

# kernel data structures

.align 16
.global gdt
gdt:
	.word 0,0,0,0
	.word 0xffff, 0x0000, 0x9a00, 0x00cf
	.word 0xffff, 0x0000, 0x9200, 0x00cf    
	.word 0xffff, 0x0000, 0xfa00, 0x00cf   
	.word 0xffff, 0x0000, 0xf200, 0x00cf  
	.word 0x0068, (tss-_start), 0x8901, 0x00cf
gdt_init:
	.word gdt_init-gdt
	.long gdt

.align 16
.global tss
tss:
	.long 0

.global total_mem_mb
total_mem_mb:
	.word 0