initial commit

prekernel.s

@@ -0,0 +1,153 @@
+.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 $2*8, %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
+
+.align 4
+.global video_info_struct
+video_info_struct:
+	.word	0
+	.byte	0,0
+	.word	0,0,0,0
+	.long	0
+.global video_xbytes
+video_xbytes:
+	.word	0
+.global video_xres
+video_xres:
+	.word	0 
+.global video_yres
+video_yres:
+	.word	0
+	.byte	0,0,0,0,0,0,0,0,0
+	.byte	0,0,0,0,0,0,0,0,0
+.global video_buffer
+video_buffer:
+	.long	0
+	.long	0
+	.word	0
+	.word	0
+	.byte	0,0,0,0,0,0,0,0,0,0
+	.long	0
+.rept 190
+	.byte	0
+.endr
+
+# some assembly stubs we can call from the kernel
+
+.global idt_load
+.extern idtr       
+idt_load:
+	lidt (idtr)     
+	ret            
+
+.global gdt_load
+gdt_load:
+	lgdt (gdt_init)
+	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