Multiboot minimalist OS

multiboot.fasm

use32
UMB_START = 0x00100000
org UMB_START

MB_MAGIC = 1BADB002h
MB_F_BOOTALIGNED	= 1
MB_F_MEMINFO		= (1 shl 1)
MB_F_VIDEOTABLE		= (1 shl 2)
MB_F_USE_MBOFFSETS	= (1 shl 16)
MB_FLAGS = MB_F_BOOTALIGNED or MB_F_MEMINFO or \
	   MB_F_USE_MBOFFSETS
MB_CHECKSUM = dword - (MB_MAGIC + MB_FLAGS)
VIDEO_MODE = 1 ; EGA text mode
VIDEO_WIDTH = 80
VIDEO_HEIGHT = 25
VIDEO_DEPTH = 0
VIDEO_BASE_ADDR = 0xB8000

multiboot_header:
		dd MB_MAGIC		; Multiboot magic number
flags:	   	dd MB_FLAGS
checksum:	dd MB_CHECKSUM
header_addr:	dd multiboot_header
load_addr:	dd multiboot_header	; start of program text
load_end_addr:	dd load_end		; end of program text+data
bss_end_addr:	dd bss+10000h
entry_addr:	dd entry_point		; 
mode_type:	dd VIDEO_MODE		; Video mode
width:		dd VIDEO_WIDTH		; Video Width
height:		dd VIDEO_HEIGHT		; Video Height
depth:		dd VIDEO_DEPTH		; Vide Depth


entry_point:
	cmp eax,0x2BADB002	; Are we multibooted
	je multiboot
	hlt			; not really sure how we'd ever get here
	jmp $-1
multiboot:
setup_stack:
	mov ecx, mbis
	mov [ecx], ebx
	add ecx, 4			; ecx = umb_end
	mov eax, [ebx]			; check memory regions
	and eax, 1			; are specified in multiboot
	jz  fail			; information structure
	mov eax, [ebx+8] 		; load upper memory size
	shl eax,10			; size is in kb, convert to bytes
	add eax,UMB_START		; eax = end of UMB
	mov esp,eax			; place stack at top of UMB
	mov [ecx], eax
	jmp write_str

mem_zero:   ; ecx = number of bytes to clear
	    ; eax = start address
	cmp ecx, 4
	jl  mem_zero_tail
	mov byte [eax],0
	add eax,4
	sub ecx,4
	jz mem_zero_end
	cmp ecx,4
	jge mem_zero
mem_zero_tail: ; 4 > ecx > 0
	mov byte [eax], 0
	inc eax
	dec ecx
	jnz mem_zero_tail
mem_zero_end:
	ret

setup_gdt:
	
write_str:
	mov edi, welcome_msg_end-welcome_msg
	mov ebx, welcome_msg
set_b:
	mov edx, VIDEO_BASE_ADDR
write_loop:
	mov al, [ebx]
	mov [edx], al
	mov [edx+1], dl
	inc ebx
	add edx,2
	cmp edx, VIDEO_BASE_ADDR+(2*VIDEO_WIDTH*VIDEO_HEIGHT)
	jge set_b
	dec edi
	jnz write_loop
	mov edi, welcome_msg_end-welcome_msg
	mov ebx, welcome_msg
	jmp write_loop

welcome_msg:
	db 'Hello there!'
welcome_msg_end:
fail:
	hlt
load_end:

bss:
mbis:	rd 0
umb_end: rd 0