nor86/boot.asm

cpu 8086
org 0x7c00

jmp short _code
nop

%ifdef F1440
; 1440K floppy
; BPB
oemidentifier     db "nor86   "
byterpersector    dw 512
sectorspercluster db 1
reservedsectors   dw 1
fats              db 2
rootdirentries    dw 224
totalsectors      dw 2880
mediadescription  db 0xf0
sectorsperfat     dw 9
sectorspertrack   dw 18
heads             dw 2
hiddensectors     dd 0
totalsectorslarge dd 0

%elifdef F360
; 360K floppy
; BPB
oemidentifier     db "nor86   "
byterpersector    dw 512
sectorspercluster db 2
reservedsectors   dw 1
fats              db 2
rootdirentries    dw 112
totalsectors      dw 720
mediadescription  db 0xfd
sectorsperfat     dw 2
sectorspertrack   dw 9
heads             dw 2
hiddensectors     dd 0
totalsectorslarge dd 0

%else
%error "No valid floppy format specified, specify -d F1440 or -d F360"
%endif

; EBPB
drivenumber       db 0 ; useless on-disk, used as a variable
reserved          db 0 ; winnt flags
signature         db 0x29 ; mkdosfs uses this, dunno how 0x28 differs
serial            dd 0
volumelabel       db "nor86 boot "
fstype            db "FAT12   "

_code:
	jmp 0:_start

_start:
	cld
	; Set up segments
	mov ax, cs
	mov ds, ax
	mov es, ax
	cli
	mov ss, ax
	mov sp, 0x7c00
	sti

	; Save bootdrive
	mov [drivenumber], dl

calc_constants:
	; Disk organization:
	; Reserved sectors (MBR)
	; FAT1
	; FAT2
	; Root dir
	; → Root dir starts at LBA reservedsectors + fats*sectorsperfat
	xor ah, ah
	mov al, [fats]
	mul word [sectorsperfat]
	add ax, [reservedsectors]
	mov [rootdir], ax

	; Data area comes after root dir, so we need to add the size of
	; the root dir in sectors
	; One root dir entry is 32 bytes, and one sector has 512 bytes
	; Therefore 512 / 32 = 16 entries correspond to a sector
	; However, we can't just do a shift, since we need to round up
	; For integers, ceil(x/y) = floor((x+y-1)/y)
	mov ax, [rootdirentries]
	add ax, 15
	shr ax, 1
	shr ax, 1
	shr ax, 1
	shr ax, 1
	mov [rootdirsectors], ax

fat:
	; FAT1 (the main one) follows right after bootsector(s)
	mov ax, [reservedsectors]
	mov bx, 0x8000
	mov cx, [sectorsperfat]
	call loadsectors

root_dir:
	mov ax, [rootdir]
	mov bx, 0x500
	mov cx, [rootdirsectors]
	call loadsectors

search_root:
	mov bx, [rootdirentries]
	shl bx, 1
	shl bx, 1
	shl bx, 1
	shl bx, 1
	shl bx, 1
	mov byte [bx + 0x500], 0
	mov si, 0x500
	.entry:
		cmp byte [si], 0
		je .end

		test byte [si + 11], 0x08 + 0x10
		jnz .skipentry

		; Make sure the file has non-zero size
		cmp word [si + 28], 0
		jne .isfile
		cmp word [si + 30], 0
		jne .isfile

	.skipentry:
		add si, 32
		jmp .entry

	.isfile:
		mov cx, 11
		mov bx, kernel_name

		.compare:
			lodsb
			cmp al, [bx]
			jne .nomatch
			inc bx
			loop .compare

		jmp found

	.nomatch:
		; Each entry is 32 bytes long
		; During each iteration of the compare loop, si is
		; incremented and cx is decremented
		; At the top of the loop, si + cx = entry_start + 11, but
		; when we jump here si has been incremented by one already
		; while cx is still the same
		; Therefore, si + cx is one more, so add extra - 1 here
		add si, 32 - 11 - 1
		add si, cx
		jmp .entry

	.end:

notfound:
	mov si, notfound_msg
	jmp fatal_error

found:
	; SI points to 11 bytes after the start of the entry, so adjust all
	; offsets
	mov ax, [si - 11 + 26] ; First cluster
	push ax

	; Load OS at the start of the memory
	mov bx, 0x500

	push bx

	.tosector:
		; Adjust the cluster number to account for first two
		; "clusters" in the FAT being used for metadata
		sub ax, 2

		; Scale by number of sectors per cluster
		xor bx, bx
		mov bl, [sectorspercluster]
		mul word bx

		; Offset by number of non-data sectors before data area
		add ax, [rootdir]
		add ax, [rootdirsectors]

	.load:
		; Load sectors
		pop bx
		xor cx, cx
		mov cl, [sectorspercluster]
		call loadsectors

	.next:
		pop ax

		; Multiply by 1.5 to get offset into the table
		; This rounds down, which is what we want in order to get
		; the first byte of the address
		mov si, ax
		shl si, 1
		add si, ax
		shr si, 1

		; Load the entry from FAT
		mov dx, [si + 0x8000]

		; Two clusters, 0xABC and 0xXYZ, are stored as
		; BC ZA XY
		; ^ where we start reading on even cluster numbers
		; ^^^^^ loading word → 0xZABC
		;    ^ where we start reading on odd cluster numbers
		;    ^^^^^ loading word: 0xXYZA
		test ax, 1
		jnz .odd

		.even:
			and dx, 0x0fff
			jmp .check_cluster

		.odd:
			shr dx, 1
			shr dx, 1
			shr dx, 1
			shr dx, 1

	.check_cluster:
		mov ax, dx

		; End of chain
		cmp ax, 0xFF8
		jg execute_kernel

		push ax
		push bx
		jmp .tosector

execute_kernel:
	mov dl, [drivenumber]
	jmp 0:0x500

; Note: bx will point to after the read data
loadsectors:
	push ax
	push cx
	push dx
	push di

	.loop:
		mov di, 3 ; Retry thrice

		.retry:

		push ax
		push cx

		.chs:
			xor dx, dx
			; cylinder (track) - head - sector
			; cylinder = LBA / sectorspertrack / heads
			; head = LBA / sectorspertrack % heads
			; sector = LBA % sectorspertrack + 1
			div word [sectorspertrack]
			; ax = LBA / sectorspertrack
			; dx = LBA % sectorspertrack

			; sector
			mov cl, dl
			inc cl

			xor dx, dx
			div word [heads]
			; ax = LBA / sectorspertrack / heads
			; dx = LBA / sectorspertrack % heads

			; head
			mov dh, dl

			; cylinder (track)
			mov ch, al
			shr ax, 1
			shr ax, 1
			and al, 0xC0
			or cl, al

		mov ah, 2
		mov al, 1
		mov dl, [drivenumber]
		int 0x13

		; TODO: Handle reset & retry
		jc .error

		pop cx
		pop ax

		inc ax
		add bx, 512

		loop .loop

	pop di
	pop dx
	pop cx
	pop ax
	ret

	.error:
		; Do we still have retries remaining?
		mov si, diskerror_msg
		test di, di
		jz fatal_error ; No, fail
		dec di

		; Yes, reset disk
		xor ah, ah
		int 0x13

		; Execute the loop again
		pop cx
		pop ax
		jmp .retry

fatal_error:
	lodsb
	test al, al
	jz hang
	mov ah, 0xe
	int 0x10
	jmp fatal_error

hang:
	hlt
	jmp hang


kernel_name: db "NOR86   KRN"

notfound_msg: db "Kernel not found", 0
diskerror_msg: db "Disk error", 0

times 510-($-$$) db 0
db 0x55, 0xaa
_end:
rootdir equ _end
rootdirsectors equ _end + 2