cpu 8086
	org 0x500
	jmp init

	%include "inc/bpb.asm"

banner:	db "rdos", 0xA, 0xD, '$'

int20h:	xor ah, ah
int21h:	; inside of kernel, direction always goes up
	; the iret will restore it to the user value later
	cld
	; set sfptr from ah
	push bx
	xor bx, bx
	mov bl, ah
	add bl, bl
	add bx, sftab
	mov bx, [cs:bx]
	mov [cs:sfptr], bx
	pop bx
	; do the actual subfunction call
	call [cs:sfptr]
	; inherit the lower 8 flag bits to userspace
	push ax
	push bp
	mov bp, sp
	lahf
	mov [bp+8], ah
	pop bp
	pop ax
	; iret frame: IP CS FLAGS
	iret

	; Subfunction ptr
	; this is used as extra register in int21h
sfptr:	dw 0

	; Subfunction table
sftab:	dw sferr, sferr, putc, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, puts, sferr, sferr
	dw sferr, sferr, sferr, sferr
	; 10
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	; 20
	dw sferr, sferr, sferr, sferr
	dw sferr, setint, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	; 30
	dw sferr, sferr, sferr, sferr
	dw sferr, getint, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr

	; DOS 1+ 2h - WRITE CHARACTER TO STANDARD OUTPUT
	; IN	dl	character to write
putc:	push ax
	push bx
	mov ah, 0x0E
	mov al, dl
	xor bx, bx
	int 0x10
	pop bx
	pop ax
	ret

	; DOS 1+ 9h - WRITE STRING TO STANDARD OUTPUT
	; IN	ds:dx	'$'-terminated string
puts:	push si
	mov ah, 0x0E
	xor bx, bx
.loop:	lodsb
	cmp al, '$'
	je .end
	int 0x10
	jmp .loop
.end:	pop si
	ret

	; DOS 1+ 25h - SET INTERRUPT VECTOR
	; IN	al	interrupt number
	;	ds:dx	entry point
setint:	push bx
	xor bx, bx
	; BX=AL*4
	mov bl, al
	add bl, bl
	add bl, bl
	; save DS:DX into vector
	mov [cs:bx], dx
	mov [cs:bx+2], ds
	pop bx
	ret

	; DOS 2+ 35h - GET INTERRUPT VECTOR
	; IN	al	interrupt number
	; OUT	es:bx	current interrupt handler
getint:	xor bx, bx
	; BX=AL*4
	mov bl, al
	add bl, bl
	add bl, bl
	; load vector into ES:BX
	les bx, [cs:bx]
	ret

	; Fallback for non-existant subfunctions
	; The carry flag is inherited to user
sferr:	stc
	ret

bpb:	times bpb_len db 0
drvnum:	db 0
	align 4
drvoff:	dd 0 ; partition offset
drvpos:	dd 0 ; absolute physical sector number
buffer:	times 512 db 0

	; Convert between drive number and BIOS dl
	; Bidirectional mapping
	; 0 <-> 0, 1 <-> 1, 2 <-> 0x80, 3 <-> 0x81
dnconv:	mov cx, 7
	ror dx, 1
	rol dl, 1
dncl:	rcl dl, 1
	rcr dh, 1
	loop dncl
	xchg dh, dl
	ret

	; Set sector number
seek:	mov [drvpos], ax
	mov [drvpos+2], dx
	ret

	; Read sector from disk
read:	; this uses the EBIOS extensions

	; qword sector number
	xor ax, ax
	push ax
	push ax
	mov ax, [cs:drvpos+2]
	push ax
	mov ax, [cs:drvpos]
	push ax

	; dword target buffer
	mov ax, buffer
	push cs
	push ax

	; word number of sectors
	mov ax, 1
	push ax

	; size & passing
	mov di, 0x10
	push di

	mov si, sp
	mov ah, 0x42
	mov dl, [cs:drvnum]
	call dnconv
	stc
	int 0x13

	add sp, di
	ret

	; Select active drive
	; IN	dl	drive number A=0, B=1, C=2, ...
select:	mov [cs:drvnum], dl
	ret
	push es
	xor ax, ax
	push cs
	pop es
	mov si, drvoff
	stosw
	stosw
	stosw
	stosw
	pop es
	call read
	; load bpb from buffer to bpb
ldbpb:	push ds
	push es
	mov ax, cs
	mov ds, ax
	mov es, ax
	mov si, buffer+0x0B
	mov di, bpb
	mov cx, bpb_len
	rep movsb
	pop es
	pop ds
	ret

main:	mov si, banner
	mov ah, 0x09
	int 0x21
end:	hlt
	jmp end

init:	cli
	xor ax, ax
	mov ds, ax
	mov es, ax
	mov ss, ax
	xor sp, sp

	mov si, 0x7C00
	mov di, $$
	mov cx, (init-$$)
	rep movsb

	call dnconv
	call select

	mov al, 0x20
	mov dx, int20h
	call setint
	mov al, 0x21
	mov dx, int21h
	call setint

	jmp 0:main