cpu 8086
	org 0
	jmp 0x07C0:init

	%include "inc/bpb.asm"

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

	; syscalls push a register set to stack
	; this is for accessing it
rsdssi: equ 0x08
rsesdi: equ 0x0C
rsbp:   equ 0x10
rslpad: equ 0x12
rscsip: equ 0x14
rsflag: equ 0x18

lpads:	times 0x30 call entry

entry:	push bp
	; BX+0C: ES:DI
	push es
	push di
	; BX+08: DS:SI
	push ds
	push si
	; BX+00: AX,CX,DX,BX
	push bx
	push dx
	push cx
	push ax

	mov ax, ss
	mov es, ax
	mov bx, sp

	; This needs to be done when coming from userspace
	mov ax, cs
	mov ss, ax
	xor sp, sp

	int 3

	; Jump back to userspace
ujump:	cli
	mov ax, es
	mov ss, ax
	mov sp, bx

	; Pop whole registerset
	; base registers
	pop ax
	pop cx
	pop dx
	pop bx
	; string ptrs
	pop si
	pop ds
	pop di
	pop es
	; BP is special
	pop bp
	; skip landing pad number
	add sp, 2
	; pop IP, CS and flags
	iret

	; Processes register set DS:BX as syscall
scall:	; Clear status flags
	and byte [bx+rsflag], 0
	ret

	; No such subfunction
sferr:	; set carry
	or byte [bx+rsflag], 1
	ret

	; Subfunction table
sftab:	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, 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

	; IN	al	number
	; OUT	bx	al * 4
times4:	push ax
	xor ah, ah
	add al, al
	add al, al
	mov bx, ax
	pop ax
	ret

	; DOS 2+ - GET INTERRUPT VECTOR
	; IN	al	interrupt number
	; OUT	es:bx	current interrupt handler
getint:	push ds
	; DS=0
	xor bx, bx
	mov ds, bx
	; BX=AL*4
	call times4
	les bx, [bx]
	pop ds
	ret

	; DOS 25h: Set interrupt vector
	; IN	al	interrupt number
	;	ds:dx	entry point
setint:	push es
	push bx
	; ES=0
	xor bx, bx
	mov es, bx
	; BX=AL*4
	call times4
	mov [es:bx], dx
	mov ax, ds
	mov [es:bx+2], ds
	pop bx
	pop es
	ret

bpb:	times bpb_len db 0
drvnum:	db 0
	align 4
drvpos:	dd 0
buffer:	times 512 db 0

	; Select active drive
	; IN	dl	drive number A=0, B=1, C=2, ...
select:	mov [drvnum], 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, [drvpos+2]
	push ax
	mov ax, [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, [drvnum]
	call dnconv
	stc
	int 0x13

	add sp, di

	ret

	; 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

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

	xor dh, dh
	call dnconv
	call select

	xor ax, ax
	xor dx, dx
	call seek

	call read

	mov ax, [buffer+0x1FE]
	int 3

main:	push cs
	pop ds
	mov si, banner
	mov ah, 0x0e
	xor bx, bx
loop:	lodsb
	test al, al
	jz end
	int 0x10
	jmp loop
end:	hlt
	jmp end