cpu 8086
	org 0x7C00

stack:	equ 0x7C00
framsz: equ 0x300

	jmp init

	; Data on user stack while in kernel (5 words):
	; AX BP IP CS flags

	; Kernel stack:
	; [frame] SP SS
	;      BP-^
	; The start of the frame is available via BP.
	; The frame is followed by a long ptr to the user stack.

	; Main entry point
int20h:	xor ah, ah
int21h:	push bp
	push ax
	; Test if SS=0, if it is, we assume we are already on kernel stack
	mov ax, ss
	test ax, ax
	jz rentry

	; Save user stack
	mov ax, ss
	mov [stack-2], ax
	mov [stack-4], sp

	; Set up kernel stack
	xor ax, ax
	mov ss, ax
	mov sp, (stack-4)

	jmp scall

	; This is like the above, except that it doesnt switch stacks.
	; The 'user' stack data is after (below on stack) the kernel frame.
        ; [frame] SP SS AX BP IP CS flags
	;          \-\--^
rentry:	mov ax, sp
	push ss
	push ax

scall:	sub sp, framsz
	mov bp, sp

	call getax
	int 3

	; Restore user stack
	mov sp, [bp+framsz]
	mov ax, [bp+framsz+2]
	mov ss, ax

	pop ax
	pop bp
	iret

	; Get AX from the user stack
getax:	push ds
	push si
	lds si, [bp+framsz]
	lodsw
	pop ds
	pop si
	ret

	; No such subfunction - set carry and exit
sferr:	stc
	ret

	; Subfunction table
	align 2
sftab:	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, setint, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr
	dw sferr, sferr, sferr, sferr

	; DOS 25h: Set interrupt vector
	; IN	al	interrupt number
	;	ds:dx	entry point
setint: push ax
	push bx
	xor ah, ah
	add al, al
	add al, al
	mov bx, ax
	mov [cs:bx], dx
	mov ax, ds
	mov [cs:bx+2], ds
	pop bx
	pop ax
	ret

; ===== end of resident, begin of transient startup code

init:	xor ax, ax
	mov ds, ax
	mov es, ax
	mov ax, 0x1000
	mov ss, ax
	xor sp, sp

	mov al, 0x21
	mov dx, int21h
	call setint

	mov ax, 0x2520
	mov dx, int20h
	call setint

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

msg:	db "rdos kernel stub", 0x0A, 0x0D, 0