; SPDX-License-Identifier: MIT
; Copyright (c) 2021 Juhani 'nortti' Krekelä.

iosegment equ 60h
dossegment equ iosegment + 1*1024/16 ; DOS starts 1KiB after IO system

dos segment at dossegment
org 0
dosinit proc far
dosinit endp
dos ends

code segment
code ends

constants segment
constants ends

data segment
data ends

iogroup group code, constants, data

code segment
assume cs:iogroup, ds:iogroup, es:iogroup
org 0

; Jump table
jmp init
jmp status
jmp getch
jmp putch
jmp unimplemented ; Output to printer
jmp unimplemented ; Serial read
jmp unimplemented ; Serial write
jmp diskread
jmp diskwrite
jmp diskchange
jmp setdate
jmp settime
jmp gettime
jmp flush
jmp mapdev

init:
	cld

	cli
	mov ax, cs
	mov ds, ax
	mov es, ax
	; Put setup stack just below 32K
	xor ax, ax
	mov ss, ax
	mov sp, 8000h
	sti

	; Figure out memory size
	int 12h
	; AX = memory size in kilobytes
	; We want it in paragraphs
	; There are 64 paragraphs in a kilobyte
	mov cx, 64
	shl al, cl
	; Memory size is passed in dx
	mov dx, ax

	; Disk table is passed in si
	mov si, offset iogroup:disks_table

	call dosinit

	mov al, 'c'
	jmp error

; OUT:
;  al = character if any
;  zf = there were no characters
status proc far
	; TODO: Implement
	xor ax, ax
	test ax, ax
	ret
status endp

getch:
	mov al, 'g'
	jmp error

; IN:
;  ax = character
putch proc far
	push ax
	mov ah, 0eh
	int 10h
	pop ax
	ret
putch endp

diskread:
	mov al, 'r'
	jmp error
diskwrite:
	mov al, 'w'
	jmp error
diskchange:
	mov al, 'c'
	jmp error
setdate:
	mov al, 'd'
	jmp error
settime:
	mov al, 't'
	jmp error
gettime:
	mov al, 'T'
	jmp error
flush:
	mov al, 'f'
	jmp error

mapdev:
	mov al, 'm'
	jmp error

unimplemented:
	mov al, '@'

error:
	mov ah, 0eh
	int 10h

hang:
	hlt
	jmp hang

code ends

constants segment

disks_table:
	db 1 ; 1 drive

	db 0 ; Physical drive 0
	dw offset iogroup:parameters_320k

parameters_320k:
	dw 512 ; Sector size in bytes
	db 2 ; Sectors per cluster
	dw 1 ; Number of reserved sectors
	dw 2 ; Number of FATs
	dw 112 ; Number of directory entries
	dw 320*2 ; Number of sectors

constants ends

end