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

debug equ 1

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
org 0

; Jump table
jmp init ; 0000
jmp near ptr status ; 0003
jmp near ptr getch ; 0006
jmp near ptr putch ; 0009
jmp unimplemented ; 000c Output to printer
jmp unimplemented ; 000f Serial read
jmp unimplemented ; 0012 Serial write
jmp near ptr diskread ; 0015
jmp diskwrite ; 0018
jmp near ptr diskchange ; 001b
jmp near ptr setdate ; 001e
jmp near ptr settime ; 0021
jmp near ptr gettime ; 0024
jmp near ptr flush ; 0027
jmp near ptr mapdev ; 002a

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, 6
	shl ax, cl
	; Memory size is passed in dx
	mov dx, ax
if debug
	call hexprint16
	call newline
endif

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

	call dosinit

	; Memory for command.com is at ds, with PSP already set up

	; Retrieve amount of memory in the program segment
	mov cx, word ptr ds:6

	; Set up DTA
	mov dx, 100h
	mov ah, 1ah
	int 21h

	; Save command.com's segment for later and revert ds for opening the FCB
	mov bx, ds
	mov ax, cs
	mov ds, ax

	; Open command.com
	mov dx, offset iogroup:command_fcb
	mov ah, 0fh
	int 21h
	test al, al
	jnz open_error

if debug
	mov al, '+'
	mov ah, 0eh
	int 10h
endif

	; Set random record field 0
	mov word ptr iogroup:command_fcb+33, 0
	mov word ptr iogroup:command_fcb+35, 0

	; Set record size to 1 byte
	mov word ptr iogroup:command_fcb+14, 1

	; Read command.com into memory
	mov ah, 27h
	int 21h

	jcxz read_error

	cmp al, 1
	jne read_error

if debug
	mov ax, cx
	call hexprint16

	mov al, '.'
	mov ah, 0eh
	int 10h
endif

	; Set up segments for command.com
	mov ds, bx
	mov es, bx
	mov ss, bx
	mov sp, 5ch ; Microsoft's documentation says to use this

	; Put a zero at top of the stack
	xor ax, ax
	push ax

	; Set default DTA
	mov dx, 80h
	mov ah, 1ah
	int 21h

	; Jump to command.com
	push bx
	mov ax, 100h
	push ax

trampoline proc far
	ret
trampoline endp

	; TODO: Better error reporting
	open_error:
		mov al, '-'
		jmp unfinished

	read_error:
		mov al, '!'

	unfinished:
		mov ah, 0eh
		int 10h
		jmp hang

; OUT:
;  al = character if any
;  zf = there were no characters
status proc far
	push bx
	push ax
	mov ah, 1
	int 16h
	mov bl, al
	pop ax
	mov al, bl
	pop bx
	ret
status endp

; OUT:
;  al = character
getch proc far
	push bx
	push ax
	xor ah, ah
	int 16h
	mov bl, al
	pop ax
	mov al, bl
	pop bx
	ret
getch endp

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

; IN:
;  al = driver number
;  ds:bx = buffer
;  cx = number of sectors to read
;  dx = LBA of first sector
; OUT:
;  TODO: Document
diskread proc far
if debug
	push ax
	mov ax, 0e00h + 'r'
	int 10h
	pop ax
	call far_caller
	push ax
	call hexprint8
	call space
	mov ax, ds
	call hexprint16
	call space
	mov ax, bx
	call hexprint16
	call space
	mov ax, cx
	call hexprint16
	call space
	mov ax, dx
	call hexprint16
	call newline
	pop ax
endif

	; TODO: Everything except sregs can be trashed
	push es
	push ax
	push bx
	push cx
	push dx
	push di

	; Save driver number
	push ax

	; BIOS uses es:bx instead of ds:bx
	mov ax, ds
	mov es, ax

	mov ax, dx

	; Put drive(r) number in dl
	pop dx

	sector_read_loop:
		jcxz ret_diskread

		mov di, 3 + 1 ; 3 retries, + 1 since we dec first

		try_sector_read:
			push ax
			push cx

			call chs

			mov ah, 2
			mov al, 1
			int 13h

			jnc sector_read_success

			dec di
			jz sector_read_fail

			reset_disk:
if debug
				mov al, '"'
				mov ah, 0eh
				int 10h
endif
				xor ax, ax
				int 13h

			pop cx
			pop ax
			jmp try_sector_read

		sector_read_fail:
			mov al, ah
if debug
			call hexprint8;debg
endif
			mov al, '?'
			mov ah, 0eh
			int 10h
			pop cx
			pop ax

			pop di
			pop dx
			pop ax ; Would be cx, don't overwrite
			pop bx
			pop ax
			pop es

			mov al, 12 ; TODO: Don't hardcode
			stc
			ret

		sector_read_success:
			pop cx
			pop ax
			dec cx
			inc ax
			add bx, 512
			jmp sector_read_loop

	ret_diskread:
		pop di
		pop dx
		pop cx
		pop bx
		pop ax
		pop es

		clc
		ret

diskread endp

; IN:
;  ax = LBA
; OUT:
;  ch = cylinder
;  cl = sector & 2 high bits of cylinder
;  dh = head
chs proc
	push ax
	push dx

	xor dx, dx
	; cylinder (track) - head - sector
	; cylinder = LBA / sectorspertrack / heads
	; head = LBA / sectorspertrack % heads
	; sector = LBA % sectorspertrack + 1
	div iogroup:sectorspertrack
	; ax = LBA / sectorspertrack
	; dx = LBA % sectorspertrack

	; sector
	mov cl, dl
	inc cl

	xor dx, dx
	div iogroup: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, 0c0h
	or cl, al

	mov ax, dx
	pop dx
	mov dh, ah
	pop ax

	ret
chs endp

diskwrite:
	mov al, 'w'
	jmp error

; IN:
;  al = drive
; OUT:
;  ah = -1 different / 0 dunno / 1 same
;  cf = 0 -> al = driver num
;  cf = 1 -> al = disk error code
diskchange proc far
	; TODO: Implement
	mov ax, 0000h
	clc
	ret
diskchange endp

; IN:
;  ax = days since 1980-01-01
setdate proc far
	mov iogroup:days_since_epoch, ax
	ret
setdate endp

; IN:
;  ch = hour
;  cl = minute
;  dh = second
;  dl = 1/100 second
settime proc far
	push ax
	push bx
	push cx
	push dx
	push si
	push di

	; Save second & 1/100 second in bx
	mov bx, dx

	; Hour
	mov al, ch
	cbw
	mul iogroup:doubleticks_per_hour
	shl ax, 1
	rcl dx, 1
	; Store the partial sum in si:di
	mov di, ax
	mov si, dx

	; Minute
	mov al, cl
	cbw
	mul iogroup:ticks_per_minute
	add di, ax
	adc si, dx

	; Second
	mov al, bh
	cbw
	mul iogroup:ticks_per_second
	add di, ax
	adc si, dx

	; 1/100 second
	mov al, bl
	cbw
	div iogroup:cs_per_tick
	xor ah, ah
	add di, ax
	adc si, 0

	; Move result to cx:ds and set timer
	mov cx, si
	mov dx, di
	mov ah, 01h
	int 1ah

	pop di
	pop si
	pop dx
	pop cx
	pop bx
	pop ax
	ret
settime endp

; OUT:
;  ax = days since 1980-01-01
;  ch = hour
;  cl = minute
;  dh = second
;  dl = 1/100 second
gettime proc far
	; TODO: Use RTC if available
	push bx

	; Get ticks since midnight and day rollover flag
	xor ah, ah
	int 1ah

	; al is nonzero if day has changed
	test al, al
	jz same_day
	inc iogroup:days_since_epoch

	same_day:

	; Divide the ticks by 2, to keep all our arithmetic within bounds
	shr cx, 1
	rcr dx, 1

	; int 1ah / ah=00 returns the ticks in cx:dx, div operates on dx:ax
	mov ax, dx
	mov dx, cx
	div iogroup:doubleticks_per_hour
	mov ch, al ; Hour

	; Remainder in dx, range [0,doubleticks_per_hour[ represents an hour
	; Multiply by 60, divide by doubleticks_per_hour to get the minute
	mov ax, dx
	mul iogroup:sixty
	div iogroup:doubleticks_per_hour
	mov cl, al ; Minute

	; Do same again to get seconds
	mov ax, dx
	mul iogroup:sixty
	div iogroup:doubleticks_per_hour
	mov bl, al ; Save seconds in bl, since dh gets destroyed by next muldiv

	; For 1/100th of a second, multiply by 100 instead
	mov ax, dx
	mul iogroup:hundred
	div iogroup:doubleticks_per_hour
	mov dh, bl ; Second
	mov dl, al ; 1/100 second

	mov ax, iogroup:days_since_epoch

	pop bx
	ret
gettime endp

flush proc far
	push ax

	flush_loop:
		mov ah, 1
		int 16h
		jz flush_ret

		xor ah, ah
		int 16h
		jmp flush_loop

	flush_ret:
	pop ax
	ret
flush endp

mapdev proc far
	; TODO: Implement
	ret
mapdev endp

unimplemented:
	mov al, '@'

error:
	mov ah, 0eh
	int 10h
	mov ah, 0eh
	mov al, '!'
	int 10h
if debug
	call far_caller
endif

hang:
	hlt
	jmp hang

code ends

; TODO: STRUC?

constants segment

sectorspertrack dw 8 ; TODO: Don't hardcode
heads dw 2 ; TODO: Don't hardcode

disks_table:
	db 1 ; 1 drive, TODO: Don't hardcode

	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
	db 2 ; Number of FATs
	dw 112 ; Number of directory entries
	dw 320*2 ; Number of sectors

doubleticks_per_hour dw 32772 ; 1800B0h / 2 / 24
ticks_per_minute dw 1092 ; 1800B0h / 24 / 60
ticks_per_second dw 18 ; 1800B0h / 24 / 60 / 60
cs_per_tick db 5 ; 24 * 60 * 60 * 100 / 1800B0h

sixty dw 60
hundred dw 100

constants ends

data segment

command_fcb:
	db 1 ; First drive, TODO: Don't hardcode
	db "COMMAND COM"
	db 25 dup (?)

days_since_epoch: dw 0

data ends

code segment
if debug

hexprint16 proc
	xchg al, ah
	call hexprint8
	xchg al, ah
hexprint8 proc
	rol al, 1
	rol al, 1
	rol al, 1
	rol al, 1
	call hexprint4
	rol al, 1
	rol al, 1
	rol al, 1
	rol al, 1
hexprint4 proc
	push ax

	and al, 0fh
	cmp al, 9
	jbe under_10
	add al, 'a' - 10 - '0'

under_10:
	add al, '0'
	mov ah, 0eh
	int 10h

	pop ax
	ret
hexprint4 endp
hexprint8 endp
hexprint16 endp
newline proc
	push ax
	mov ax, 0e0dh
	int 10h
	mov ax, 0e0ah
	int 10h
	pop ax
	ret
newline endp
space proc
	push ax
	mov ax, 0e20h
	int 10h
	pop ax
	ret
space endp

far_caller proc
	push ax
	push bx
	mov bx, sp
	mov ax, [ss:bx + 8]
	call hexprint16
	mov al, ':'
	mov ah, 0eh
	int 10h
	pop bx
	pop ax

near_caller proc
	push ax
	push bx
	mov bx, sp
	mov ax, [ss:bx + 6]
	call hexprint16
	call space
	pop bx
	pop ax
	ret
near_caller endp
far_caller endp

logaddr proc
	push ax
	push bx
	mov al, '>'
	mov ah, 0eh
	int 10h
	mov bx, sp
	mov ax, [ss:bx + 4]
	call hexprint16
	call space
	pop bx
	pop ax
	ret
logaddr endp

endif
code ends

end