From 9288ad46a4554e9bb4137ad7bc0e2b87b4cebbbc Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Mon, 2 Mar 2020 19:44:37 +0000
Subject: [PATCH] fat: rework bootloader

---
 fat.asm | 441 ++++++++++++++++++++++++++++----------------------------
 1 file changed, 221 insertions(+), 220 deletions(-)

diff --git a/fat.asm b/fat.asm
index 71a2ab4..771856a 100644
--- a/fat.asm
+++ b/fat.asm
@@ -1,241 +1,242 @@
-; Memory layout:
-%define segment   0x00100
+	; Bootsector for FAT12 filesystems
+	; Memory layout:
+	; 0000-03FF  IVT
+	; 0400-0500  Bios data area
+	; 0501-0600  Stack for us
+	; 0600-0800  This code
+	org 0x0600
+	; 0800-0A00  Buffer for disk
+buf:	equ 0x0800
 
-%define self      (0x7C00-(segment<<4)) ; 1 sector
-%define fattab    (self+0x200) ; variable size
-%define rootdir   0x00100 ; variable size
-%define prog      0x0F000 ; 4K at the end for OS
+	jmp near init
 
-org self
+	; current cluster number
+cclus:	dw 0
+	; sector number in cluster
+csec:	dw 0
 
-jmp short init
+	times (0x0B - ($-$$)) db 0
+fdc:	; sector size
+.ss:	dw 0x200
+	; sectors per cluster
+.sc:	db 2
+	; reserved sector count
+.rsc:	dw 1
+.fn:	; number of file allocation tables
+	db 2
+.rde:	; number of root directory entries
+	dw 0x70
+.ts:	; total number of sectors
+	dw 720
+.mid:	db 0xFD
+.sf:	dw 2
+.spt:	dw 9
+.nos:	dw 2
+.po:	dd 0
+.lrgts:	; large sector count - relevant if .ts is 0
+	dd 0
 
-cluster_offset:
-  dw 0
+	; mformat keeps writing until here
+	; if we place init earlier, code gets overwritten
+	times (62 - ($-$$)) nop
 
-times (0x0B - ($-$$)) db 0
+init:	xor ax, ax
+	mov ds, ax
+	mov es, ax
+	mov ss, ax
+	mov sp, 0x0600
 
-fdc:
-.ss:
-  dw 0x200	; sector size
-.sc:
-  db 2		; sectors per cluster
-.rsc:
-  dw 1		; reserved sector count
-.fn:
-  db 2		; number of file allocation tables
-.rde:
-  dw 0x70	; number of root directory entries
-.ts:
-  dw 720	; total number of sectors
-.mi:		; medium identifier
-  db 0xFD	; 5.25-inch Double sided, 40 tracks per side, 9 sectors per track (360 KB)
-.sf:		; sectors per fat
-  dw 2
-.spt:
-  dw 9		; sectors per track
-.nos:
-  dw 2		; number of sides (heads)
-.po:
-  dd 0		; partition offset (in LBA blocks)
-.lrgts:
-  dd 0
-.drv:
-  db 0		; drive number
-  db 0
-  db 0x29	; efdc signature
-.vid:
-  dd 0		; volume id
-.vlabel:
-  db "2B"
-  times (54 - ($-$$)) db " "
-.fstype:
-  db "FAT12"
-  times (62 - ($-$$)) db " "
+	; Relocate and jump
+	mov di, sp
+	mov si, 0x7C00
+	mov cx, 0x0100
+	rep movsw
+	jmp 0:main
 
-; mformat keeps writing until here
-; if we place init earlier, code gets overwritten
-times (62 - ($-$$)) nop
+	; Set adress for sector read/write
+	; Number is relative to current partition
+	; IN	dx:ax	sector number
+seek:	push ax
+	push dx
+	push ds
+	push cs
+	pop ds
+	; add partition offset (required for HDD)
+	add ax, [fdc.po]
+	adc dx, [fdc.po+2]
+	; ax:temp     = (lba / spt)
+	; dx:sector   = (lba % spt) + 1
+	div word [fdc.spt]
+	inc dx
+	; Sector number (bits 0-5) into CL
+	; This is blindly assumed to never exeed 63
+	mov byte [.cl], dl
+	; ax:cylinder = (tmp / heads)
+	; dx:head     = (tmp % heads)
+	xor dx, dx
+	div word [fdc.nos]
+	; Cylinder number bit 0-7 into CH
+	mov byte [.ch], al
+	; Head number into DL
+	mov byte [.dh], dl
+	; Bits 8-9 of cylinder number into bits 6-7 of CL
+	and ah, 0x03
+	ror ah, 1
+	ror ah, 1
+	or byte [.cl], ah
+	pop ds
+	pop dx
+	pop ax
+	ret
+.cl:	db 0
+.ch:	db 0
+.dl:	db 0
+.dh:	db 0
 
-init:
-  cli
-  jmp segment:main
+	; Read sector into ES:BX
+	; Sector number has been previously set with seek
+read:	push ax
+	push cx
+	push dx
+	; ah = int13 subfunction 2
+	; al = read 1 sector
+	mov ax, 0x0201
+	; load seek data into registers
+	mov cx, word [cs:seek.cl]
+	mov dx, word [cs:seek.dl]
 
-main:
-  ; Stack grows down from 64k
-  mov ax, cs
-  mov ss, ax
-  mov sp, prog
-  mov ds, ax
-  mov es, ax
+	int 0x13
 
-  mov [fdc.drv], dl ; save drive number in fd
-  sti
+	pop dx
+	pop cx
+	pop ax
+	ret
 
-  ; load fat table into memory
-  mov ax, [fdc.rsc]
-  mov cx, [fdc.sf]
-  xor dx, dx
-  mov bx, fattab
-  call load_sectors
+	; Get start sector of root dirrectory
+	; OUT	ax	sector number
+dirsec:	; Multiply sectors per fat with number of fats
+	mov ax, [cs:fdc.sf]
+	mul byte [cs:fdc.fn]
+	; Add number of reserved sectors
+	add ax, [cs:fdc.rsc]
+	ret
 
-  ; calculate length of rootdir
-  mov ax, [fdc.rde]
-  mov cl, 4
-  shr ax, cl ; 32 bytes per entry
-  mov cx, ax
+	; Get root directory length in sectors
+	; OUT	cx	sectors
+dirlen: push ax
+	mov ax, [cs:fdc.rde]
+	mov cl, 4
+	shr ax, cl ; 32 bytes per entry
+	mov cx, ax
+	pop ax
+	ret
 
-  ; load root dir
-  xor dx, dx
-  mov ax, [fdc.sf]
-  mul byte [fdc.fn]
-  add ax, [fdc.rsc]
-  mov bx, rootdir
-  call load_sectors
+	; Get first sector of data area
+	; OUT	ax	sector number
+clusec:	push cx
+	call dirsec
+	call dirlen
+	add ax, cx
+	pop cx
+ret:	ret
 
-  ; remember where we left off
-  ; clusters start after rootdir
-  mov [cluster_offset], ax
+	; Get total number of sectors
+	; OUT:	dx:ax	number of sectors
+secnum: xor dx, dx
+	mov ax, [cs:fdc.ts]
+	test ax, ax
+	jnz ret
+	mov ax, [cs:fdc.lrgts]
+	mov dx, [cs:fdc.lrgts+2]
+	ret
 
-  ; Load kernel
-  mov bx, prog
-  mov ax, kernel_name
-  call load_file
-  mov bp, 0x3332
-  jc error
+	; Get number of clusters
+	; OUT:	ax	number of clusters
+	; Cant overflow for FAT16 :P
+clunum:	push cx
+	push dx
+	call clusec
+	mov cx, ax
+	call secnum
+	sub ax, cx
+	sbb dx, 0
+	mov cl, byte [cs:fdc.sc]
+	xor ch, ch
+	div cx
+	pop dx
+	pop cx
+	ret
 
-  mov dl, [fdc.drv]
+	; Scan current buffer for matching file
+	; IN	ds	= cs
+	;	es	= cs
+	; OUT	bx	start cluster if file found
+	;		trashed if no match
+	; carry is set if no file was found
+scadir:
+	push cx
+	push si
+	mov si, buf
+	mov cx, 0x10
+.loop:	push cx
+	push si
+	mov bx, [si+0x1A]
+	mov di, .name
+	mov cx, 11
+	repe cmpsb
+	pop si
+	pop cx
+	je .found
+	add si, 0x20
+	loop .loop
+	stc
+.found:	pop si
+	pop cx
+	ret
+.name:	db "HELLO   "
+.ext:	db "COM"
 
-  ; jump into kernel
-  jmp segment:prog
+open:
+	push ax
+	push cx
+	push dx
+	push bx
+	push ds
+	push es
+	xor ax, ax
+	mov ds, ax
+	mov es, ax
+	mov [csec], ax
+	call dirsec
+	call dirlen
+.loop:	xor dx, dx
+	mov bx, buf
+	call seek
+	call read
+	call scadir
+	jnc .found
+	inc ax
+	loop .loop
+	stc
+.found:
+	mov [cclus], bx
+	pop es
+	pop ds
+	pop bx
+	pop dx
+	pop cx
+	pop ax
+	ret
 
-; Load a file into memory
-; IN	AX	pointer to 8.3 filename
-;	ES:BX	pointer to target area
-; OUT	CF	flag set if error
-;	DI	file size in bytes (<64K)
-load_file:
-  mov si, rootdir
-  mov cx, [fdc.rde]
-.search:
-  push cx
-  push si
-  mov di, ax
-  mov cx, 11
-  repe cmpsb
-  pop si
-  pop cx
-  je .read
-  add si, 0x20
-  loop .search
-  xor di, di
-  stc
-  ret
-.read:
-  mov ax, [si+0x1A]
-  mov di, [si+0x1C]
-  jmp read_clusters
+main:	mov [seek.dl], dl ; save drive number
 
-; Read the file given by cluster number
-; into the target program area
-; in	ax	cluster number
-read_clusters:
-  ; read cluster into area for target file
-  push ax
-  sub ax, 2
-  mul BYTE [fdc.sc]
-  add ax, [cluster_offset]
-  xor dx, dx
-  mov cx, [fdc.sc]
-  xor ch, ch
-  call load_sectors
-  pop ax
+	int 3
+	call open
+	int 3
 
-  ; calculate index in FAT
-  mov si, ax
-  shr si, 1
-  add si, ax
-  add si, fattab
+.hlt:	hlt
+	jmp .hlt
 
-  ; load entry from FAT, truncate to 12 bit
-  mov dx, [si]
-  test ax, 1
-  jz .noshift
-  mov cl, 4
-  shr dx, 4
-.noshift:
-  mov ax, dx
-
-  and ax, 0x0FFF
-  cmp ax, 0x0FF8
-  jc read_clusters
-
-  ret
-
-; Read sectors from disk
-; Does not return on error
-; ax and bx will be incremented, cx decremented
-; in    dx:ax   sector number
-;       es:bx   buffer
-;	cx	number of sectors to read
-; out	dx:ax	next sector to read
-;	es:bx	next free buffer
-;	cx	zero
-load_sectors:
-  ; fail instantly if reading sectors > 16 bit
-  test dx, dx
-  mov bp, 0x3330
-  jnz error
-
-.loop:
-  push ax
-  push cx
-  push dx
-
-  ; add partition offset (required for HDD)
-  add ax, [fdc.po]
-  adc dx, [fdc.po+2]
-
-  ; calculate CHS data
-  div word [cs:fdc.spt]    ; ax:temp     = (lba / spt)
-  inc dx                   ; dx:sector   = (lba % spt) + 1
-  mov cl, dl ; sector number
-  xor dx, dx
-  div word [cs:fdc.nos]    ; ax:cylinder = (tmp / heads)
-                           ; dx:head     = (tmp % heads)
-  mov ch, al ; cylinder number
-  mov dh, dl ; head number
-  mov dl, [cs:fdc.drv] ; driver number
-  mov ax, 0x0201 ; ah=0x02 al=0x01
-  int 0x13
-  mov bp, 0x3331
-  jc error
-
-  pop dx
-  pop cx
-  pop ax
-
-  ; count up for next sector
-  add bx, 0x0200
-  inc ax
-
-  loop .loop
-  ret
-
-error:
-  mov ax, bp
-  mov ah, 0x0e
-  mov bx, 7
-  int 0x10
-  mov al, 0x21
-  int 0x10
-  xor ax, ax
-  int 0x16
-  int 0x19
-
-kernel_name:
-  db "BDOS    BIN"
-
-times (0x1FE - ($-$$)) db 0
-
-dw 0xAA55
+	; Padding and signature
+	times (0x1FE - ($-$$)) db 0
+	dw 0xAA55