From bbe006c9bce06c380e9adb9a21d830ce3ad35d08 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Mon, 30 Mar 2020 15:07:09 +0000
Subject: [PATCH] Improve FAT offset calculation

---
 boot/fat32.asm | 139 +++++++++++++++++++++++--------------------------
 1 file changed, 66 insertions(+), 73 deletions(-)

diff --git a/boot/fat32.asm b/boot/fat32.asm
index 78e9ca7..6d7f8d5 100644
--- a/boot/fat32.asm
+++ b/boot/fat32.asm
@@ -4,15 +4,23 @@
 	nop
 
 	; WORD reserved sector count
-rsc:    equ ( $$ + 0x00E )
+rsc:	equ ( $$ + 0x00E )
+
 	; BYTE number of sectors per cluster
 sc:	equ ( $$ + 0x00D )
+
 	; BYTE number of FATs
 fn:	equ ( $$ + 0x010 )
+
+	; WORD number of root directory entries
+rde:	equ ( $$ + 0x011 )
+
 	; DWORD hidden sector count (partition offset)
 po:	equ ( $$ + 0x01C )
+
 	; DWORD sectors per FAT
 sf:	equ ( $$ + 0x024 )
+
 	; BYTE drive number (we set it from dl)
 dn:	equ ( $$ + 0x040 )
 
@@ -43,34 +51,67 @@ mend:	push si
 	ret
 
 err:	call msg
-	db "DISKERR", 0x0A, 0x0D, 0
+	db "ERR", 0x0A, 0x0D, 0
 hlt:	hlt
 	jmp hlt
 
-	; Requires DWORD ptr and QWORD sector on the stack
-read:	pop bp
+	; offsets relative to FS
+offd:	; add offset of cluster data area to DX:AX
 	push cx
-	mov ax, 0x10
+	mov di, [rde]
+	mov cl, 4
+	shr di, cl ; 32 bytes per entry
+	add ax, di
+	adc dx, 0
+	pop cx
+offr:	; add offset to rootdir to DX:AX (FAT12/FAT16 only)
+	push cx
+	xor ch, ch
+	mov cl, byte [fn]
+fatlp:	add ax, [sf]
+	adc dx, [sf+2]
+	loop fatlp
+	pop cx
+offf:	; add offset to FAT table to DX:AX
+	add ax, word [rsc]
+	adc dx, 0
+	ret
+
+readp:	; read sector DX:AX from partition
+	add ax, word [po]
+	adc dx, word [po+2]
+	jc err
+
+readd:	; read sector DX:AX from disk
+	; qword sector number DX:AX
+	push cs
+	push cs
+	push dx
 	push ax
+
+	; dword target buffer: BX:0
+	push bx
+	push cs
+
+	; word sector number
+	push cx
+
+	; size & passing
+	mov bp, 0x10
+	push bp
+
+	mov si, sp
 	mov ah, 0x42
 	mov dl, 0x80
-	mov si, sp
-
-	mov di, [si+4]
+	stc
 	int 0x13
 	jc err
 
-	pop ax
-	pop cx
-	push bp
+	add sp, bp
+
 	ret
 
-	; Advances [clus] to next FAT entry
-next:	; push upper two words for sector num
-	xor ax, ax
-	push ax
-	push ax
-
+next:	; Advances [clus] to next FAT entry
 	; get current cluster number
 	mov ax, [clus]
 	mov dx, [clus+2]
@@ -83,21 +124,11 @@ shftl:	clc
 	rcr ax, 1
 	loop shftl
 
-	; add reserved sector count
-	add ax, word [rsc]
-	adc dx, 0
-
-	; push lower dword of sector number
-	push dx
-	push ax
-
-	; push ptr to target buffer
-	push cs
-	mov ax, buf
-	push ax
+	call offf
 
+	mov bx, 0xA0
 	mov cx, 1
-	call read
+	call readp
 
         ; get lower part of cluster number
         mov si, [clus]
@@ -112,8 +143,6 @@ shftl:	clc
 	movsw
 	movsw
 
-	; 1x QWORD, 1x DWORD = 12 bytes
-	add sp, 12
 	ret
 
 	; reads current cluster into [dest]
@@ -136,48 +165,18 @@ l02:	shr cl, 1
 	jmp l02
 l02e:
 	; dx:ax is now sector num in data area
-
-	; get number of FATs
-	xor ch, ch
-	mov cl, byte [fn]
-	; add their sizes up
-floop:  add ax, [sf]
-	adc dx, [sf+2]
-	loop floop
-
-	; add reserved sector count
-	add ax, [rsc]
-	add dx, 0
-
-	; push sector num QWORD
-	xor cx, cx
-	push cx
-	push cx
-	push dx
-	push ax
-
-	; push target buffer DWORD
-	; dest gives the segment register
-	mov ax, [dest]
-	push ax
-	push cx
-
-	; push sector count
-	; always an full cluster
+	call offd
+	mov bx, [dest]
 	mov cx, [sc]
 	xor ch, ch
 
-	call read
-
-	cmp cl, [sc]
-	jne err
+	call readp
 
 	xchg bx, cx
 	mov cl, 5
 	sal bx, cl
 	add [dest], bx
 
-	add sp, 12
 	ret
 
 loadf:	call readc
@@ -187,7 +186,8 @@ loadf:	call readc
 	ret
 
 main:	call loadf
-	mov ax, [dest]
+	mov ax, [0x7C00]
+	mov cx, [0x7C02]
 	int 3
 
 	xor ah,ah
@@ -195,16 +195,9 @@ main:	call loadf
 	int 0x19
 	jmp hlt
 
-	; the packing is so that it looks nice in a hexdump
-	times (0x1E0 - ($-$$)) db 0
-	; directory the kernel is in
-tdir:	db "RDOS       ", 0
 	; current cluster number
 clus:	dd 2
 
-	times (0x1F0 - ($-$$)) db 0
-	; filename for the kernel
-tfile:	db "KERNEL  BS ", 0
 	; segment register for data
 dest:	dw 0x07c0