From 1981a8efba2e6d63faf993a854940b106f7b19a5 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Tue, 31 Mar 2020 20:03:05 +0000
Subject: [PATCH] fat32 bs: use BPB include header and base off BP

We now set BP once at the start to point to the beginning of the BPB.
Encoding an direct address access can then be done relative to BP, which
saves an immediate byte per access.
---
 boot/fat32.asm | 61 ++++++++++++++++++--------------------------------
 1 file changed, 22 insertions(+), 39 deletions(-)

diff --git a/boot/fat32.asm b/boot/fat32.asm
index 60dad99..d1523ae 100644
--- a/boot/fat32.asm
+++ b/boot/fat32.asm
@@ -4,29 +4,12 @@
 	jmp short init
 	nop
 
-	; WORD reserved sector count
-rsc:	equ ( $$ + 0x00E )
+	%include "inc/bpb.asm"
 
-	; 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 )
+bpb:	equ 0x080B
 
 	; Area for BPB
-	times (0x5A - ($-$$)) db 0
+	times ( 0x5A - ($-$$) ) db 0
 
 init:	xor ax, ax
 	mov ss, ax
@@ -66,7 +49,7 @@ hlt:	hlt
 	; offsets relative to FS
 readd:	; add offset of cluster data area to DX:AX
 	push cx
-	mov di, [rde]
+	mov di, [bp+bpb_rde]
 	mov cl, 4
 	shr di, cl ; 32 bytes per entry
 	add ax, di
@@ -75,17 +58,17 @@ readd:	; add offset of cluster data area to DX:AX
 readr:	; 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]
+	mov cl, byte [bp+bpb_fn]
+fatlp:	add ax, [bp+bpb7_lsf]
+	adc dx, [bp+bpb7_lsf+2]
 	loop fatlp
 	pop cx
 readf:	; add offset to FAT table to DX:AX
-	add ax, word [rsc]
+	add ax, word [bp+bpb_rsc]
 	adc dx, 0
 readp:	; read sector DX:AX from partition
-	add ax, word [po]
-	adc dx, word [po+2]
+	add ax, word [bp+bpb_po]
+	adc dx, word [bp+bpb_po+2]
 	jc err
 read_:	; read sector DX:AX from disk
 	; qword sector number DX:AX
@@ -102,17 +85,17 @@ read_:	; read sector DX:AX from disk
 	push cx
 
 	; size & passing
-	mov bp, 0x10
-	push bp
+	mov di, 0x10
+	push di
 
 	mov si, sp
 	mov ah, 0x42
-	mov dl, [dn]
+	mov dl, [bp+bpb7_dn]
 	stc
 	int 0x13
 	jc dskerr
 
-	add sp, bp
+	add sp, di
 
 	ret
 
@@ -159,7 +142,7 @@ readc:	; load cluster number
 
 	; convert cluster number to sector number
 	; this is some cheapo multiplication with 2^n
-	mov cl, [sc]
+	mov cl, [bp+bpb_sc]
 l02:	shr cl, 1
 	jz l02e
 	clc
@@ -168,15 +151,15 @@ l02:	shr cl, 1
 	jmp l02
 l02e:
 	; dx:ax is now sector num in data area
-	mov cx, [sc]
+	mov cx, [bp+bpb_sc]
 	xor ch, ch
 
 	call readd
 
-	xchg bp, cx
+	xchg di, cx
 	mov cl, 5
-	sal bp, cl
-	add bx, bp
+	sal di, cl
+	add bx, di
 
 	pop dx
 	pop ax
@@ -219,7 +202,8 @@ loadf:	call search
 	mov dx, [si+0x14]
 	jmp load
 
-main:	mov [dn], dl
+main:	mov bp, bpb
+	mov [bp+bpb7_dn], dl
 	; load root directory
 	call loadr
 	; search for first system directory
@@ -234,10 +218,9 @@ main:	mov [dn], dl
 	xor dx, dx
 	xor bx, bx
 	xor di, di
-	xor bp, bp
 
 	; restore drive number
-	mov dl, [dn]
+	mov dl, [bp+bpb7_dn]
 
 	; restore potential partition table
 	pop si