From e34accfcff969c617fb84ace814fdb6c24642dd9 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Sat, 4 Apr 2020 21:05:44 +0000
Subject: [PATCH] Improve support for auto-selecting LBA addressing

---
 boot/fat32.asm | 87 +++++++++++++++++++++++++++-----------------------
 1 file changed, 47 insertions(+), 40 deletions(-)

diff --git a/boot/fat32.asm b/boot/fat32.asm
index 77d61ba..8531cf1 100644
--- a/boot/fat32.asm
+++ b/boot/fat32.asm
@@ -54,7 +54,39 @@ readp:	; read sector DX:AX from partition
 	add ax, word [bp+bpb_po]
 	adc dx, word [bp+bpb_po+2]
 	jc dskerr
-	jmp [bp-2]
+
+	; This jump is overwritten with NOP's if LBA support
+dskjmp:	jmp rchs
+
+rebios:	; read sector DX:AX from disk
+	; this uses the EBIOS extensions
+	; 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 di, 0x10
+	push di
+
+	mov si, sp
+	mov ah, 0x42
+	mov dl, [bp+bpb7_dn]
+	stc
+	int 0x13
+	jc dskerr
+
+	add sp, di
+
+	ret
 
 rchs:	; read sector DX:AX from disk
 	; uses basic CHS disk IO
@@ -98,36 +130,6 @@ chslp:	push ax
 	loop chslp
 	ret
 
-rebios:	; read sector DX:AX from disk
-	; this uses the EBIOS extensions
-	; 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 di, 0x10
-	push di
-
-	mov si, sp
-	mov ah, 0x42
-	mov dl, [bp+bpb7_dn]
-	stc
-	int 0x13
-	jc dskerr
-
-	add sp, di
-
-	ret
-
 	; error handling for disk errors
 dskerr: call errcll
 
@@ -231,23 +233,28 @@ lloop:	call readc
 	jne lloop
 	ret
 
-main:	mov bp, bpb
+main:	; Set up BP to point to BPB
+	; Relative addressing saves one instruction byte per access
+	mov bp, bpb
+	; Save DL contents
 	mov [bp+bpb7_dn], dl
 
-	; check if EBIOS is supported
+	; EBIOS is ignored for floppies (dl < 80h)
+	test dl, 0x80
+	jz noebio
+
+	; check for EBIOS support
 	mov bx, 0x55AA
 	mov ah, 0x41
+	; set carry to assume failure if int didnt do anything
 	stc
 	int 0x13
-	; default: use CHS read
-	mov ax, rchs
 	jc noebio
-	; if EBIOS: use LBA read
-	mov ax, rebios
-noebio:	lea di, [bp-2]
-	stosw
 
-	; load root directory
+	; disable CHS routine, fall into LBA routine instead
+	mov word [dskjmp], 0x9090
+
+noebio: ; load root directory
 	call loadr
 	; search for first system directory
 	mov ah, 0x14