From 1e99086fc628a9e8928bd3c3cb540b70f1d1953a Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Sat, 4 Apr 2020 14:29:33 +0000
Subject: [PATCH] Add CHS+LBA dual support for boot sector

---
 boot/fat32.asm | 82 ++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 70 insertions(+), 12 deletions(-)

diff --git a/boot/fat32.asm b/boot/fat32.asm
index 0f1b9dd..fe5d20c 100644
--- a/boot/fat32.asm
+++ b/boot/fat32.asm
@@ -54,7 +54,52 @@ readp:	; read sector DX:AX from partition
 	add ax, word [bp+bpb_po]
 	adc dx, word [bp+bpb_po+2]
 	jc dskerr
-read_:	; read sector DX:AX from disk
+	jmp [bp-2]
+
+rchs:	; read sector DX:AX from disk
+	; uses basic CHS disk IO
+	; this cant reliable read over track boundaries
+	; so we read each single sector in loop over CX
+chslp:	push ax
+	push cx
+	push dx
+	push bx
+
+	; calculate CHS data
+	div word [bp+bpb_spt]    ; ax:temp     = (lba / spt)
+	inc dx                   ; dx:sector   = (lba % spt) + 1
+	mov cl, dl ; sector number
+	xor dx, dx
+	div word [bp+bpb_nos]    ; ax:cylinder = (tmp / heads)
+	                         ; dx:head     = (tmp % heads)
+	mov ch, al ; cylinder number
+	mov dh, dl ; head number
+	mov dl, [bp+bpb7_dn] ; drive number
+
+	; set up read buffer
+	mov es, bx
+	xor bx, bx
+
+	; call bios
+	mov ax, 0x0201
+	int 0x13
+	jc dskerr
+
+	pop bx
+	pop dx
+	pop cx
+	pop ax
+
+	; increment sector num + write offset
+	add ax, 1
+	adc dx, 0
+	add bx, 0x20
+	; loop on CX
+	loop chslp
+	ret
+
+rebios:	; read sector DX:AX from disk
+	; this uses the EBIOS extensions
 	; qword sector number DX:AX
 	push cs
 	push cs
@@ -153,6 +198,15 @@ l02e:
 
 	ret
 
+loadr:	mov ax, 2
+	xor dx, dx
+	jmp load
+
+loadf:	call search
+	jne fserr
+	mov ax, [si+0x1A]
+	mov dx, [si+0x14]
+
 	; load cluster chain DX:AX to [dest]:0
 load:	mov bx, 0x07C0
 lloop:	call readc
@@ -179,18 +233,22 @@ loop:	add si, 0x20
 	jne loop
 	ret
 
-loadr:	mov ax, 2
-	xor dx, dx
-	jmp load
-
-loadf:	call search
-	jne fserr
-	mov ax, [si+0x1A]
-	mov dx, [si+0x14]
-	jmp load
-
 main:	mov bp, bpb
 	mov [bp+bpb7_dn], dl
+
+	; check if EBIOS is supported
+	mov bx, 0x55AA
+	mov ah, 0x41
+	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
 	call loadr
 	; search for first system directory
@@ -213,7 +271,7 @@ main:	mov bp, bpb
 	pop si
 	pop ds
 
-	jmp 0:0x7C00
+	jmp 0x7C00
 
 	; error handling for file not found
 fserr:	call errcll