From a472c396e7a17a29602eaa9023ce772227a29b69 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Thu, 7 May 2020 22:56:37 +0000
Subject: [PATCH] Implement reading of BPB from FAT partition

---
 kernel/diskio.asm | 34 +++++++++++++++++++++++++++-------
 1 file changed, 27 insertions(+), 7 deletions(-)

diff --git a/kernel/diskio.asm b/kernel/diskio.asm
index 1536985..1ff7504 100644
--- a/kernel/diskio.asm
+++ b/kernel/diskio.asm
@@ -65,6 +65,7 @@ select:	mov byte [disk_current], 0xFF
 	jmp select_hdd
 
 select_floppy:
+	push dx
 	call seek_zero
 	call read
 	; load disk geometry
@@ -76,17 +77,30 @@ select_floppy:
 	; we dont rely on the 'total sectors' given in the BPB
 	; instead, we assume maximum cylinder
 	; 1023 is the maximum encodable for int 13h
-	mov di, 1023
-	mul di
-	mov [part_size], ax
-	mov [part_size+2], dx
-	; set partition offset to zero
+	lea di, [part_offset]
 	xor ax, ax
-	mov [part_offset], ax
-	mov [part_offset+2], ax
+	; partition offset = 0
+	stosw
+	stosw
+	; partition size = 0x00010000 = 32MB
+	stosw
+	inc ax
+	stosw
+
+	pop dx
 	ret
 
 select_hdd:
+	push dx
+	; load CHS geometry for drive
+	mov ah, 0x8
+	int 0x13
+	xor ah, ah
+	mov al, cl
+	mov [disk_spt], ax
+	mul byte dh
+	mov [disk_spc], ax
+	; load offset and size of first partition
 	call seek_zero
 	call read
 	mov bx, disk_buffer+0x1BE
@@ -94,6 +108,12 @@ select_hdd:
 	lea di, [part_offset]
 	mov cx, 4
 	rep movsw
+	; read bpb
+	mov dx, [part_offset+2]
+	mov ax, [part_offset]
+	call seek
+	call read
+	pop dx
 	ret
 
 	; Set absolute sector number