From 27e4dec5802deaba96b257c3032c3ad270213319 Mon Sep 17 00:00:00 2001
From: Ain <41307858+nero@users.noreply.github.com>
Date: Wed, 15 Apr 2020 23:51:07 +0200
Subject: [PATCH] Implement loading BPB from media

---
 boot/kernel.asm | 38 ++++++++++++++++++++++++++++----------
 1 file changed, 28 insertions(+), 10 deletions(-)

diff --git a/boot/kernel.asm b/boot/kernel.asm
index d68eef7..baa20f5 100644
--- a/boot/kernel.asm
+++ b/boot/kernel.asm
@@ -139,12 +139,36 @@ setint:	push es
 bpb:	times bpb_len db 0
 drvnum:	db 0
 	align 4
-drvpos:	dd 0
+drvoff:	dd 0 ; partition offset
+drvpos:	dd 0 ; absolute physical sector number
 buffer:	times 512 db 0
 
 	; Select active drive
 	; IN	dl	drive number A=0, B=1, C=2, ...
 select:	mov [drvnum], dl
+	push es
+	xor ax, ax
+	push cs
+	pop es
+	stosw
+	stosw
+	stosw
+	stosw
+	pop es
+	call read
+	call ldbpb
+	ret
+
+	; load bpb from buffer to bpb
+ldbpb:	push ds
+	push es
+	mov ax, cs
+	mov si, buffer+0x0B
+	mov di, bpb
+	mov cx, bpb_len
+	rep movsb
+	pop es
+	pop ds
 	ret
 
 	; Set sector number
@@ -207,17 +231,11 @@ init:	cli
 	mov ss, ax
 	xor sp, sp
 
-	xor dh, dh
 	call dnconv
+	int 3
 	call select
-
-	xor ax, ax
-	xor dx, dx
-	call seek
-
-	call read
-
-	mov ax, [buffer+0x1FE]
+	int 3
+	mov ax, [cs:bpb+bpb_ss]
 	int 3
 
 main:	push cs