From 391a1aa7d551047d3230cc2c21352582c28c1d48 Mon Sep 17 00:00:00 2001
From: Ain <41307858+nero@users.noreply.github.com>
Date: Tue, 26 Mar 2019 20:46:40 +0000
Subject: [PATCH] vbr: CHS reading seems to work so far

---
 Makefile     |   3 +-
 boot/vbr.asm | 119 ++++++++++++++++++++++++++++++++++++++-------------
 2 files changed, 91 insertions(+), 31 deletions(-)

diff --git a/Makefile b/Makefile
index b7e6411..ac41d13 100644
--- a/Makefile
+++ b/Makefile
@@ -7,8 +7,7 @@ vbr.bin: boot/vbr.asm
 	nasm -s -o $@ -I lib $<
 
 fdimage.img: vbr.bin kernel.com
-	dd if=/dev/zero bs=512 count=720 of=$@
-	dd if=vbr.bin bs=512 count=1 conv=notrunc of=$@
+	cat vbr.bin kernel.com > $@
 
 clean:
 	rm -f *.com
diff --git a/boot/vbr.asm b/boot/vbr.asm
index cf77145..45fccba 100644
--- a/boot/vbr.asm
+++ b/boot/vbr.asm
@@ -29,15 +29,66 @@ bpb:
   dd 0		; number of hidden sectors
 .lrgsectors:
   dd 0		; large sector count
+.drivenum:
+  db 0		; drive number
+.winntflags:
+  db 0
+.signature:
+  db 0x28
 
-times (0x3E - ($-$$)) nop
+times (0x5A - ($-$$)) nop
 
 _startup:
-  call printgeo
+  call adjust_bpb
+  mov ax, 0x0500
+  mov es, ax
+
+  mov si, 1
+  mov di, 0x7D00
+  mov cx, 0x10
+  call loadblk
+  call dump
+.end:
+  hlt
+  jmp .end
+
+dump:
+  mov si, 0x0000
+  mov cx, 0x10
+.nextline:
+  mov ax, si
+  call print16
+  mov al, 0x3A
+  call printc
+  mov al, 0x20
+  call printc
+
+  push cx
+  mov cx, 0x0020
+.words:
+  es lodsb
+  call print8
+  loop .words
+  pop cx
+
+  mov al, 0x0A
+  call printc
+  mov al, 0x0D
+  call printc
+
+  loop .nextline
+
+  ret
+
+; Adjust the bdp data from BIOS-supplied data (if)
+; No args, no output
+adjust_bpb:
+  push ax
+  push cx
+  push dx
   mov ah, 0x08
-  mov dl, 0x00
   int 0x13 ; required, QEMU detects 360kB as 1200kB with CHS 80 2 15
-  jc .noadjust
+  jc .end ; skip if function does not exist
   inc dh
   mov [bpb.heads], dh
   mov ax, cx
@@ -51,39 +102,49 @@ _startup:
   mul word [bpb.heads] ; number of tracks = number of cylinders * heads
   mul word [bpb.spt] ; number of sectors = number of tracks * sectors per track
   mov [bpb.sectors], ax
-.noadjust:
-  call printgeo
 .end:
-  hlt
-  jmp .end
+  pop dx
+  pop cx
+  pop ax
+  ret
+
+; in	si	sector number,
+;	dl	drive number
+;	es:0000	buffer
+; out	al	error code,
+;	carry set if error
+loadblk:
+  push bx
+  push cx
+  push dx
+  xor dx, dx
+  mov ax, si
+  div word [bpb.spt]	; ax:temp     = (lba / spt)
+  inc dx		; dx:sector   = (lba % spt) + 1
+  mov cl, dl ; for int 0x13
+  xor dx, dx
+  div word [bpb.heads]  ; ax:cylinder = (tmp / heads)
+			; dx:head     = (tmp % heads)
+  mov ch, al ; for int 0x13
+  mov ax, dx
+  pop dx     ; drive number comes back into dl
+  mov dh, al ; for int 0x13
+
+  mov bx, 0x0000
+  mov ax, 0x0201 ; ah=0x02 al=0x01
+  int 0x13
 
-printgeo:
-  mov ax, [bpb.heads]
-  call print16
-  mov ax, [bpb.spt]
-  call print16
-  mov ax, [bpb.sectors]
   call print16
+  lahf
+  xchg ah, al
+  call print8
   mov al, 0x0A
   call printc
   mov al, 0x0D
   call printc
-  ret
-
-; in	bx	sector number,
-;	dl	drive number
-;	ds:di	buffer
-; out	al	error code,
-;	carry set if error
-loadblk:
-  mov ax, bx
-  div byte [bpb.spt]
-  call print16
-  ; Temp = LBA / (Sectors per Track)
-  ; Sector = (LBA % (Sectors per Track)) + 1
-  ; Head = Temp % (Number of Heads)
-  ; Cylinder = Temp / (Number of Heads)
 
+  pop cx
+  pop bx
   ret
 
 %include "print.asm"