From c88395c12db8509e13c3e1aabed9ba2f810909a3 Mon Sep 17 00:00:00 2001
From: Ain <41307858+nero@users.noreply.github.com>
Date: Wed, 27 Mar 2019 22:05:00 +0000
Subject: [PATCH] vbr: work in progress on block reading

---
 boot/vbr.asm | 184 +++++++++++++++++++++++----------------------------
 1 file changed, 82 insertions(+), 102 deletions(-)

diff --git a/boot/vbr.asm b/boot/vbr.asm
index 45fccba..6895d15 100644
--- a/boot/vbr.asm
+++ b/boot/vbr.asm
@@ -1,57 +1,104 @@
 org 0x7C00
 
-bpb:
-  jmp _startup ; 2 bytes
-  nop ; 1 byte
+fdc:		; FDC Descriptor as per ECMA-107
+  jmp _startup
+  times (0x03 - ($-$$)) nop
 .oem:
-  db "OEM     "	; OEM
-.bps:
-  dw 0x200	; bytes per sector
-.spc:
-  db 1		; sectors per cluster
-.rsectors:
-  dw 2		; number of reserved sectors
-.fats:
-  db 2		; number of file allocation tables
-.dirents:
-  dw 2		; Number of directory entries
-.sectors:
-  dw 720	; total number of sectors
-.mtd:		; media descriptor type
+  db "2B"	; creating system identifier
+  times (0x0B - ($-$$)) db " " ; space-padded
+.ss:
+  dw 0x200	; sector size
+.sc:
+  db 0		; sectors per cluster
+.rsc:
+  dw 2		; reserved sector count
+.fn:
+  db 0		; number of file allocation tables
+.rde:
+  dw 0		; number of root directory entries
+.ts_old:
+  dw 0		; legacy: total number of sectors
+.mi:		; medium identifier
   db 0xFD	; 5.25-inch Double sided, 40 tracks per side, 9 sectors per track (360 KB)
-.spfat:		; sectors per fat
+.sf:		; sectors per fat
   dw 0
 .spt:
   dw 9		; sectors per track
-.heads:
-  dw 2		; number of heads
-.hidsecs:
-  dd 0		; number of hidden sectors
-.lrgsectors:
-  dd 0		; large sector count
-.drivenum:
+.nos:
+  dw 2		; number of sides (heads)
+.po:
+  dd 0		; partition offset (in LBA blocks)
+.ts:
+  dw 720
+.drv:
   db 0		; drive number
-.winntflags:
   db 0
-.signature:
-  db 0x28
-
-times (0x5A - ($-$$)) nop
+  db 0x29	; efdc signature
+.vid:
+  dd 0		; volume id
+.vlabel:
+  db "2B"
+  times (54 - ($-$$)) db " "
+.fstype:
+  db "FAT12"
+  times (62 - ($-$$)) db " "
 
 _startup:
-  call adjust_bpb
+  mov [fdc.drv], dl ; backup driver number
+  mov ah, 0x08
+  int 0x13 ; required, QEMU detects 360kB as 1200kB with CHS 80 2 15
+  jc .noadjust ; skip if function does not exist
+  inc dh
+  mov [fdc.nos], dh
+  mov ax, cx
+  and ax, 0x003F
+  mov [fdc.spt], ax ; no adjustment because sectors are 1-indexed
+  mov ax, cx
+  xchg al, ah
+  mov cl,6
+  shr ah,cl
+  inc ax ; convert from maximum number (0-based) to total number (1-based) of cylinders
+  mul word [fdc.nos] ; number of tracks = number of cylinders * heads
+  mul word [fdc.spt] ; number of sectors = number of tracks * sectors per track
+  mov [fdc.ts], ax
+.noadjust:
   mov ax, 0x0500
   mov es, ax
-
-  mov si, 1
-  mov di, 0x7D00
-  mov cx, 0x10
+  mov bx, 0x0000
+  mov ax, 1
   call loadblk
   call dump
+
+  mov al, [fdc.sc]
+  call print8
 .end:
   hlt
   jmp .end
 
+; Load a cluster from cluster offset
+loadcluster:
+  ret
+
+; Load a single cluster into memory
+; in    ax      sector number
+;       es:bx	buffer
+; out   al      error code,
+;       carry set if error
+loadblk:
+  xor dx, dx
+  div word [fdc.spt]    ; ax:temp     = (lba / spt)
+  inc dx                ; dx:sector   = (lba % spt) + 1
+  mov cl, dl ; sector number
+  xor dx, dx
+  div word [fdc.nos]    ; ax:cylinder = (tmp / heads)
+                        ; dx:head     = (tmp % heads)
+  mov ch, al ; cylinder number
+  mov dh, dl ; head number
+  mov dl, [fdc.drv] ; driver number
+  mov ax, 0x0201 ; ah=0x02 al=0x01
+  int 0x13
+  ret
+
 dump:
   mov si, 0x0000
   mov cx, 0x10
@@ -80,73 +127,6 @@ dump:
 
   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
-  int 0x13 ; required, QEMU detects 360kB as 1200kB with CHS 80 2 15
-  jc .end ; skip if function does not exist
-  inc dh
-  mov [bpb.heads], dh
-  mov ax, cx
-  and ax, 0x003F
-  mov [bpb.spt], ax ; no adjustment because sectors are 1-indexed
-  mov ax, cx
-  xchg al, ah
-  mov cl,6
-  shr ah,cl
-  inc ax ; convert from maximum number (0-based) to total number (1-based) of cylinders
-  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
-.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
-
-  call print16
-  lahf
-  xchg ah, al
-  call print8
-  mov al, 0x0A
-  call printc
-  mov al, 0x0D
-  call printc
-
-  pop cx
-  pop bx
-  ret
-
 %include "print.asm"
 
 times (0x1FE - ($-$$)) nop