vbr: Parse CHS from BIOS to optionally overwrite hardcoded CHS data

boot/vbr.asm

@@ -4,18 +4,18 @@ bpb:
   jmp _startup ; 2 bytes
   nop ; 1 byte
 .oem:
-  db "2A      "	; OEM
+  db "OEM     "	; OEM
 .bps:
   dw 0x200	; bytes per sector
 .spc:
   db 1		; sectors per cluster
 .rsectors:
   dw 2		; number of reserved sectors
-.fatcnt:
+.fats:
   db 2		; number of file allocation tables
 .dirents:
   dw 2		; Number of directory entries
-.seccnt:
+.sectors:
   dw 720	; total number of sectors
 .mtd:		; media descriptor type
   db 0xFD	; 5.25-inch Double sided, 40 tracks per side, 9 sectors per track (360 KB)
@@ -23,26 +23,69 @@ bpb:
   dw 0
 .spt:
   dw 9		; sectors per track
-.headcnt:
+.heads:
   dw 2		; number of heads
-.hidseccnt:
+.hidsecs:
   dd 0		; number of hidden sectors
-.lrgseccnt:
+.lrgsectors:
   dd 0		; large sector count
 
 times (0x3E - ($-$$)) nop
 
 _startup:
+  call printgeo
   mov ah, 0x08
   mov dl, 0x00
-  int 0x13
+  int 0x13 ; required, QEMU detects 360kB as 1200kB with CHS 80 2 15
+  jc .noadjust
+  inc dh
+  mov [bpb.heads], dh
   mov ax, cx
   and ax, 0x003F
-  call print16
+  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
+.noadjust:
+  call printgeo
 .end:
   hlt
   jmp .end
 
+printgeo:
+  mov ax, [bpb.heads]
+  call print16
+  mov ax, [bpb.spt]
+  call print16
+  mov ax, [bpb.sectors]
+  call print16
+  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)
+
+  ret
+
 %include "print.asm"
 
 times (0x1FE - ($-$$)) nop