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From: "Michal H. Tyc" <mht AT bttr-software DOT de>
Organization: BTTR Software
Date: Fri, 05 Dec 2003 18:57:47 +0100
X-Mailer: Arachne V1.73;GPL
To: opendos AT delorie DOT com
Subject: Re: DR Dos on a P4
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Message-Id: <E1ASKD3-0000gQ-Bg@mailout.artfiles.de>
Reply-To: opendos AT delorie DOT com

Oh, my first message to the list was rejected too...

Let's try again:

; CLRVDISK.COM
; Clear VDISK signature in HMA if A20 enabled at boot time,
;   which happens on some systems (as Toshiba Satellite 300CDS)
;   and causes memory manager initialization error
; Add INSTALL=CLRVDISK.COM in [D]CONFIG.SYS before the memory
;   manager; this works under DR-DOS only, as MS-DOS processes
;   INSTALL= directives too late
; Intel syntax (NASM), 80386+

  org 100h

  xor bx, bx
  mov ds, bx              ; ds = 0
  dec bx
  mov es, bx              ; es = 0FFFFh
  mov bx, 200h            ; word to compare: unused INT 80h vector
  mov di, 10h             ; offset of HMA in segment 0FFFFh
  mov ax, [bx]
  cmp ax, [es:bx + di]    ; 20-bit address wrap test
  jne clear               ; words differ, A20 enabled
  not dword [es:bx + di]  ; not sure yet, try modifying HMA
  cmp ax, [bx]
  jne done                ; 0:200h changed, so A20 is disabled
clear:
  cld
  mov cx, di              ; length = 10h words, dest = HMA start
  xor ax, ax
  rep stosw               ; clear VDISK header (if any)
done:
  ret                     ; return via INT 20h

The binary is attached in a private message.

Hope this helps,

Michal