DBWIN Won't Work with Text on First Line of Mono Display

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1

SUMMARY

The DBWIN sample shipped with the Microsoft Windows Software Development Kit version 3.1 allows your system to redirect its debugging output to a variety of locations, including a secondary monochrome monitor.

Due to the design of DBWIN, if the monochrome monitor contains any text on the first line of its display when DBWIN attempts to write to it, DBWIN will not output any text.

MORE INFORMATION

Because Windows does not control access to the secondary monochrome monitor, it is possible for more than one application to write to the monitor at the same time, producing garbled results.

To reduce the possibility of this occurring, DBWIN attempts to detect when another application is using the monochrome display, and will stop output if this is detected.

The method DBWIN uses to detect whether another application is using the monochrome monitor is to check for output on the first line of the monitor. If there is any text on this line, DBWIN assumes that another application is using the monitor and discontinues output to the monitor.

All output that DBWIN sends to the monochrome monitor begins on the second line of the monitor, so it will never write on the first line. Each time a request is made to output to the monochrome monitor [either through an OutputDebugString() call or through a debugging message from Windows], a check for any text on the first line is made. If any text exists on the first line, DBWIN will not attempt to write to the monitor because a second application might be using it.

Note that this method of checking for monochrome monitor usage is not foolproof and may not work in all cases. In certain situations, it may be necessary to clear the monitor manually to allow DBWIN to send text to the monochrome monitor again. The following batch file clears the monochrome monitor and allow DBWIN to work again:

   @echo off
   mode mono
   cls
   mode co80

extern   WORD _B000h; // Selector exported by KERNEL
void ClearMono(void) {
  _asm{
    push di               // DI is not saved automatically
    mov ax, OFFSET _B000h // Get selector to mono video memory
    mov es,ax             // Move into ES
    xor di,di             // ES:DI points to mono video memory
    mov ax, 0720h         // 0x07 = attributes, 0x20 = space character
    mov cx, 80*25         // Repeat for 80 columns by 25 rows
    rep stosw             // Store AX into (ES:DI)++, CX times
    pop di
    }