VGAONOFF: Enable/disable writes to VGA memory from DOS

This DOS utility modifies a flag in a VGA register which controls whether the VGA memory can be updated. This can be used to suppress output from command(s) (and then from the COMMAND.COM prompt if you don't re-enable the flag!).

Usage

• vgaonoff off disables VGA memory updates. The cursor will still be visible unless a separate utility is used to control it, and it will still move around the screen under the control of whatever program(s) run after vgaonoff returns.

• vgaonoff on restores normal behavior.

When invoked as shown above, it generates no output except in case of an error. If invoked without any arguments or with an unsupported argument, it displays a usage message.

Supported hardware and emulators

A VGA adapter is required. This utility attempts to detect whether one is present. An error is reported if one is not detected.

This has been tested successfully with a small number of VGA adapters ranging from an older ISA one to the Intel 865G chipset. It is assumed that this is fairly standard VGA behavior.

This VGA behavior is not emulated by Bochs 2.7.0, DOSBox 0.74-3, PCem 16 or QEMU 6.1.0, so there's a good chance it's not emulated by any emulator or hypervisor.

GitHub issues reporting exceptional behavior - a real VGA adapter that this utility does not work with or an emulated one that it does work with - are most welcome.

Example

The example directory contains a test.bat which uses vgaonoff and the accompanying progress.bat to display a splash screen with a progress bar while suppressing output from the accompanying spam.bat. spam.bat uses the Batch Enhancer utility from Norton Utilities for DOS (tested with version 8) to introduce delays.

Building

This utility was built using Borland's Turbo Assembler 3.1 as included with Borland C++ 3.1. build.bat can be used to perform the build.

License

VGAONOFF is Copyright 2024 David O'Shea

VGAONOFF is licensed to you under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. See the COPYING file for details.

I'm willing to consider licensing it under a less restrictive license. Please file a GitHub issue if you would like this, explaining why it would be useful.