Retro DOS Clock Program

A 16-bit x86 assembly program that displays a real-time digital clock in VGA graphics mode on DOS systems. Written for educational purposes to demonstrate low-level graphics programming and system time access.

What it does (high level)

This program creates a digital clock display that:

• Switches the display to 640×480, 16-color VGA graphics mode
• Reads the current system time from DOS
• Draws digital clock digits using custom line-drawing routines
• Displays hours, minutes, and seconds in HH:MM:SS format
• Updates the display in real-time as time changes
• Uses pixel-level graphics to render seven-segment style digits

The clock runs continuously, updating the display whenever the seconds change, providing a live digital timepiece in a retro graphics environment.

Requirements

Development Tools

• Turbo Assembler (TASM) 5.0 - for assembling the source code
• Turbo Linker (TLINK) - for linking the object file into an executable

Runtime Environment

• DOSBox (recommended) - DOS emulator for modern systems
• FreeDOS or MS-DOS - on real hardware or virtual machine
• VGA-compatible graphics adapter - supports 640×480, 16-color mode

System Requirements

• 16-bit real-mode DOS environment
• VGA graphics capability (standard on most systems since the 1990s)
• Minimal memory requirements (uses small memory model)

Quick Setup with DOSBox

1. Prepare directories on your host system:

   C:\dos\
   ├── bin\          (place TASM.EXE and TLINK.EXE here)
   └── src\          (place digi_clock.asm here)
   

2. Launch DOSBox and mount the directories:

   mount c c:\dos
   c:

3. Build the program:

   cd \src
   ..\bin\tasm digi_clock.asm
   ..\bin\tlink digi_clock.obj

   This creates DIGI_CLOCK.EXE

4. Run the clock:

   digi_clock

Expected Behavior

When run, the program will:

• Clear the screen and switch to VGA graphics mode
• Display a framed area for the clock
• Show the current time as large digital numbers
• Update automatically as time progresses
• Continue running until manually terminated

Troubleshooting

Build Issues:

• Ensure TASM and TLINK are accessible in your PATH or use full paths
• Verify you're using TASM 5.0 or compatible version

Runtime Issues:

• Black screen: Ensure DOSBox supports VGA graphics mode
• Program doesn't start: Check that you're running in a DOS-compatible environment
• Graphics corruption: Verify VGA compatibility settings in DOSBox

DOSBox Configuration:

• Default DOSBox settings should work fine
• Ensure machine=svga_s3 or similar VGA-compatible setting
• No special CPU speed or memory configuration needed

Technical Features

• Real-time updates: Reads system time using DOS interrupt 21h, function 2Ch
• Custom graphics: Draws digits using pixel-level line drawing routines
• VGA graphics mode: Uses standard 640×480, 16-color VGA mode
• Efficient rendering: Only updates display when time actually changes

Educational Value

This program demonstrates:

• 16-bit x86 assembly language programming
• VGA graphics programming techniques
• DOS system calls for time access
• Real-mode memory management
• Interrupt-based hardware interface programming

Perfect for learning about retro computing, low-level graphics, and DOS programming fundamentals.

License

Educational/demonstration code. Consider adding appropriate license if redistributing.