MS DOS Cursor Size

Set the Microsoft DOS cursor to a block cursor. █

📑 Contents

• 🔎 Overview
• ✨ Features
• 🚀 Quick Start
• ⌨️ Usage
• ⚙️ How It Works
• 📂 Project Structure
• 💻 Building From Source
• 🙋‍♂️ Acknowledgements
• 📄 License

🔎 Overview

cursor is an MS-DOS command-line utility that sets the text-mode cursor shape. cursor -block gives a full-height block cursor; cursor -line restores the default underline (line) cursor. It reprograms the cursor through the BIOS and exits immediately.

The shape is sized adaptively: the program measures the character-cell height at run time and fits the cursor to it, so a -block is always a genuinely full-cell block and a -line always sits on the bottom row. This enables it to set the cursor size correctly on CGA, EGA/MDA, and VGA alike, with no hard-coded scan-line values.

Written in C, compiled with Borland Turbo C++, targeting DOS 3.3 / 8086 real mode.

C:\> cursor -block      ...Sets a full-height block cursor
C:\> cursor -line       ...Restores the default underline (line) cursor

✨ Features

Feature	Description
Two shapes	-block for a full-cell block, -line for the conventional underline.
Adaptive sizing	Fits the cursor to the measured character-cell height, so the block fills the whole cell and the underline sits on the very bottom row. Works correctly on CGA (8), EGA/MDA (14), and VGA/DOSBox (16) with no per-adapter table.
Forgiving CLI	Switches accept a - or / prefix, are case-insensitive, and come in long and short forms (-block/-b, -line/-l).
Silent on success	Prints nothing when a shape is applied; all output is reserved for the usage screen.
Batch-friendly	Meaningful ERRORLEVEL (0 = applied, 1 = usage/error) so it composes cleanly inside batch files.
No dependencies	A standalone cursor.exe — BIOS/DOS only, no runtime, no TSR footprint.

🚀 Quick Start

Download the executable from the latest release:

File	Download	Use case
cursor.exe	download	Run on an MS-DOS machine, or under a DOS emulator like DOSBox.

Copy cursor.exe somewhere on your DOS PATH (or into the current directory), then:

cursor -block
cursor -line

⌨ Usage

cursor -block
cursor -line

Switch	Also accepted	Action
-block	-b, /block, /b	Set a full-height block cursor.
-line	-l, /line, /l	Restore the default underline (line) cursor.
-help	-?, -h, /?, /h, /help	Show the usage screen.

• Switches may use a - or / prefix and are case-insensitive (-BLOCK, /Block, -b are all valid).
• On a recognised shape switch the program is silent and exits with ERRORLEVEL 0.
• Running cursor with no argument, with the wrong number of arguments, with an unrecognised switch, or with a help switch prints the usage screen and exits with ERRORLEVEL 1.

The usage screen:

┌──────────────────────┐
│ Cursor (version 1.5) │
└──────────────────────┘

Usage:
  cursor -block    Set a full block cursor.
  cursor -line     Set the default underline (line) cursor.

Switches may use - or / and are case-insensitive (-b, -l also accepted).

⚙ How It Works

The PC text-mode cursor is a horizontal band of scan lines inside the character cell, programmed into the CRT Controller's Cursor Start/End registers. cursor never touches the CRTC directly — it goes through the BIOS (INT 10h), the documented, portable interface.

Rather than hard-code scan-line values, the program first measures the character-cell height with INT 10h (AH=11h, AL=30h), then derives the cell's bottom scan line B and computes:

• block — scan line 0 to B (the whole cell), and
• line — scan line B − 1 to B (the bottom row or two).

Because the shape is computed from the actual cell height, it is correct on every common adapter without a per-adapter lookup table:

Adapter	Cell height	Block (scan lines)	Line (scan lines)
CGA	8	0–7	6–7
EGA/MDA	14	0–13	12–13
VGA / DOSBox	16	0–15	14–15

On the VGA/DOSBox target this yields a block of CX=000Fh and a line of CX=0E0Fh. The program never emits a shape whose start scan line exceeds its end, and it falls back to a fixed 8-line cell on pre-EGA BIOSes that do not implement the cell-height query.

📂 Project Structure

cursor
├─ src
│  ├─ cursor.c    C source -- CLI parsing + BIOS cursor control (single translation unit).
│  ├─ build.bat   Compile + link with bcc -> cursor.exe (output logged to build.log).
│  └─ clean.bat   Remove cursor.obj, cursor.exe, build.log.
├─ assets
│  └─ images      Screenshots and the demo GIF used by this README.
├─ README.md      This file.
└─ LICENSE        MIT licence.

💻 Building From Source

The project builds under DOS on a real machine, or under an emulator, like DOSBox, with the Borland Turbo C++ command-line compiler.

Tool	Needs	Role
Borland Turbo C++ (bcc)	A DOS environment (real DOS or DOSBox) on the PATH	Compiles and links cursor.c into cursor.exe.
DOSBox	—	DOS emulation, if needed.

1. Build

From the src directory:

build

build.bat invokes the compiler in a single step and logs everything to build.log:

bcc -ms -ecursor.exe cursor.c

-ms selects the small memory model; the -2 (80286) flag is deliberately omitted so the default 8086 code generation is used.

2. Run

cursor -block
cursor -line

Remove the build artifacts (cursor.obj, cursor.exe, build.log) with clean, also run from src.

🙋‍♂ Acknowledgements

Author	Tool / Source
Borland	The Turbo C++ command-line compiler bcc).
The DOSBox Team	DOSBox for DOS emulation.

📄 License

Released under the MIT License — Copyright © 1991 Rohin Gosling.