Largest small system emulator

Adrian Cable

Judges' comments:

To build:

make cable3

To run:

./cable3 bios-image-file floppy-image-file [harddisk-image-file]



Selected Judges Remarks:

This entry weighs in at a magical 4043 bytes (8086 nibbles, 28,301 bits). It manages to implement most of the hardware in a 1980’s era IBM-PC using a few hundred fewer bits than the total number of transistors used to implement the original 8086 CPU.

If you are using OS X, the included sc-ioccc.terminal configuration file will correctly display console applications that use ANSI graphics.

Update: This entry now has its own web page

Author’s comments:

A tiny but highly functional PC emulator/virtual machine

The author hereby presents, for the delectation (?) of the judges, a portable PC emulator/VM written specifically for the IOCCC which runs DOS, Windows 3.0, Excel, MS Flight Simulator, AutoCAD, Lotus 1-2-3 …

In just 4043 bytes of C source, you get a complete mid-late 1980s-era IBM-compatible PC, consisting of:

The emulator uses the SDL graphics library for portability, and compiles for Windows, Mac OS X, Linux and probably most other 32-bit/64-bit systems too.

If you like living on the edge you can try building the emulator on a big endian machine, and you will get an emulation of a big endian 8086, a rather bizarre and somewhat useless beast. For everyone else, please run the emulator on a little endian machine.



Why is this entry obfuscated/interesting?

Compiling on different platforms

This entry has been tested on Windows (compiled with MS Visual Studio 2010 and 2013), Mac OS X (clang and gcc), and Linux (clang and gcc). The Makefile supplied is good for Mac OS X, Linux and probably other UNIXes. I have received reports that the emulator works on Raspberry Pi/Android/ARM (you will need to compile with -fsigned-char) and iOS. You will need to adjust the Makefile if your system lacks sdl-config to correctly point to the SDL libraries and header files.

On UNIX-based systems we can get raw keystrokes using stty. However Windows has no stty. Therefore the Makefile includes a -D entry to define a “keyboard driver” KB which as it stands is suitable for UNIXes, but maybe not non-UNIX platforms. For example, for Windows/MS Visual Studio, instead of the Makefile definition of KB, use something slightly different - add the following entry to the Preprocessor Definitions list in the Project Properties page:


POSIX portability note

The code as supplied uses implicit function declarations for POSIX file I/O and in doing so assumes that file offsets are the same bit width as your architecture. For some systems (e.g. 32-bit Mac OS X, which uses 64-bit file offsets) this is not the case, and to run successfully on these systems, you will need to explicitly declare these functions by adding the appropriate include to the top of the source:

#include <unistd.h>


./cable3 bios-image-file floppy-image-file [harddisk-image-file]

PLEASE NOTE that under UNIXes the keyboard must be in raw mode for the emulator to work properly. Therefore the emulator is best run from a shell script that looks something like:

stty cbreak raw -echo min 0
./cable3 bios floppy.img harddisk.img
stty cooked echo

To run the emulator - floppy mode only

The simplest use of the emulator is with a single floppy boot disk image, like the fd.img provided, which is a FreeDOS boot disk.

Before running the emulator on a Unix-type system, stty needs to be used to put the keyboard into raw mode (and afterwards it needs to be put back to cooked). So, run the emulator using something like this script (provided as “runme”):

stty cbreak raw -echo min 0
./cable3 bios fd.img
stty cooked echo

To run the emulator - floppy + HD mode

Easiest to start with is to try a ready-made 40MB hard disk image containing a whole bunch of software:

For the more adventurous, you can start off with (for example) a blank 40MB image file called hd.img made using e.g. mkfile. Then use:

stty cbreak raw -echo min 0
./cable3 bios fd.img hd.img
stty cooked echo

Preparing the hard disk for use in the emulator is done just like a real PC. Boot the emulator, and use FDISK to partition the hard disk. When it’s done FDISK will reboot the emulator. Then you can use FORMAT C: and you are done. The resulting disk image is in the right format to be mounted on a real Windows PC using e.g. OSFMount, on a Mac using hdiutil, or on Linux using mount, providing an easy way to copy files and programs to and from the disk image. Or, you can install programs from regular floppy disk images (see “Floppy disk support” below).

Keyboard emulation

The emulator simulates an XT-style keyboard controlled by an Intel 8042 chip on I/O port 0x60, generating IRQ1 and then interrupt 9 on each keypress. This is harder than it sounds because a real 8042 returns scan codes rather than the ASCII characters which the C standard I/O functions return. Rather than make the emulator less portable and use ioctl or platform-dependent equivalents to obtain real scan codes from the keyboard, the emulator BIOS does the reverse of a real PC BIOS and converts ASCII characters to scancodes, simulating press/release of the modifier keys (e.g. shift) as necessary to work like a “real” keyboard. The OS (DOS/Windows) then converts them back to ASCII characters and normally this process works seamlessly (although don’t be surprised if there are issues, for example, with non-QWERTY e.g. international keyboards).

Most of the time you can just type normally, but there are special sequences to get Alt+xxx and Fxxx.

To send an Alt+XXX key combination, press Ctrl+A then the key, so for example to type Alt+F, press Ctrl+A then F.

To send an Fxx key, press Ctrl+F then a number key. For example, to get the F4 key, press Ctrl+F then 4. To get F10, press Ctrl+F then 0.

To send a Page Down key, press Ctrl+F then O. To send a Page Up key, press Ctrl+F then E. Other key combinations are left for the discovery of the user.

Text mode support

The emulator supports both text output via the standard BIOS interrupt 0x10 interface, and also direct video memory access (one page, 4KB video RAM at segment B800) in 80x25 CGA 16-color text mode.

BIOS text output calls are converted to simple writes to stdout. Direct video memory accesses for the 80x25 CGA color text mode are converted to ANSI terminal escape sequences. If you are using a terminal which does not support ANSI (e.g. you are compiling the emulator with MS VC++ and running in a Windows console window) then PC applications that directly write to video memory in text mode may be unusable.

Most CGA I/O ports are not supported except for the CGA refresh register at 0x3DA, which some applications use for timing or synchronisation.

The regular PC character code page (437) includes various extended ASCII characters for things like line drawing. You might want to set the font in your terminal program to something that includes these (e.g. on Mac OS X there is a freeware font called Perfect DOS VGA 437 which does the trick).

Occasionally a DOS application on exit will leave the video hardware in an odd state which confuses the emulator, resulting in subsequent text output being invisible. If this happens, just use the DOS CLS command to clear the screen and all will be well again.

Graphics mode support

Hercules 720x348 monochrome graphics mode emulation is implemented using SDL. Most Hercules features are supported via the normal I/O interface on ports 0x3B8 and 0x3BA including video memory bank switching (segments B000/B800), which some games use for double-buffered graphics. CGA graphics modes are not supported.

When an application enters graphics mode, the emulator will open an SDL window (which will be closed when the application goes back to text mode). Including code to redirect keystrokes from the SDL window to the main terminal window would have busted the IOCCC size limits, so you need to keep the main emulator terminal window in focus at all times even when you are doing graphics (sounds a little odd but you will get used to it).

On UNIXes, SDL will automatically output graphics via X11 if the DISPLAY environment variable is set up.

Dual graphics card support

Some applications (e.g. AutoCAD) support a PC configuration with a CGA card and a Hercules card, for simultaneous text and graphics output on different displays. The emulator simulates this configuration, too, using separate windows for the (terminal) text and (SDL) graphics displays.


Like a real PC, the emulator needs a BIOS to do anything useful. Here we use a custom BIOS, written from scratch specifically for the emulator. Source code for the BIOS (written in 8086 assembly language) which compiles with the freely-available NASM x86 assembler is available from the author on request.

The BIOS implements the standard interrupt interfaces for video, disk, timer, clock and so on, much as a “real” PC BIOS does, and also a small timer-controlled video driver to convert video memory formatting into ANSI escape sequences when the emulator is in text mode.

CPU and memory emulation

Memory map is largely as per a real PC, with interrupt vector table at 0:0, BIOS data area including keyboard buffer at 40:0, CGA text video memory at B800:0, Hercules dual-bank graphics memory at B000/B800:0, and BIOS at F000:100. Unlike a real PC, in the emulator the CPU registers are memory-mapped (at F000:0), which enables considerable optimisation of the emulator’s instruction execution unit by permitting the unification of memory and register operations, while remaining invisible to the running software.

CPU supports the full 8086/186 instruction set. Due to the complexities of the 8086’s arbitrary-length instruction decoding and flags, 8086 instructions are first converted to a simpler intermediate format before being executed. This conversion, along with instruction lengths and how each instruction modifies the flags, is assisted by some lookup tables which form part of the BIOS binary.

The CPU also implements some “special” two-byte opcodes to help the emulator talk with the outside world. These are:

0F 00 - output character in register AL to terminal
0F 01 - write real-time clock data (as returned by localtime) to memory location ES:BX
0F 02 - read AX bytes from disk at offset 512*(16*SI+BP) into memory location ES:BX. Disk is specified in DL (0 = hard disk, 1 = floppy disk)
0F 03 - write AX bytes at memory location ES:BX to disk at offset 512*(16*SI+BP). Disk is specified in DL as per 0F 02

Emulator exit is triggered if CS:IP == 0:0 (which would be nonsensical in real software since this is where the interrupt vector table lives). The supplied Dos6.22.img disk includes a small program QUITEMU.COM which contains a single JMP 0:0 instruction, to allow the user to easily quit the emulator without shutting down the terminal.

Floppy disk support

Emulates a 3.5" high-density floppy drive. Can read, write and format 1.44MB disks (18 sectors per track, 2 heads) and 720KB disks (9 sectors per track, 2 heads).

If you want to install your own software from floppy images (downloaded from e.g. Vetusware), the easiest way to “change disks” is to copy each disk image in turn over the floppy image file you specify on the command line. Don’t forget to put your original boot disk back at the end!

Hard disk support

Supports up to 1023 cylinders, 63 sectors per track, 63 heads for disks up to 528MB.

Disk image format used is a subset of the standard “raw” format used by most disk image mount tools. In general, disk images prepared by the emulator will work with disk image tools and other emulators, but not the other way around.

The emulator uses a particularly dumb algorithm to derive a simulated cylinder/sector/head geometry from the disk image file’s size. This algorithm often results in not all the space in the image file being available for disk partitions. For example, creating a 40,000,000 byte image file results in DOS FDISK seeing only 31.9MB as the volume size.

Note that unlike a real PC, the emulator cannot boot from a hard disk (image). Therefore, you will always need to use a bootable floppy image, even if after boot everything runs from the HD.


No mouse is emulated.

Real-time clock

Reading the RTC (both time and date) is emulated via the standard BIOS clock interface, pulling the time/date from the host computer. Setting the time or date is not supported.


A countdown timer on I/O port 0x40 is simulated in a broken way which is good enough for most software. On a real PC this has a default period of 55ms and is programmable. No programmability is supported in the emulator and the period may be about right or completely wrong depending on the actual speed of your computer.

On a real PC, IRQ0 and interrupt 8 are fired every 55ms. The emulator tries to do the same but again, the delay period is uncalibrated so you get what you get.

PC speaker

Beeps only, through the console.

Software supported

The emulator will run practically any software a real PC (of the spec listed at the top of this file) can. The author has tested a number of OSes/GUIs (MS-DOS 6.22, FreeDOS 0.82pl3, Windows 3.0, DESQview 2.8), professional software (Lotus 1-2-3 2.4 and AsEasyAs 5.7 for DOS, Excel 2.1 for Windows, AutoCAD 2.5, WordStar 4), programming languages (QBASIC, GWBASIC, Turbo C++), games (Carrier Command, Police Quest, and a bunch of freeware Windows games), and diagnostic/benchmark software (Manifest, Microsoft MSD, InfoSpot, CheckIt) and all of them run well.

Screenshots of some of these applications running (on Mac OS X) are provided for the impatient.

Compiler warnings

A lot of compiler warnings are produced by clang. Missing type specifiers, control reaching the end of functions without returning values, incompatible pointer type assignments, and some precedence warnings, all necessary to keep the source size down. Other compilers are likely to produce similar warnings.

Creative Commons License

© Copyright 1984-2015, Leo Broukhis, Simon Cooper, Landon Curt Noll - All rights reserved
This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.