ntvdm - An NT Virtual DOS Machine.

Not the real one, but this one can be compiled and executed on Windows (x64 and ARM64), Linux (ARM32, ARM64, RISC-V, x64 and x32), and MacOS (tested with an M1).

I wrote it so I could test my BASIC compiler and it emulates an Intel 8086 processor and MS-DOS 3.00 APIs well enough to be able to run some command line and text mode applications.

There are many better DOS emulators available, including DOSBox, but what makes this one different is that it provides a way to run a DOS application directly from the command line or shell/batch script in the current console window.

The emulator will work with applications that hook interrupts via the DOS mechanism or by directly writing to memory. This includes support for int9 and int0x1c in applications like Microsoft QuickBasic and Brief Editor.
However it does not provide support for graphics, sound, mouse, or anything else that is not needed for simple text-mode apps.

It also includes a disassembler that is used when tracing program execution which is useful when debugging why apps don't work properly.

The performance of the 8086 processor emulation is about the same as other emulators written in C/C++, and is about 30% faster than DOSBox for the tic-tac-toe benchmark written in 8086 assembler.

I can't vouch for 100% 8086 emulation because I can't find any applications that perform such validation, unlike the 6502, 8080, Z80, and other earlier CPUs that have validation suites. But I have done a fair bit of testing of different applications and compared instruction and register traces with other emulators.

The emulator can simulate running at a given clock rate, however the number of cycles needed for each instruction varies widely between various sources and the code doesn't check for misaligned memory access, not get details of mult/div operations correct, so the results are only an aproximatiion.

Using mips version 1.20 (a benchmark application from 1986 written by Chips and Technologies) then if the clock is set to /s:4500000 (4.5 Mhz) the 8086 emulator runs at about the same speed an actual 8088 running at 4.77Mhz.

Given the wide variability online regarding the performance differences between the 8088 and 8086 (5%-50%) this seems quite close.

Applications Tested

• Microsoft Macro Assembler Version 1.10
• Microsoft Link Version 2.00
• DEBUG 2.0 (Breakpoints and single-stepping tested).
• GWBASIC
• Microsoft QuickBasic 1.1
• Microsoft QuickBasic 7.1 (Including Microsoft Basic compiler 7.10)
• Borland Brief 3.1. (Requires -k to be passed on the Brief command line)
• WordStar Professional Release 4 for DOS
• Microsoft 8086 Object Linker Version 3.01 (C) Copyright Microsoft Corp 1983, 1984, 1985
• Microsoft C Compiler Version 1.04 & Microsoft Object Linker Version 1.10 (1981)
• Microsoft C Compiler Version 2.03 & Microsoft Object Linker Version 2.40 (1983)
• Microsoft C Compiler Version 3.00 (1984, 1985)
• Microsoft Segmented-Executable Linker 5.10
• Aztec (CG65 Version 3.2c ) cross-compiler (1989)
• Turbo Pascal Version 1.00A, 3.02A, 5.5, and 7.0. (Debugger)
• Turbo Basic Version 1.1
• Turbo C Version1.0 and 2.0 (Including debug breakpoints and single-stepping)
• Turbo Assembler Version 3.2
• Turbo Link Version 2.0
• Microsoft Quick C Version 1.0 & 1.01. (Compiling, editing, breakpoints, single-stepping, etc). Requires -h flag
• Microsoft Quick C Version 2.01 and v2.51 (Incremental linking must be is disabled). Requires -h flag
  • ilink.exe reads memory control blocks, which don't exist, so use link.exe instead.
• Microsoft Quick Pascal Version 1.0
• Microsoft Word Version 6.0 for DOS. (Set view / preferences / cursor control / speed to 0 to avoid key repeats).
• Microsoft Works Version 3.0 for DOS.
• Lotus 1-2-3 Release 1A
• PC-LISP V3.00
• LOGITECH MODULA-2 Compiler, DOS, Rel. 3.40, Mar 90
• muLISP v5.10 interpreter (released by Microsoft).
• IBM Personal Computer Pascal Compiler Version 2.00 (generated apps require 8086 hma address wrapping)
• Microsoft COBOL Compiler Version 5.0 (compiler and generated apps). Linker requires 286 but QBX's linker works fine.
• Digital Research PL/I-86 Compiler Version 1.0, link86, and generated apps.
• Microsoft FORTRAN77, the linker, and generated apps.
• Microsoft (R) C Optimizing Compiler Version 5.10 with Microsoft (R) Overlay Linker Version 3.65
• Microsoft Pascal v1.10 (this requires using ntvdm's -h flag due to bugs in pas2.exe and generated apps)
• Digital Research CB-86 v2.0 BASIC compiler. Note that this compiler requires source files be 0x1a / ^z terminated.
• Mark Williams Company MWC86 Let's C Version 3.1.1 and Version 4.0.124.0.12

Software

The repository includes a several folders containing copies of old MS-DOS programs that were used to test the emulator.

• gwbasic contains GWBasic 3.22

• msc_v3 contains Microsoft C Compiler Version 3.00 and command.c, a greatly simplified replacement for command.com that can be built with that compiler. It's handy for when apps like WordStar and QBX shell out to command.com.

• turbodos contains Turbo Pascal 1.00A

• turbo3dos contains Turbo Pascal 3.

• qbx contains Quick Basic 7.1

• wordstar contains WordStar Professional Release 4

• tasm contains Turbo Assembler and several assembly language routines for various interrupts. mint.bat generates the machine code in C arrays that are copied into ntvdm.cxx and loaded into DOS RAM at runtime.

Tested Platforms

The emulator has been compiled and tested on the following platforms:

• Windows 11, Visual Studio 2022, x64

• Windows 11, Visual Studio 2022, ARM64

• MacOS on M1 and M3 MacBook Air, ARM64.

• Debian 12 (Bookworm), g++ 12.2.0, x64

• Debian 12 (Bookworm), g++ 12.2.0, x32

• Debian 12 (Bookworm), g++ 12.2.0, RISC-V

• Debian 10 (Buster), g++ 8.3.0, x64

• Open SUSE Leap 15.4, g++ 7.5.0, x64

• Ubutnu 20.04, gcc 9.4.0, x64 (WSL2)

• Raspberry PI OS, g++, ARM64 and ARM32

Windows

Build with m.bat or mr.bat for debug and release versions on Windows. Or build with g++ instead by using mg.bat or mgr.bat.

G++ versions are 20% faster than Microsoft C++ versions.

Usage

To display the command line options:

C:\> ntvdm -?
NT Virtual DOS Machine: emulates an 8086 MS-DOS 3.00 runtime environment enough to run COM/EXE apps
usage: ntvdm [arguments] <DOS executable> [arg1] [arg2]
  notes:
     -b               load/run program as the boot sector at 07c0:0000
     -c               tty mode. don't automatically make text area 80x25.
     -C               make text area 80x25 (not tty mode). also -C:43 -C:50
     -d               don't clear the display on exit
     -e:env,...       define environment variables.
     -h               load high above 64k and below 0xa0000.
     -i               trace instructions to ntvdm.log.
     -m               after the app ends, print video memory
     -p               show performance stats on exit.
     -r:root          root folder that maps to C:\
     -t               enable debug tracing to ntvdm.log
     -s:X             set processor speed in Hz.
                        for 4.77 MHz 8086 use -s:4770000.
                        for 4.77 MHz 8088 use -s:4500000.
     -v               output version information and exit.
     -?               output this help and exit.

To execute app.com with debuging output in ntvdm.log:

C:\>ntvdm -c -t app.com foo bar

To execute Turbo Pascal with am emulated clock speed of 4.77MHz:

C:\>ntvdm -s:4770000 turbo.com

To assemble and link a 'small' program:

C:\>ntvdm s:\github\MS-DOS\v2.0\bin\masm small,,,small
C:\>ntvdm s:\github\MS-DOS\v2.0\bin\link small,,,small

To execute a program and show the performance information on exit:

C:\>ntvdm -c -p ttt8086.com
    3.3 seconds
    moves: 6493
    iterations: 1000

    elapsed milliseconds:            3,331
    8086 cycles:             4,761,338,635
    clock rate:                  unbounded
    approx ms at 4.77Mhz:          998,184  == 0 days, 0 hours, 16 minutes, 38 seconds, 184 milliseconds
    unique first opcodes:               63
    app exit code:                       0

Linux

• The code assumes VT-100 support is available in your terminal window and uses ASCII characters for instead of the MS-DOS code page 437 character set which can be ugly. Anyone know how to enable codepage 437 on Linux?

• Linux is case-sensitive and DOS isn't. To minimize the potential issues this causes you can force DOS to use UPPERCASE names by using -u. If you still have problems use a seperate root directory for any DOS applications with just uppercase directory and file names. The '-r' argument can be used to specify a folder that maps to C:. e.g. -r:. or -r:.. or -r:/this/that

• Apps that use the Alt key generally work, but you will need to configure the terminal application to disable menu access using the Alt key.

• Most DOS applications require lines to end with CR/LF. Ensure your input files have this on Linux systems or appliations will fail in odd ways.

Usually View - Keyboard Shortcuts

• Keyboard handling adds some small (10ms) delays, which means it isn't possible to paste text into an application.

Prerequisites

The following packages are required to build the emulator.

• Debian : g++, libstdc++-8, libstdc++6

• SUSE : gcc-c++

• Ubuntu : g++, libstdc++-8, libstdc++6

Compiling

Release

g++ -ggdb -Ofast -fno-builtin -D NDEBUG -I . ntvdm.cxx i8086.cxx -o ntvdm -fopenmp

Debug

g++ -ggdb -Og -fno-builtin -D DEBUG -I . ntvdm.cxx i8086.cxx -o ntvdm -fopenmp

Usage

To display the command line options:

$ ntvdm -?
Usage: ntvdm [OPTION]... PROGRAM [ARGUMENT]...
Emulates an 8086 and MS-DOS 3.00 runtime environment.

  -b               load/run program as the boot sector at 07c0:0000
  -c               tty mode. don't automatically make text area 80x25.
  -C               make text area 80x25 (not tty mode). also -C:43 -C:50
  -d               don't clear the display on exit
  -u               force DOS paths to be uppercase
  -l               force DOS paths to be lowercase
  -e:env,...       define environment variables.
  -h               load high above 64k and below 0xa0000.
  -i               trace instructions to ntvdm.log.
  -t               enable debug tracing to ntvdm.log
  -p               show performance stats on exit.
  -r:X             X is a folder that is mapped to C:\
  -s:X             set processor speed in Hz.
                     for 4.77 MHz 8086 use -s:4770000.
                     for 4.77 MHz 8088 use -s:4500000.
  -v               output version information and exit.
  -?               output this help and exit.

To compile and link the Microsoft C 3.0 demo application:

$ cd msc_v3
$ ../ntvdm -u -e:include=.\\inc msc.exe demo.c,,\;
$ ../ntvdm -u -e:lib=.\\lib link.exe demo,,\;

Note the use of the -e option to define the environment vairables used by the compiler and linker and the use of a \ to escape any \ characters in any DOS pathnames.

Running the applicatiion with the same environment vairables defined will display the application pathname, command line, arguments and environment vairables used above.

$ ../ntvdm -u -e:include=.\\inc,lib=.\\lib demo.exe one two three
C:\msc_v3\DEMO.EXE
one
two
three
COMSPEC=COMMAND.COM
INCLUDE=.\INC
LIB=.\LIB
$

To run QuickBASIC.

$ cd qbx
$ ../ntvdm -u -r:. -C:50 qbx

QuickBASIC, like other apps on Linux, works best with -r:. and -u flags.

43 and 50 line modes

By default NTVDM runs in 25 line mode. Some DOS apps support 43 and 50 line mode and others don't. Some apps like Quick Pascal have UI to change the line count, and that just works. Other apps like Brief support more than 25 lines, but require you use NTVDM's -C argument and Brief's line count argument to make that work. For example:

    ntvdm -C:50 -r:.. b -L50 foo.txt

Many apps assume 25 lines and won't use more than that even with the -C:50 flag.