NTVDM

NTVDM stands for NT Virtual DOS Machine. It is a subsystem in Windows NT-based operating systems that provides a virtualized environment for running 16-bit DOS programs and the 16-bit Windows 3.x subsystem within a protected Windows session. NTVDM allows legacy software to operate on modern Windows by exposing a compatible DOS/16-bit interface while isolating it from the 32-bit host.

In practice, NTVDM runs as a separate process (on 32-bit Windows it appears as ntvdm.exe) and emulates the necessary 16-bit environment, including memory, interrupts, basic I/O, and video output. It routes calls from 16-bit code to the Windows host as needed and supplies a contained DOS or Windows 3.x runtime, enabling many older programs to launch and run without a full emulator.

Platform availability has changed over time. NTVDM is present in 32-bit Windows editions to support 16-bit DOS and Windows 3.x software. On 64-bit Windows editions, NTVDM is not included, so native execution of 16-bit DOS or Windows 3.x programs is not supported. Users needing to run such software on 64-bit systems typically rely on virtualization with a 32-bit guest or third-party emulators such as DOSBox.

Limitations exist. Some programs depend on direct hardware access, specific BIOS features, or timing that NTVDM cannot faithfully reproduce, which can prevent certain applications from running correctly. NTVDM is a legacy compatibility component, and modern Windows environments favor virtualization or alternative emulation approaches for legacy software.