DOS (MS-DOS / DR-DOS)

Boots DOSBox (em-dosbox / js-dos) to a DOS C:\> command prompt. Click the screen, then type at the prompt (try DIR, then MEM). Add ?debug, or click Debug, to inspect the LIVE x86 registers, read physical memory, and single-step the CPU. Pausing installs a dedicated frame loop that yields to the browser, so the debugger stays responsive; Step runs exactly one instruction (DOSBox's own CPU_Cycles=1 primitive).

Boots an IBM PC/XT (Intel 8088) running GlaBIOS, which loads DOS from drive A: to the A:\> prompt. Click the screen, then type at the DOS prompt (try DIR, then VER). Add ?debug, or click Debug, to inspect the live 8088 registers, read physical memory, disassemble, and single-step. Step here is a TRUE one-instruction step (MartyPC has a native single-instruction debugger step), unlike the burst step of a JIT core.

Boots FreeDOS(98) to the A:\> command prompt on an emulated NEC PC-98. Click the screen, then type at the DOS prompt (try DIR, then VER). Add ?debug, or click Debug, to inspect the live Intel i386 registers, read physical memory, and step the CPU. Step runs one NP2kai frame (see the notes: a WASM core with a private state struct is stepped a frame at a time, not one instruction).

A full x86 PC in WebAssembly, booting a menu of real operating systems from self-hosted disk images: FreeDOS (A:\> prompt), Windows 3.0 / 3.1, Windows 95, Windows 98 and ReactOS to their desktops, plus a small Buildroot Linux. Windows 98 and ReactOS resume from a pre-booted state snapshot, so they come up already on the desktop; Windows 3.x and 95 cold-boot from disk (give 95 a minute). Click the screen, then use the keyboard and mouse. Add ?debug, or click Debug, to inspect the live x86 registers, read physical memory, and step the CPU. Step runs one v86 time-slice (see the notes: a JIT-in-WASM core has no clean one-instruction step).