Note: This article is part of my Testing stuff with QEMU series.
MenuetOS is an operating system with a monolithic preemptive, real-time kernel, including video drivers, all written in FASM assembly language, for 64-bit and 32-bit x86 architecture computers, by Ville Mikael Turjanmaa.
MenuetOS development has focused on fast, simple, efficient implementation. It has a graphical desktop, games, and networking abilities (TCP/IP stack), yet still fits on one 1.44MB floppy disk. It also facilitates easy, full-featured assembly language programming. This stands in marked contrast to the (as of 2007) widespread view that assembly languages are useful mainly for old and embedded systems.
Testing (the GPL'd) MenuetOS in QEMU is easy:
qemu -fda M32-084.IMG -m 384
There's also Menuet 64, written in 64-bit assembly, but that's not open source'd for some strange reason I don't understand. But you can try that one, too (the binary images, that is), using QEMU:
qemu-system-x86_64 -fda M64-059.IMG