The emulator is the product of playing around with "bare metal" coding on the Raspberry Pi.
It's written in C and assembly and boots in less than one second, which is pretty close to the real deal. No USB keyboard support yet, to interact you need to connect a serial terminal to the UART pins on the GPIO header.
Current screen modes supported are text, HGR and HGR2 in monochrome and paging support. It loads a DSK image embedded in the kernel, next up is SD card support to access floppy images on the fly.