Wikipedia
SymbOS
SYmbiosis Multitasking Based Operating System (SymbOS) is a multitasking operating system for Zilog Z80-based 8-bit computer systems.
Contrary to early 8-bit operating systems it is based on a microkernel, which provides preemptive and priority-oriented multitasking and manages random-access memory (RAM) with a size of up to 1024 KB. SymbOS contains a Microsoft Windows like graphical user interface (GUI), supports hard disks with a capacity of up to 128 GB and can already be booted on an unexpanded Amstrad CPC-6128, a 128K- MSX2 and an Amstrad PCW.
At present it is available for the Amstrad CPC series of computers, as well as for all MSX models starting from the MSX2 standard and for most Amstrad PCW models.