I dare to say that MVS and its successors (OS/390, z/OS) are also, in some ways, microkernels. Sadly, I am not an OS expert to make a proper comparison. But I

I dare to say that MVS and its successors (OS/390, z/OS) are also, in some ways, microkernels. Sadly, I am not an OS expert to make a proper comparison. But I think one of the main mechanisms is basically non-kernel services install their own system calls (SVCs) and this SVC will schedule a light-weight thread (SRB) in the requestor's address space. There was an older book that described how to create a simple OS along similar lines to MVS, but I don't remember the name.