It was called, unimaginatively, 'CTOS' for "Convergent Technologies OS". When the 286 came along the original syscall trick quit working, so we invented another one. We simply used an invalid segment and trapped; the offset became the function number. Otherwise it worked identically.
CTOS was message-passing. All system calls were for timers, memory or message manipulation. The messages were a tiny header including a 'request code', some scalar arguments and a set of buffers. OpenFile, Read, everything. On the top of the OS were Services that received messages. There was an OS message dispatch mechanism where Services registered for particular message codes. So the client Sending a message didn't know where their message would end up. The original feature that made money was the 'diskless workstation'. The file system codes were dispatched to Client Agent that simply packaged them up and sent them to a designated Server. Brought down the client workstation cost dramatically.