AIX on RT PC had a rather interesting architecture: the AIX kernel ran on top of a microkernel called VRM. VRM also supported two other operating systems, AOS (IBM’s port of 4.3BSD) and PICK. VRM could run multiple operating systems simultaneously, with a hotkey to switch which had access to the keyboard, mouse and screen. Similar idea to Windows NT’s environment subsystems, and IBM’s later Mach-based Workplace OS (cancelled before it ever went GA, but shipped in beta form to customers as OS/2 PowerPC Edition).
VRM was written, not in C, but in a PL/I dialect (not sure which, but probably PL.8). So AIX for RT PC is a rare example of a Unix whose kernel is (partially) written in a language other than C. The only other example of that I know of is IBM z/OS (which most people don’t think of as a Unix, but officially it is one, since it passes the certification test suite and IBM pays The Open Group the Unix trademark license fees.)
I say partially because the actual AIX kernel is in C, but it then calls to the VRM microkernel which isn’t in C. But a lot of basic functions which normally belong in a Unix kernel are in the VRM instead and the AIX kernel is just a thin wrapper over them.
When AIX moved to the RS/6000, the VRM microkernel was dropped, and AIX from then onwards was a classic monolithic kernel all in C.
Well, historically “AIX” has been a brand for (many but not all) IBM Unixes, with several unrelated code bases being labelled “AIX”. One of those, AIX/ESA (for IBM mainframes), was actually based on OSF/1, and hence used the Mach microkernel. But it was discontinued in the first half of the 1990s, when MVS gained Unix compatibility support. And all the other “weird” AIX variants are also now long dead, so in practice AIX for RS/6000’s descendant is the only AIX left.
AIX is a bit like DB2, in that both were brand names for multiple distinct code bases. The difference is all but one AIX code base died, whereas multiple DB2-branded code bases are actively supported today.
VRM was written, not in C, but in a PL/I dialect (not sure which, but probably PL.8). So AIX for RT PC is a rare example of a Unix whose kernel is (partially) written in a language other than C. The only other example of that I know of is IBM z/OS (which most people don’t think of as a Unix, but officially it is one, since it passes the certification test suite and IBM pays The Open Group the Unix trademark license fees.)
I say partially because the actual AIX kernel is in C, but it then calls to the VRM microkernel which isn’t in C. But a lot of basic functions which normally belong in a Unix kernel are in the VRM instead and the AIX kernel is just a thin wrapper over them.
When AIX moved to the RS/6000, the VRM microkernel was dropped, and AIX from then onwards was a classic monolithic kernel all in C.
Well, historically “AIX” has been a brand for (many but not all) IBM Unixes, with several unrelated code bases being labelled “AIX”. One of those, AIX/ESA (for IBM mainframes), was actually based on OSF/1, and hence used the Mach microkernel. But it was discontinued in the first half of the 1990s, when MVS gained Unix compatibility support. And all the other “weird” AIX variants are also now long dead, so in practice AIX for RS/6000’s descendant is the only AIX left.
AIX is a bit like DB2, in that both were brand names for multiple distinct code bases. The difference is all but one AIX code base died, whereas multiple DB2-branded code bases are actively supported today.