virtual memory 4519

Jan VorbrYggen

VMS on Alpha could binary recompile VMS-VAX applications and-or emulate the VMS-VAX environment as needed to run old code. The OS proper had little knowledge that this was happening and just exported the VMS-Alpha native APIs. The emulation environment took care executing the programs and translating any system calls (and a bunch of other stuff). The fact that the ISA changed radically certainly encouraged Digital to take this approach.

Contrast this with the approach where the OS supports both a old (eg. 32 bit) and new (64 bit API) natively, or tries to do the opposite mapping (translating 64 bit API calls to 32 bit APIs). This is obviously feasible only if the ISA doesn't change.

There is much to commend the first approach, if the OS isn't too hard to port, and there aren't too many low level hooks into the OS that applications are using (which can make the emulation excessively difficult). OTOH, the latter approach allows a gradual implementation and better compatibility with existing applications. As examples, the NT version of Windows, Linux (to an extent), and VMS took the former approach, Win3.x-9x, Solaris, AIX, HP-UX and zOS took the latter in various iterations.