gnu@sun.uucp (John Gilmore) (05/09/85)
> Gee, maybe some day Motorola will catch up to Intel and integrate the > CPU and the MMU. > > expecting lots of flames on this one... > Phil Ngai Here's a little candle then, Phil... It is certainly a pain for people porting Unix systems, as well as those trying to come up with a 68000 Unix-binary standard, that there is no decent standard MMU for the 68000 series. Motorola is fixing that, though I believe it is lower priority than either the 68020 CPU or the 68881 floatbox, since most customers already have an MMU but don't have a 32-bit processor or a well-integrated float box. Just think if they'd integrated the 68451 loser onto the 68010 -- we'd all be stuck with it. Instead, people were free to come up with their own idea of how to build an MMU. We at Sun have a no-wait-state TTL&RAM MMU that we're very happy with, though it costs us in board area. Motorola is building one (68851 PMMU) that snarfs the page table entries out of RAM into a TLB like a VAX, but it adds a wait state unless you walk a verry narrow path (so I hear), and TLB misses are expensive. Signetics is building a great MMU (the 68920 MAC) that runs no wait states, fetches page table entries from RAM, deals with multiprocessors and local/global memories, and also provides complete control of a cache built out of external static RAMs. Its problem is that it won't run past 12-16MHz and will not be out for a while. Various other companies (Apple Lisa, Fortune, ...) have designed their own idea of a good MMU. The point is that the customer gets lots of options and can trade off time-to-market vs cost vs features vs risk vs software work. Out of curiosity, is anyone building National xx0xx based systems using any MMU except the National MMU? Or was it good enough that it stuck as a standard?
doug@terak.UUCP (Doug Pardee) (05/11/85)
> Out of curiosity, is anyone building National xx0xx based systems > using any MMU except the National MMU? Or was it good enough that it > stuck as a standard? The NatSemi MMU interface is tailored so specifically to the 32082 MMU chip that if you tried to build a custom MMU, you'd basically end up with a functional duplicate of the '082. The '082 is tolerable, so... -- Doug Pardee -- Terak Corp. -- !{ihnp4,seismo,decvax}!noao!terak!doug ^^^^^--- soon to be CalComp