x8086 (02/26/83)
To anyone looking for more information on the 16032. This week's issue
of Electronics magazine has a good article on the 16032 MMU. A short summary
follows. The MMU takes the 24bit virtual 16032 address and maps it onto
512byte pages. The interesting part is how it is done. National decided to
use a 32 page, page table cache kept in the MMU itself. According to the
specs 97% of all address accesses will hit the cache which has a 100ns access
time. The other 3% of the accesses require the MMU to go to the real memory
page table which itself is paged. Getting the idea yet? Memory accesses
for not present page table entries takes approximately 3 microseconds. The
new page discriptor is put in the cache by LRU rule. i.e. most recently used
pages only are kept in the cache.
The unit also supports all the usual protection fields like execute only,
read, etc. Since it contains a refrence bit it should be easy to implement
a demand paged virtual memory on a system which uses it.
And last but not least, the page table itself is pointed to by a virtual
register in the MMU. Each job on the system can have its own page table and
its own 16megabyte address space. And they call this a microprocessor!
Tim Tucker
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