PDP-8 /e/f/m memory

Don THX1138 at dakotacom.net
Mon Aug 14 23:45:57 CDT 2006


Ethan Dicks wrote:
> On 8/15/06, Don <THX1138 at dakotacom.net> wrote:
>> Outrageous overkill.  What's that, 10ns memory?  What's the 8's
>> memory cycle time??  ;-)
> 
> The machine cycle time for an -8/e is 1.2uS.  ISTR that a core read
> cycle on an MM8E (Omnibus core stack), including replacing the old
> value after the read destroys it, is on the order of 950ns, but that's
> from memory, not from looking it up.  In any case, *any* SRAM, even
> stuff from the mid-1970s is fast enough.  As for overkill, it's not
> about memory access time, it's about modern availability and package
> count - if you want to use a 32Kx8 SRAM, your choices are,
> essentially, uber-fast cache RAM in a skinny DIP or still-too-fast
> CMOS SRAM (62256) in a wide DIP form factor.

But cache RAM is power hungry and has little practical chance
of being converted to BBSRAM

> One could even wire up a socket with 3 rows of pins so the user could
> choose skinny or wide - the RAMs have, essentially, the same pinout.




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