100ns .vs. 120ns

[pri=-10 Stuart Lynne]

In article <8687 at alice.UUCP> debra at alice.UUCP () writes:
>In article <377 at fantasci.UUCP> jep at fantasci.UUCP (Joseph E Poplawski) writes:
>>How bad of a speed difference would my UNIX V.3 for the 80386 system incur if
>>I added 120ns chips instead of the 100ns chips it already has 2 meg of?  The
>>system is a personal system with no more than 3-5 users on at the extreme most.
>>
>>The reason I am considering the slower chip is mainly the price.
>>
>
>There is no magic in the PC and AT compatibles (including 80386's) to detect
>what speed the memory can handle. There are 2 possibilities: either your
>system doesn't really need 100ns chips and will run just as fast with 120ns
>chips, or else the system will not run at all with 120ns chips (it will give
>parity errors at least). Considering the price of memory chips it is very
>unlikely that you got the machine with 100ns chips when it only needs 120ns.
>
>The only way to make your system work with slower chips is to lower the
>clock-frequency (like from 20Mhz to 16Mhz).
>
>What ram-chips your system needs is very motherboard-dependent. Some boards
>use very fast rams (60ns) and no cache memory, others use slow ram (up to
>120ns) and compensate with cache memory. In any case the hardware expects
>a minimal speed of ram which is really necessary at the highest clock rate.

Actually the speed of your RAM is dependant on the clock-rate *AND* the
number of wait states. Generally speaking you can use slower memory chips
with high speed machines by adding wait states.

Depending on your machine you will have one or more of the following
possibilities:

	- the new chips work great (try and run a good memory test though)
	- they don't work

If they don't work:

	- reduce the clock rate
	- add global wait states 
	- add wait states for the new memory only

The last is best, but probably not supported on too many 386 systems.

The actual impact lowering the clock-rate or adding wait states will have is
dependant on the system:

	- does it have four way interleave
	- does it have a cache

The bottom line, is RTFM. Find out what options you have in terms of setting
your system up, can you:

	- modify clock-rate
	- add wait states
	- add wait states for specific memory only

BTW these are issues that are best dealt with *before* you buy the system,
unless you are never going to be expanding the system after you buy it (or
are willing to return to the vendor for all upgrades).



-- 
Stuart.Lynne at wimsey.bc.ca {ubc-cs,uunet}!van-bc!sl     Vancouver,BC,604-937-7532