Ram chips wanted!

[Paul De Bra]

In article <6800049 at cpe> tif at cpe.UUCP writes:
>>I need memory chips for my 12MHz 286 so I can run Xenix better.  The chips I
>>am looking for are 256k 80ns DRAM's.
>
>Obviously you can buy whatever you want but isn't 80ns faster than necessary
>for a 12MHz machine?  Seems like we put 100ns in our 16MHz 386's.
>
How fast rams have to be to work at a given clock-speed depends on how much
time they get from the CPU, and how much time the surrounding hardware (mmu
and such) already consume. A rough estimate is that the surrounding hardware
takes 1 clock tick and the memory 1 or 2 clock ticks (depending on 0 or 1 wait
state). The reason that this is a useful estimate is that machines with faster
clocks use faster mmu's too. It boils down to the following table:
clock	0-wait	1-wait
6Mhz	165ns	250ns
8Mhz	125ns	185ns
10Mhz	100ns	150ns
12Mhz	 83ns	125ns
16Mhz	 62ns	 94ns
20Mhz	 50ns	 75ns
25Mhz	 40ns	 60ns
This is only a ROUGH estimate, but it shows that a 12Mhz AT with 0 wait states
needs 80ns rams, and if a 16Mhz machine is using 100ns rams that means it is
running with at least 1 wait state (not accounting for a possible cache)

Paul.

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|Paul De Bra              | I am completely surrounded by giant bugs !    |
|debra at research.att.com   | There's millions of them, all over this code! |
|uunet!research!debra     | Beam me up quickly...Please...                |
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