Micronics 386 motherboards

Cliff C Heyer cliffhanger at cup.portal.com
Sun Oct 8 08:27:30 AEST 1989

Toby A David asks... 
>1. Advantages to the 25mhz with cache memory board?  
The cache is more useful with DOS because it is more 
likely that your code will  stay in the cache. With UNIX 
the cache may get flushed at each context switch. But  I 
suppose if you have a 128KB cache it may not all get 
flushed, but this would take  some internals hacking to 
know for sure. 25MHz has a clock time of 40ns. If the 
code  your running fits all in the cache you'll cook. 
BUT if your accessing slow main memory  with 100ns DRAM 
which has 200ns cycle time, obviously you'll have many 
wait states.  In fact, if your applications are not 
going to make use of the cache, then you might as  well 
buy a 16MHz 386 with a clock time of 62ns, because the 
25MHz will be waiting for  memory all the time and only 
be running as fast as a 16MHz machine. 

>2. 100ns ram, or do you prefer the 80ns ? 
For 25MHz, clock time is 40ns. For 0 wait states you 
would have to be able to do a memory access in 40ns. The 
fastest DRAM is 60ns, BUT if you get 4MB 4-way 
interleaved 160ns DRAM then your cycle time would be 
40ns ASSUMING you are reading bytes sequentially from 
memory. If your application does lots of random memory 
reads, though, you'll have wait states again. (PS 
interleaved memory is not new - computers like the 
DECsystem-10 and 20s of the 70s used it for the same 
reason.) If you want 0 wait states all the time, you'll 
need 40ns main memory which would have to be SRAM 
(static-RAM). But no board makers are making 40ns boards 
for 386 PCs - they are too busy making them to sell 
$50,000 workstations. I think if they made them for PCs 
the economy of scale would drive the price down, but 
then who would pay $50,000 for a workstation?

>3. Any problems with the memory interleave? 
See #2.

>4. Optimum bus speed & wait states for the above 
operating systems? 
You might as well stick with the 8MHz AT bus so you'll 
be sure all the boards you use will work. However, what 
you might want is a board that has a SCSI or ESDI 
controller "on board" BYPASSING the AT-bus with a direct 
channel to memory. On the other hand, I've been told 
that a "good" DMA board design will allow you to pump 
1MB/sec through the AT bus, but most boards are slow 
because they are cheaper to make and customers arn't 
aware enough about speed to complain.

>5. Any problems with any particular MFM or ESDI 
controllers/drives ? 
YES! You will find machines advertized as having 
1.2MB/sec ESDI, but when you get the machine in your 
office and benchmark it you find it does only 300KB/sec
raw I/O. I have yet to find out what is wrong with these 
mahcines. All I can say is don't buy before you see 
someone with an ACTUAL MACHINE that gives you raw
I/O of 900KB/sec before you buy!

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