Xref: utzoo comp.sys.ibm.pc:35895 comp.unix.i386:702 Path: utzoo!attcan!uunet!portal!cup.portal.com!cliffhanger From: cliffhanger@cup.portal.com (Cliff C Heyer) Newsgroups: comp.sys.ibm.pc,comp.unix.i386 Subject: Re: Micronics 386 motherboards Message-ID: <22861@cup.portal.com> Date: 7 Oct 89 22:27:30 GMT References: <10871@dasys1.UUCP> Organization: The Portal System (TM) Lines: 59 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!