At 23:40 -0500 08/10/2002, Robyn Lyons wrote: >Speculative Processing SHOULD be turned off if you are using one of the >'classic' versions of the OS. Under OSX only, Speculative Processing >works like it should on 'old world' machines.
Speculative processing seems to mainly have problems with some (all?) Adaptec SCSI cards and Retrospect. There may be other issues, but those are the ones I know about. For example, if you have an Adaptec 2940UW installed and a G3/G4 card and Speculative Processing enabled the machine will freeze at the gray screen. Old World machines with the Kansas ROMs work fine with Speculative Processing, and, in fact, I've installed the Kansas ROMs in a few S900s and PowerTower Pros and they work fine with Speculative Processing after the transplant. > I'm not sure about the >Write-Thru cache setting though, I don't even know what the difference >is; can anyone enlighten us? There are two commonly used caching schemes called Write-Back caching and Write-Through caching. In simplest terms, Write-Back is faster and more dangerous and Write-Through is slower and safer. G3/G4 CPUs have at least two levels of cache. Whenever information is read from memory that information is stored in the cache on its way to the CPU. Then, if it is needed again, the CPU gets it from the much faster cache rather than from memory. A similar scheme is used when the CPU writes to memory. The item written is stored in the cache,. so that if it is needed again soon, it can be read quickly from the cache without waiting for a RAM access. If the CPU moves far enough afield, eventually the contents of the cache will be replaced with other material. This makes sense as the RAM memory is several times as large as the cache, so only recently used (and hopefully needed again soon) items can be kept in the cache. When the CPU writes information out, it is much faster to just store that new data in the cache. This is Write-Back caching. Writes only go to the cache, until it is time to use that segment of cache for some other memory location. Only at that time, are the up-to-date contents of the cache written out to the memory. Notice that if something goes wrong here, the memory has out of date information in it. Only the cache has the up-to-date stuff. A machine using write-through caching sends all writes to both the memory and to the cache. There's still some benefit, because the CPU can read the updated info from teh cache, without going all the way out to memory. But there's a time penalty for all those extra writes to memory. So write-through is slower. On the other hand, Write-through is less prone to errors. RAM memory always has the up to date information. In a well designed system, write-back caching should work just fine and it seems to give as much as a 20% improvement in CPU performance in some cases. But, there are instances where it just isn't reliable for some reason, probably having to do with the fact that our machines were not designed with backside cache in mind. Jeff Walther -- SuperMacs is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Refurbished Drives | Service & Replacement Parts [EMAIL PROTECTED] | & CDRWs on Sale! | Support Low End Mac <http://lowendmac.com/lists/support.html> SuperMacs list info: <http://lowendmac.com/supermacs/list.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive: <http://www.mail-archive.com/supermacs%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
