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In article <30f80e73.896741@news.innotts.co.uk>
           eddy@innotts.co.uk "Eddy Hood" writes:

> I am thinking about buying a new motherboard and CPU, and in adverts for
> them I have seen things such as W/B Cache, and 256k Cache.. I think I
> understand W/B to mean Write Back Cache. What is one of these? Is it better
> to have a Write Back Cache than a 256k Cache or it is best to have both?
> Would a motherboard with just a Write Back Cache be alright?
> I hope someone can help me out here !
> Thanks alot,
> 
Basic difference from memory is that (if I've got them the write way round),

Write Through cache write the data to cache and to main memory.  It
then continous processing.  This means that any data fetched from
main memory is guarranted to be updated and correct.

Write Back cache writes the data to cache and then continous procossing
the next instruction. In this situation, the logic controlling the cache
has to be better designed to ensure that old data is not fetched from
main memory when there is an updated version in cache.  Not having to 
wait for the data to be written to main memory as well allows slight 
increase in speed.

Level 1 (L1) cache refers to the cache builtin onto the CPU die.  This 
is the fastest access of memory store and runs at the same speed as the 
CPU.  Typical size of this cache is 8k and 16k (eg AMD DX4-100 uses 8k
while the Intel DX4-100 uses 16k).  I think that all Pentium processors
use 16k, I'm am not sure about Cyrix 586 (16k ?).

Level 2 (L2) cache refers to the cache on the motherboard.  This is
faster than main memory (60-80ns) but slower than L1 cache.  Typical
speed is 8-20ns (lower number being better).  Typical size is 64k
in older M/B to 256k used in most systems in the last couple of years.

In newer Pentium systems (typically using Triton chipset), you may
be offered [Pipiline]Burst-cache.  Without going into details, this
offers upto 8-10% improvment in speed.  256kk is sufficent to effectively
cache upto 32 meg of an average system.  You will see little improvement
upgrading to 512k unless you have more than 32meg of RAM or are using
very CPU intensive applications.  Note some motherboards are not able
to cache more than the first 32 meg of RAM.

Both types of caching types (WB & WT) can be applied to L1 and L2. Some 
DX4-100/120 systems are known to have problems if write-back cache is 
enabled in either L1 and/or L2 cache.

Further to confuse the situation, there are many caching techniques
for how the main memory and cache memory are mapped (this applies
to caches in hard disks as well), thus a smaller but better designed
cache may be able to out-preform a larger but worse design cache.

In answer to your final question, 256k of L2 cache is better than no
L2 cache.  WB can offer some (maybe a few percent) improvement over
WT.

If antbody has anything to add/correct then please feel free.

-- 
Rajesh Varia     raj@c4rv.demon.co.uk