Cache Structure and Packaging



System cache can come in many different physical forms. This 
section describes the different types of packaging that cache is 
normally found in. Which type your system uses is a function of 
your processor, chipset and motherboard.


Integrated Level 2 Cache

The Intel Pentium Pro processor comes with an integrated level 2 
cache. The "chip" that you plug into the motherboard is really two 
chips. One is the processor itself (including the level 1 cache) 
and the other is the level 2 cache. These processors are available 
with 256 KB, 512 KB and 1 MB of level 2 cache. This is a very 
performance-enhancing design, because it allows the level 2 cache 
to run at the processor's internal speed (usually 180 or 200 MHz) 
instead of just the system bus speed (60 or 66 MHz). It also 
gives you one less thing to worry about in setting up a new 
system, because all of the support circuitry, tag RAM etc., is 
inside the chip.

One drawback of this design is that it is not possible to 
increase the level 2 cache without replacing the processor. These 
processors are also very expensive due to the difficulty of 
manufacturing the large chip required for the level 2 cache. 
Regular cache is made of many small chips, whereas this one is 
made from one large chip. In addition, defects in the level 2 
cache often are not discoverable until after the processor and 
cache are put into their shared package; this means the processor 
has to be discarded as well if a defect is found in the cache 
chip. This is the main reason that Intel moved away from putting 
integrated cache on its Pentium II processor. No other CPUs 
currently use this design and it is unlikely that any more 
ever will.

The integrated level 2 cache of the Pentium Pro is also faster 
than the older cache used with fifth generation systems due to 
performance enhancements. The main one is that the cache is 
transactional, or non-blocking .


Daughterboard Cache

Starting with the Pentium II processor (a.k.a. "Klamath") Intel 
has introduced a new form of packaging, called SEC (Single Edge 
Contact) . The integrated cache of the Pentium Pro processors ran 
at processor speed and offered very high performance, but was
very expensive to manufacture. The motherboard cache of the 
regular Pentium was easy and cheap to produce but offered lower 
performance. SEC is a compromise where the processor and cache 
are mounted together on a small "daughterboard" that plugs into 
the motherboard. This greatly reduces manufacturing costs, and 
also means that a bad cache chip doesn't result in the processor 
being wasted.

This type of cache runs at a faster speed than it would if it were 
on the motherboard, but slower than an integrated cache; this is 
why it is a compromise between the other two designs. On the 
Pentium II the level 2 cache runs at half the processor speed. So 
a 266 MHz Pentium II will have a 133 MHz level 2 cache. Not as 
good as the 200 MHz Pentium Pro's integrated cache, but a lot 
faster than running it at 66 MHz. The Pentium II's cache is also 
non-blocking , like the Pentium Pro's.

 Note: Even though the Pentium II has an architecture very similar 
to that of the Pentium Pro, due to a design limitation it will 
only cache the first 512 MB of system memory. The Pentium Pro will 
cache up to 4 GB of system memory.


Motherboard Cache

The most common cache design places the chips directly on the 
motherboard. On some older designs the cache is several SRAM chips 
in sockets (which means it can be replaced, but also means it is 
more prone to certain types of failures). On most newer 
motherboards it is in the form of 1 to 4 chips soldered directly 
to the board. If the cache is socketed, you can in some cases add 
extra SRAM chips to increase the size of the data store. The exact 
chips you need to add depend on the motherboard; your manual is a 
necessity here.

Some motherboard support the use of both soldered cache and also a 
COASt module. To use both you may need to change a jumper setting 
on the motherboard.

 Warning: There are some motherboards that actually have fake 
level 2 cache on them. These are most common on 486 motherboards 
with two or so flat cache chips soldered directly to the 
motherboard. In some cases, these chips are actually just empty 
plastic packages! In many cases the BIOS is even hacked so that it 
will report external (level 2) cache even when it doesn't exist. 
You can test for this by disabling the external cache . If you 
disable it and see no performance difference in a good benchmark 
program, the cache may be fake. 


COASt Modules

Some motherboards use a cache packaging format called COASt, which 
stands for "Cache On A Stick". This is a silly name for what is in 
effect a small circuit board similar to a single inline memory 
module (SIMM) that contains cache SRAM chips on it. It is inserted 
into a special socket on the motherboard often called a CELP 
("card edge low profile"). Some motherboards only use this socket 
for cache, some have only motherboard cache, and some have both. 
Usually jumpers are used in this last case to tell the board what 
is being used, although some boards will autodetect when a COASt 
module is added. See this procedure for instructions on adding a 
COASt module to the motherboard.

The CELP socket could have evolved into a standard of sorts for 
COASt modules, much the way SIMMs and DIMMs are (mostly) 
standardized in the memory area. However, this has not happened. 
Despite standard-sounding names like "COASt V1.2" and whatnot, you 
cannot rely on just any old COASt working in your motherboard. 
While many manufacturers share COASt module types, many others use 
proprietary designs. It's important to contact your motherboard 
vendor or manufacturer to ensure you obtain the correct type for 
your PC.
 Note: The COASt module often contains not just more data store 
for holding cached entries, but also more tag RAM to allow for 
more system memory to be cached. See here for more details.
