Last time on Byte Size,
we spoke about Level 1 and Level 2 cache on processors up to Pentium specifications.
But what about the Pentium Pro, and its cousins, the Pentium 2, the Celeron, and the Xeon chips?
Well, let's find out.
With the Pentium Pro, Intel introduced the revolutionary step of making a sort of double sized chip
that look like two Pentium chips side-by-side. The second "chip", however was a 512k L2 memory,
a built-in cache RAM.
This cache was terrific in that you could talk to the CPU at half of the CPU's full internal speed.
That's the core processor frequency, not the external bus speed.
Motherboards could be designed with even more external cache, and some Pentium Pro systems
sold with as much as 1024k of external cache, a mix of built-in L2 cache and some chips on the motherboard.
As any chips of the motherboard can only communicate with a processor at the external bus speed
rather than the Intel's speed, you might call that on-motherboard cache a sort of L3 cache.
The Pentium Pro has 16k of L1 cache, as did the original Pentium.
With the Pentium II, Intel created a larger
rectangular package called a Single Edge Cartridge which no longer allows external cache.
The Single Edge Contact Cartridge was used at the beginning of the Slot 1 era of the Pentium 2 CPUs.
Inside the cartridge, the CPU itself is enclosed in a hybrid plastic and metal case.
Previously, with the Pentium Pro, Intel had combined processor and cache dies in the same Socket A
package, and these were connected by a full-speed bus resulting in significant performance benefits.
Unfortunately, this method required that the two components be bonded together early in the
production process before testing was possible. As a result, a single tiny flaw in either die
made it necessary to discard the entire assembly, causing a low production yield and high cost.
Intel subsequently designed a circuit board where the CPU and cache remained closely integrated,
but were mounted on a printed circuit board, and this is where we get the Single Edge Contact Cartridge from.
The CPU and cache can be tested separately before final assembly into a package, reducing cost and
making the CPU more attractive to market. The result of this is you can't design a motherboard for a Pentium 2
that contains any cache. The Pentium 2 has a built-in 512k of L2 cache, but that's it.
I mean, a Pentium 2 also has more L1 cache than the Pentium Pro's 16k of data and
16k of instruction cache, for a total of 32, but that leads to an interesting
comparison for the older Pentium Pro versus the Pentium 2. As some of the Pentium Pro motherboards
had room for a megabyte of cache, one can benchmark a Pentium Pro at 200 Megahertz, versus
a Pentium 2 at 333 Megahertz and the Pentium Pro can actually be faster because it's got
1024k of cache versus the Pentium 2's half megabyte of cache.
Now the Pentium 2 Xeon chip addresses that problem and offers considerably improved
performance for two reasons.
But, the Xeon communicates between its built-in L2 cache and its processor at full core processor speeds,
a 400 Megahertz Xeon processor talks to its L2 cache at a full 400 Megahertz.
Second, the Xeon comes up with 1024k of built-in L2 cache, as you can buy the Xeon in
up to 400 megahertz speeds, it's the ideal Quake platform, or at least it was.
But, I haven't yet mentioned the Celeron. What kind of L2 cache does that come with? None.
Yep, that's right, Intel figured that they could build a cheap Pentium 2 to make it keep Cyrix, AMD, and other
competitors from carving out a place on the under
$1000 PC market back in the late 90s.
I mean, it does have the same 32k of L1 cache as the other Pentium 2 processors,
but that is where it ends.