Cache (continued...)

Cache Level
When we talk about the level of cache we're referring to it's connecting circuits or electronic pathways (bus) and physical proximity to the CPU. What does this have to do with performance? First off, think about the actual physical distance. Does it take you longer to walk 50 feet as opposed to 10 feet? Data that flows as electricity seems almost instantaneous, but it does have to travel along those wires. How fast can it travel along the bus? Remember the clock?

Internal Cache (Level I or L1)
Internal cache is a small amount of SRAM that is placed inside the actual CPU (internally) that is accessed directly. It runs at the same speed as the processor. With the introduction of the 486 CPU, Intel placed 8K of internal cache memory in the processor. Today's computers have 16K - 64K of L1 cache memory. With the newer processors, this means the L1 cache can be accessed at speeds up to 650+ MHz. This can't help but enhance performance.

External Cache (Level II or L2)
External Cache is separate (or external) from the CPU. It's often socketed on the motherboard in DIP chips or in COAST slots ('Cache On A Stick' is SRAM that is socketed on a small circuit board or module and installs in a long thin slot on the system board). If your system bus speed is 66 MHz, then that's how fast the bus speed is to your External Cache. No matter how fast your CPU runs internally, the external cache is limited to the speed of the system bus. External cache is often called Level II, or L2 cache. Newer computers will typically have 256K or 512K of level II cache, but check your documentation because sizes do vary, and so does the possibility of upgrade.

To shorten the bus length and increase the speed to L2 cache, modern computers have the level II cache inside the CPU package, whether its inside the actual CPU chip or in the CPU cartridge (in the case of an Intel PII or PIII). It's still separate from the CPU core and connected by a bridge or what is termed the 'backside bus'. Speeds range from CPU core speed to regular system bus speed. In the case of the PII and PIII, the backside bus allows for transfers at half the processor's core speed.

Intel has always set the standards, so let's use their processors as an example. The Pentium II and the Pentium III typically have 512K of Level II cache separate from the CPU, but in the same slot 1 cartridge. It's connected to the CPU by a backside bus running at half the processor's speed. The Celeron CPU was aimed at the lower end market and had the same core as the PII but no external cache on the backside bus. Not a big seller! Intel revised the Celeron and came out with the Celeron 'A', which had 128K on the backside bus. They've continued that practice with the newer Celerons in the PGA format.

By the way, now that Level II cache has been placed inside the chip or cartridge on the 'backside bus', I've seen cache levels redefined by some authors. Level I being attached directly to the core and running at CPU speed, Level II on the backside bus or bridge running at half the processor speed, and Level III cache attached to the motherboard with a bus speed the same as the system bus. I don't know if this redefinition is going to be accepted or not, but I thought it worth mentioning in case you ever come across this type of reference. What have you heard?