Computer Gaming Hardware

A computer is completely useless if you don’t tell the processor (i.e. the CPU) what to do. This is done thru a program, which is a list of instructions telling the CPU what to do.
The CPU fetches programs from the RAM memory. The problem with the RAM memory is that when it’s power is cut, it’s contents are lost – this classifies the RAM memory as a “volatile” medium. Thus programs and data must be stored on non-volatile media (i.e. where the contents aren’t lost after your turn your PC off) if you want to have them back after you turn off your PC, like hard disk drives and optical media like CDs and DVDs.
The CPU can’t fetch data directly from hard disk drives because they are too slow for it, even if you consider the fastest hard disk drive available. Just to give you some idea of what we are talking about, a SATA-300 hard disk drive – the fastest kind of hard disk drive available today for the regular user – has a maximum theoretical transfer rate of 300 MB/s. A CPU running internally at 2 GHz with 64-bit internal datapaths* will transfer data internally at 16 GB/s – over 50 times faster.
The difference in speed comes from the fact that hard disk drives are mechanical systems, which are slower than pure electronics systems, as mechanical parts have to move for the data to be retrieved (which is far slower than moving electrons around). RAM memory, on the other hand, is 100% electronic, thus faster than hard disk drives and optimally as fast as the CPU.
The problem is not only the transfer rate, i.e. the transfer speed, but also latency. Latency (a.k.a. “access time”) is how much time the memory delays in giving back the data that the CPU asked for – this isn’t instantaneous. When the CPU asks for an instruction (or data) that is stored at a given address, the memory delays a certain time to deliver this instruction (or data) back. On current memories, if it is labeled as having a CL (CAS Latency, which is the latency we are talking about) of 5, this means that the memory will deliver the asked data only after five memory clock cycles – meaning that the CPU will have to wait.
Waiting reduces the CPU performance. If the CPU has to wait five memory clock cycles to receive the instruction or data it asked for, its performance will be only 1/5 of the performance it would get if it were using a memory capable of delivering data immediately. In other words, when accessing a DDR2-800 memory with CL5, the performance the CPU gets is the same as a memory working at 160 MHz (800 MHz / 5). In the real world the performance decrease isn’t that much because memories work under a mode called burst mode where from the second data on, data can be delivered immediately, if it is stored on a contiguous address (usually the instructions of a given program are stored in sequential addresses). This is expressed as “x-1-1-1” (e.g. “5-1-1-1” for the memory in our example), meaning that the first data is delivered after five clock cycles but from the second data on data can be delivered in just one clock cycle – if it is stored on a contiguous address, like we said.
However, increasing the memory cache isn’t something that guarantees increase in performance. Increasing the size of the memory cache assures that more data will be cached, but the question is whether the CPU is using this extra data or not. For example, suppose a single-core CPU with 4 MB L2 cache. If the CPU is using heavily 1 MB but not so heavily the other 3 MB (i.e. the most accessed instructions are taking up 1 MB and on the other 3 MB the CPU cached instructions are not being called so much), chance is that this CPU will have a similar performance of an identical CPU but with 2 MB or even 1 MB L2 cache.