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Choosing a CPU
by Alan Lynn, ebytes@bigfoot.com - September 07, 2001 at 17:24:22:
We started our exploration with a listing of some of the available CPUs and their relative prices. If choosing a CPU were as simple as choosing a megahertz range and a price it would be simple. As mentioned, there is more to it than that.Without getting too technical, the Pentium III has a top speed of 1 GHZ, its successor, the Pentium 4 has some major problems. The Pentium 4 is fast but it is like a race car, you throw out anything that hinders speed, the heater, the radio, extra seats. The P 4 is optimized for software that might be written in the future, it is very inefficient for current applications. At 1.3 GHZ it is only about the same as an 800 MHZ Athlon in many benchmarks The new 1.7 MHZ P4 runs hot enough that it slows itself down to keep cool. Intel is using RamBus, which is expensive, they are making a new type of Pentium 4, which will use either SDRAM or DDRAM. This will leave the current Pentium 4s as orphans with no upgrade path. If you must have Intel, then stick with the Pentium III for now, if software catches up, or if Intel corrects the problems their might be a new chip in the future.
AMD is releasing their 1.4 GHZ chip soon, they have also released a dual processor chipset, and the Athlon 4 is now out in notebooks. The Duron is now at 950 MHZ.
For those who insist on more gigahertz, CPUs are getting faster, but you need to look at several criterion.
Ancient history, one of the differences between a 386 and a 486, a difference that made a 486 faster, was the cache. Cache is a small amount of fast memory close to the processing unit of the CPU. With the 386 and previous processors, data was stored in RAM, random access memory. The type of RAM which is used is called DRAM or dynamic RAM, This is a relatively large, inexpensive type of memory, but it "leaks" and must be refreshed every few milliseconds. During the refresh cycle, RAM is unavailable, the processor cannot write to or read from the RAM. The CPU inserts "wait states" or wasted cycles until RAM is available again. With slow processors (8088, 286) this was not a problem, as speeds increased this slowed the computer. It was found that often the data needed for the next operation was often the data put out by the last operation. Speed in memory has variables, they include, the type of memory, the size and the distance from the processor. SRAM, Static RAM, has no wait states, but it is more expensive If you put a small amount of SRAM very close to the processor, then a small amount of SRAM can hold data for use during the refresh cycle.
The 486 has an SRAM cache, and thus is more efficient than the 386. The cache in the processor is called Level 1 or L1 cache. This is expensive because it is built right into the CPU. What if we could have a less expensive cache? With the P5, Pentium and X586 class machines there is more L1 cache memory than in the 486, cache is also placed on the motherboard. It is called Level 2 or L2 cache. Being further away from the processor it is slower, also it runs at the speed of the motherboard, not at CPU speed. The CPU in 486SX2 and DX2 and up is "clock doubled," they run at a higher speed than the motherboard.
The first P6 CPU was the Pentium Pro. If speed is important then the L2 should be closer, and it should be faster. The Pro put the L2 "on die" it was close to but not a part of the CPU, on a separate silicon wafer. Closer is faster, also the L2 now ran at CPU speed. This made for a large, expensive chip. Also having the L2 on die meant you were locked into one kind of L2 chip.
Next, in cache development is the Pentium II the MMX is just a refinement of the P5 Pentium, while the PII is a P6. If you put the processor on a circuit board then you can put the L2 close without putting it on die. SRAM limitations reduced the speed to half the CPU speed. The PII has 512 KB of L2 cache on the circuit board running at half CPU speed. This is less expensive than the Pro, but you need a slot to hold the board, and the plastic cartridge to cover it, all expensive compared to a chip in a socket.
To see how important cache is look at the first Celeron, without an L2 cache. This was a very slow processor. Intel changed that with the Celeron Mendocino. The Mendocino has a 128 KB full CPU speed L2 cache. Despite its small size, the speed makes it comparable to the PII at 66 MHZ bus speed.
Manufacturing progress made smaller circuits possible. CPUs have gone from .35 microns to .18 microns, this means more circuits and less size. The Celeron was the first PII class chip to migrate from slot to socket. All new PIIIs are socket chips, the AMD CPUs have followed this evolution, the first Athlon used the slot A to hold it, the new Thunderbird and Duron chips are in the socket A configuration.
Notes: The Athlon, Duron and Thunderbird use a doubled data rate front side bus. This means the motherboard is running at half the speed of the memory connection to the CPU. Also, RamBus (RDRAM) runs at 800 MHZ, but with a 16 bit path, instead of a 64 bit path used by other CPUs, this makes an effective throughput of 200 MHZ.
Memory is important, the two contenders are DDRAM, double data rate RAM and RamBus. RamBus has not fully lived up to its potential, it is more expensive and it must often be run in pairs, which doubles the price and complicates buying memory. DDRAM is about the same price as SDRAM and works in single DIMM banks.
The next important decision is the motherboard, which we will go into next time.
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