There are four main components in a CPU:
- Memory Cache —Memory caches can internal or external to the CPU die.
- Control Unit (CU)
- Register
- Arithmetic Logic Unit (ALU)
The computer's memory bus (also called a data bus) connects the CPU with the server’s RAM. In general, the wider the memory bus, the more data it can transfer.
A Note on CPU Frequency
When upgrading a processor, processing frequency—rated in megahertz (MHz) or gigahertz (GHz)—is usually considered first. Though important, frequency should not be the only consideration.
Some processors have better designs and run with much greater efficiency (though at a lower frequency) than other chips. For example, a processor may have a larger onboard cache that allows for significantly increased performance. In this case, although the processor frequency is lower, the CPU may still outperform higher frequency models.
A Note on Cache Evolution
Memory caches have evolved quite a bit since the 80386 CPU was released. Caches are named based how “close” they are to the CPU. The first, and fastest, memory cache the CPU will use is called the Level 1 (L1) cache. The next closest cache (with lower performance) would be the L2 cache, then the L3 cache, and so on. Usually, as the levels increase, the caches’ performance becomes slower, but the size increases.
Back when the 80386 CPU was in use, the L1 cache was located on the motherboard. The 80486 introduced cache directly on the CPU core. The 80486’s on-die CPU cache is the L1 cache and any cache on the motherboard is the L2 cache. The Pentium Pro was Intel’s first CPU to move the L2 cache off of the motherboard and onto the same packaging as the CPU. The Pentium II followed this pattern by having the L2 cache integrated into the Slot 1 CPU packaging, but not directly on the CPU core. Later, with the introduction of the Pentium II Celeron, a small amount of L2 cache was integrated into the CPU die itself, introducing Intel’s first CPU with both the L1 and L2 cache integrated directly into the CPU core.
Integrated L1 and L2 caches have been the trend over the last few years for both AMD and Intel, but recently Intel started introducing Pentium IV CPUs with integrated L3 caches up to 2MB in size. The Pentium IV Extreme Edition CPU features an estimated 20KB L1 cache (Intel does not reveal the exact size), and L2 cache of 512KB, and a 2MB L3 cache. In general, CPUs with bigger caches and more cache levels will outperform those with smaller caches and smaller cache levels. However, the performance benefits realized from the bigger cache will depend on the applications used.
A couple of other terms associated with caches are inclusive and exclusive cache management schemes. Inclusive cache management always duplicates any data stored in a higher cache level to lower cache levels.
- The Pentium IV uses inclusive cache management. If the 512KB of data is stored in the L2 cache on the Pentium IV Extreme Edition, an exact duplicate of the data will be stored in the L3 cache also.
- AMD's Athlon CPU family uses exclusive cache management. In exclusive cache management, data is not duplicated from one level of cache to another level, unless needed.
Each method has benefits and disadvantages. Inclusive cache management can cause some waste in cache memory via duplication, but management of the cache is easier. Exclusive cache management allows more cache memory to be available by reducing duplication, but management of the caches is more difficult since unique data can reside in each cache.
