Microprocessors

The following set of notes corresponds to the Microprocessor / CPU module of the PC Repair course, providing students an outline of the information they are expected to understand. The corresponding reading for this module is chapter two of All In One A+ Certification Exam Guide by Michael Meyers, pages 50 - 12.

Physical Properties

• CPUs come in three basic physical types: PGA (Pin Grid Array), SPGA (Scattered Pin Grid Array), and SEC (Single Edge Contact).
• 286, 386, and 486 CPUs are usually Pin Grid Arrays.
• Pentium-class and above CPUs are usually Scattered Pin Grid Arrays.
• The Single Edge Contact design is used for Pentium II, Pentium III CPUs. AMD Athlons use a similar design.
• Processors get very hot when they run. Heat sinks, fans, and case ventilation are required to keep them from burning out.

CPU Sockets and Slots

• A ZIF or Zero Insertion Force socket is designed to make the removal of the CPU chip easier.
• Socket 3 is perhaps the most common socket type for 486 CPUs. This is a 237 pin PGA socket.
• Socket 4 is a 273 pin PGA used exclusively for Pentium 60 / 66.
• Socket 5 is a 320 pin SPGA used for Pentium 75 / 90 / 100 / 120.
• Socket 7 is a 321 pin SPGA used for Pentium class 75MHz and higher CPUs.
• Socket 8 is a 387 pin SPGA specifically for the Pentium Pro.
• Socket 370 is a 370 pin Plastic Pin Grid Array socket for the Intel Celeron and Pentium III.
• Socket A is a 462 pin socket for AMD processors.
• Slot 1 and Slot 2 are Intel-specific slot designs for Pentium II and Pentium III SEC CPUs. This slot connection is licensed by Intel, so don't expect to see it being used by any other company.
• Slot A is AMDs version of Intel's Slot 1 and is used for the AMD Athlon series of CPUs.
• With the battle between AMD and Intel heating up, motherboards.org or a similar site can help keep up to date.

CPU History

• The first CPU was the 8086, a 16 bit CPU capable of addressing up to 1MB of RAM. The CPU was too advanced for contemporary machines, however, and was overshadowed by the 8088.
• The 8088 was an 8 bit CPU that powered the first IBM PCs. The chips ran at clock speeds of 5 MHz up to 8MHz. Microsoft DOS was built for this CPU.
• The 286 was a 16 bit enhancement over the 8088. The 286 was capable of addressing up to 16MB of RAM in protected mode, but since the dominant software at that time, DOS, ran in real mode, the practical improvements provided by the 286 were limited to its faster clock cycles (8-20MHz).
• The 386 was a significant improvement over the 286, providing in its 386 protected mode such features as multi-tasking and the ability to address up to 4 GB of RAM.
• The 386 supported real mode (8088 emulation), 286 protected mode (286 emulation), and a new 386 protected mode. There have been no new protected modes since the 386.
• The 386 was offered in a DX version (32 bit), SX version (16 bit, more compatible with 286), and an SL version (3.3volt instead of typical 5 volt).
• The 486 was a 32 bit CPU that introduced a math coprocessor and an on-board cache.
• The 486 was sold in an SX version (math coprocessor disabled) or a DX version. The DX version was also availble in clock doubling versions, known as the DX2 (two times the system clock speed) and DX4 (four times the system clock speed).
• Pentium CPUs provided significant improvements in CPU architecture. A Pentium class CPU is a 64 bit CPU with dual-pipelining and branch prediction.
• The Pentium Pro CPU improved upon the Pentium by offering quad pipelining, dynamic processing, and a built-in L2 cache. This processor was designed especially for 32 bit code, however, and did not run as quickly when processing legacy 16 bit code (DOS or Windows 3.x programs).
• The Pentium II provides MMX technology and faster speeds than the Pentium Pro.

CPU Features

• Real mode allows the 286 and all subsequent computers to be backward compatible with the old 8088. While running in this mode, the CPU may not be able to take advantage of some of its higher functions.
• Pipelining is a method that allows a CPU to process two or more commands simultaneously. In this type of system, there is a main pipeline capable of performing all commands, and secondary pipelines designed to handle the most basic commands and arithmetic.
• In branch prediction, a CPU guesses what might come next in a series of commands, does the processing ahead of time, and stores the results for faster retrival.
• Dynamic processing allows a CPU to run commands out of order so that it can use time that might otherwise be lost to a wait state. So, if the CPU has to wait for some information to be retrieved from the system RAM, it can hold the command that requires the RAM information and run other commands out of their original sequence until the desired data is returned.
• MMX is a set of 57 commands that enable a CPU to process multimedia (graphics and sound) more efficiently.
• In a split voltage CPU, the inside of the CPU runs at a lower voltage level than the outside of the CPU.
• Some motherboards include voltage regulator modules to allow customized CPU voltage regulation. Other motherboards set the voltage with jumpers. Since each CPU has its own voltage requirements, be certain to set these parameters correctly.

Clock Cycles

• In a computer system, all actions are performed to the rhythm of the system's internal clock. This clock determines how frequently the electricity is refreshed, or how often a wire is sent a charge. Each tick of the clock is called a clock cycle
• Clock cycles are measured in MegaHertz, with 1MHz being 1 million cycles per second.
• The clock speed of a CPU is the maximum number of cycles per second the CPU is designed to handle.
• Each processor command takes a minimum of 2 clock cycles to process.
• Over clocking is attempting to run a CPU at a clock cycle faster than it is rated for. This can cause the CPU to overheat and possibly burn up.
• Under clocking is running a CPUat a clock speed slower than it is rated for.
• Most motherboards are designed to run at multiple clock speeds. If replacing the CPU on a motherboard, be certain to check and set the jumpers that control the clock speed.
• Clock doubling allows the motherboard to run at one speed and the CPU to run at some multiple of that speed.

Cache

• A level one or L1 cache is a set of SRAM on the CPU that preloads commands for the CPU to process and stores them so the CPU can retrieve them more quickly then if it had to wait for the instructions to come from the system RAM.
• Level two or L2 cache is additional cache storage for the CPU. Originally this cache was located on a different part of the motherboard, but more modern CPUs have a level 2 cache incorporated into their design.
• Write through cache is a cache design that writes directly from the cache to RAM. This may involve wated time as the system waits for a free RAM cycle.
• Write back cache is a cache design that keeps the data in the cache as long as possible, not writing to RAM until it either has a free moment or absolutely has to write. This type of cache provides a little better performance than write through cache.