This chapter discusses features processors and associated support chips on motherboards. The objectives important to this chapter are:
A discussion of the Central Processing Unit (CPU) opens this chapter. Features of CPUs that are used to rate and compare them:
Regarding the older 386 and 486 chips: you should be aware that there were 386SX chips which had a 16 bit external data path, and 386DX chips which had a 32 bit external data path. They also came in several speeds. 486 chips came as 486SX (in which the built-in math coprocessor did not work) and 486DX (in which the built-in math coprocessor did work). To make it more complicated, some 486 chips were labeled as 486DX2, which meant that the chip doubled the clock speed, running at twice the speed rated for the motherboard. Confusingly, there were also 486DX4 chips, which ran at 3 times the motherboard clock speed, not 4 times.
Many early processors ran on 5 volts. Late 486SX models and 486DX4 chips used 3.3 volts. Voltage regulators were needed to step down the voltage until system boards became available with 3.3 volt lines. Pentium I chips running at 60 or 66 MHz used 5 volts, while Pentium Pro and Pentium II chips used 3.3 or 2.8 volts.
Be aware that Pentium chips have a 64 bit external data path and a dual 32 bit internal data path. This is because they have two arithmetic logic units (ALUs), making it possible for them to do two math functions at once .
The text goes on to discuss several buses. The most significant is the one used to connect devices directly to the CPU. It may be called the system bus, system board bus, the memory bus, or the frontside bus. Common speeds are 100, 133, 200, 400, 533, and 800 MHz.
A processor may often run faster internally than the bus it connects to. The text gives an example of a 3.2 GHz processor connected to an 800 MHz bus. In this example, the factor that you would multiply times the bus speed to get the processor speed is called the multiplier. When installing a CPU on a motherboard, it may be necessary to set a dip switch or a jumper to the proper value for the multiplier. (In this example, the multiplier is 4.) Some users (usually gamers) set a system bus or a processor to run faster than rated by the vendor. This is called overclocking. It is NOT recommended. It often results in components burning out before their time.
Memory cache is discussed next. Be aware that cache that is included on the CPU itself is called primary, internal, L1, or Level 1 cache. Cache that is separate from the processor is called external, secondary, L2, or Level 2 cache.
This is confusing in to many students. Consider a diagram on Mike Karbo's Online Service site that shows a diagram of a Pentium II module. In this example, the module (sometimes compared in size and shape to a Hershey bar) contains the processor chip (which contains L1 cache) and a separate L2 cache chip. This separate L2 chip is inside the module, but not part of the processor, so it is Level 2. It is connected to the processor by a fast bus line, referred to as a backside bus because you cannot see it. An L2 cache chip that connects to the system board would connect to the processor through a frontside bus, which simply means that it can be seen on the system board.
Ready for some more confusion? A Pentium III contains L2 cache directly on the same die (chip) as the processor core. This is called Advanced Transfer Cache (ATC). It is still L2 cache, but it is hard to see that it is separate. ATC allows Pentium III chips to be smaller and less expensive than Pentium II chips. The ATC Cache bus is 256 bits wide. It runs at the same speed as the processor. Pentium III L2 cache stored on a separate microchip within the CPU housing is called discrete L2 cache. With discrete L2 cache, the Pentium III cache bus is 64 bits wide and runs at half the speed of the processor. All Pentium III processors have either 512K of discrete cache or 256K of ATC cache.
A discussion of various Pentium models follows. You should be familiar with some details about each model:
Two competitors to Intel are mentioned in the text: VIA/Cyrix and AMD. AMD marketed a 64 bit processor that outperformed Intel Pentium 4 processors. Intel countered with versions of its Itanium processors. A brief discussion is provided about the Itanium chip, the current generation chip from Intel.
A distinction is made about Complex Instruction Set Computer (CISC) chips and Reduced Instruction Set Computer (RISC) chips. Essentially, most legacy chips and programs assume a CISC chip is being used. Pentium chips use some of each instruction set, AMD's K5 and K6 chips use RISC, and most other Intel chips are CISC. Itanium chips use Explicitly Parallel Instruction Processing (EPIC). The EPIC system is meant to send instructions to the chip, and instructions about how to perform those instructions on the two processors in the chip.
Processors may attach to a motherboard by an array of pins, by a long slot connector, or by an array of lands, which is a newer technology. The Intel web site features a page that explains the difference between pins and lands.
Several CPU packages have evolved since the first "hershey bar" design for the Pentium II. Several designs are compared in the text. Some of the variations seem to be changes for change's sake, rather than improvements. We will not dwell on this list for this class.
Several CPU socket designs have existed as processors have evolved, as well. Be aware of the progression from sockets to slots, and back to sockets again. Two of the terms used: Pin Grid Array (PGA) implies a fairly symmetrical array of rows and columns of pins. Staggered Pin Grid Array (SPGA) implies an asymmetrical array of pins, with different numbers of pins in different rows. A desirable feature to find on any socket is the Zero Insertion Force (ZIF) lever. A ZIF equipped socket allows the user to lift the lever, which releases the chip. Lowering the lever causes the socket to securely grip the chip's pins.
The text discusses the voltage regulator mentioned above. Some processors require two voltages, some require one. Setting the jumpers on a voltage regulator will provide different voltages. See motherboard documentation for settings.
The text defines a chip set as "a set of chips on the motherboard that controls the memory cache, external buses, and some peripherals". Various chip sets are available. The text notes that the i800 chip set (and others since) have featured a hub interface between the I/O buses and the system bus. The hub is said to have two ends. The faster end is the North Bridge, which connects to the system bus and RAM. It contains the graphics and memory controller. The slow end of the hub is called the South Bridge. It interfaces with ports, hard drives, and the expansion slots. It contains the I/O controller. This information allows us to understand the improved version of PCI, PCI Express. Standard PCI slots are all controlled through a single bus communicating through the South Bridge. PCI Express attaches the PCI devices to a shared switch, which connects to both Bridges.
The next section discusses ROM BIOS, which is somewhat badly named. Originally, the BIOS chips in a system were Read Only Memory (ROM), but often you can Flash a BIOS chip, meaning you can run a program to upgrade the instructions in the chip. This makes one question the use of "Read Only". A system may have several BIOS chips in it, some of which may be on expansion cards. Any BIOS chip may need to have its instructions shadowed, which means that the instructions are copied to a section of RAM so that the operating system may interface with it more readily. As previously noted, if the system halts or behaves oddly, try turning this shadowing off.
Some older BIOS chips are simply incompatible with newer components. Upgrading the chip or upgrading the programs on it are often required. If you are upgrading a Flash ROM, you may be able to download a new program from the vendor or manufacturer's web site. Other times, you may have to have a diskette sent to you.
CPU cooling fans are an important part of the computer. It should be remembered that air flow through a computer will be disrupted if openings are made in the case that are not intended to be open. As noted above, the user might remove a metal cover on the back of a computer, intending to install an expansion card whose ports would be accessed through the new opening. If the user never installs the card and never replaces the metal cover, the new hole in the case will cause air to flow in unintended directions, improperly cooling (or not cooling) various components. The text describes other optional cooling equipment:
A discussion of buses and slots describes how to mount processors of various types on a system board. At this point, the students in this class should have performed a lab that required removing and replacing a processor chip. Since we do not have several models to work on in the lab, students should review the procedures for each processor type in the text, and should perform an internet search for more illustrated material on this topic. Mr. Karbo's illustrations are recommended. Follow the link above, and select PC Architecture.