Articles and Information
by Members for Members

The CPU: Your Computer's Core

by Lee Alexander

The core of a PC is its CPU (Central Processing Unit). What is a CPU? It is an IC (Integrated Circuit) of tiny transistors arranged in logic gates to perform the Boolean operations that carry out the instructions of the software code. Popularly, the type of the PC is identified by the CPU's development. A 386 PC meant the CPU was an 80386 unit. The next progression was the 80486 and Intel dropped the five digit numerical designation with its Pentium (5- series) chip.

IC's are fabricated by a complex process involving photolithography and the doping (adding positive and negative materials) of a thin, single crystal, silicon wafer. The generations of the CPU are astounding in their growth parameters. The aforementioned 80286, in 1982, was comprised of 134,000 transistors using 1.5 micron technology. The 1.5 microns refers to the smallest line width of the process. A micron is one millionth of a meter (0.000001 m), equivalent to 0.000004 inches. A human hair is about 0.002 inches thick; one hair would cover 2000 side-by-side lines. Jumping ahead to 1993, Intel's Pentium CPU contained 3.1 million transistors and used 0.8 micron lines. The present top Pentium, the P4, has 55 million transistors and employs 0.13 micron technology and runs at just over 3 GHz (Gigahertz).

What's inside and what does it do?

• Address – a number used within the CPU to locate a specific piece of data. Often used in place of the actual value, it can be more compact and act quicker.

• Bus – an arrangement of conductors to carry data between logical units in the CPU. Buses are specified in terms of bit-width, the number of bits transmitted simultaneously. Intel's buses are 32 bits wide.

• Cache Memory – there are two types: L1 (primary) resides on the CPU chip and is much faster than the off-chip L2 in the Pentium II and III CPU's. The latest P4 PGA (Pin Grid Array) has the L2 cache on-chip but in a separate location for efficiency.

• Clock Speed – the frequency of cycles performed by the CPU in the tick of the clock on the mother board. Noted in MHz or GHz (Mega/Gigahertz), it is not always a direct indication of performance.

• Logic Gate – a circuit of transistors that outputs a signal (data) depending upon the data input. Gates come in a variety of configurations: AND, OR, NAND (not AND), and NOR. Basically, the computer sees the output as TRUE or FALSE.

• RAM – not part of the CPU. It is the destination for the instructions and data from the CPU.

• Registers – small elements of memory within the CPU. They hold the temporary data for the internal workings of the CPU. This allows for a much faster operation than looking up an address of main RAM and fetching its contents.

• Transistor – in the case of the most common FET's (Field Effect Transistors), three elements comprise the solid state switch: the Source, the Drain, and the Gate. The signal to the gate toggles the device on/off (open/closed).

• ALU (Arithmetic Logic Unit) – the area in the CPU where arithmetic (!) and Boolean logic operations are carried out; such as incrementing a counter or summing values. The intermediate results are kept in registers.

• Control Unit – implements the four basic steps for an instruction:

•  Fetch -sending the address of the next word in the instruction over the address bus and placing the value in the proper register.

•  Decode -transferring the data to the Decoder where the machine language is translated into binary code.

•  Execute -performing the math and logic that controls the data flow.

•  Store -recording the results in the correct registers.

• FPU (Floating Point Unit) - the ALU can only perform integer math, using whole numbers. Some applications, notably CAD and other graphic intensive programs (games) require fractional values. Prior to the 486 family, this was a separate chip on the mother board.

• Instruction Set – machine language code stored permanently within the CPU. It comes in two flavors: CISC (Complex Instruction Set Computer) and RISC (Reduced Instruction Set Computer). RISC has fewer commands and they are all the same word length for faster operations - but require more transmissions. Intel CPUs use CISC.

• Prefetch unit – a small core of memory holds the next instruction and compares it to the contents of the cache and CPU's main memory. This occurs while the Decode and Execute units go about their tasks.

Let's follow some action on your PC. Suppose you reach a point where the OS (Operating System) requests a password and you have previously set the PW to AardVark. Hitting the A on the keyboard sends the data through RAM to the processor. As this is not an instruction, the CPU recognizes it as data but does not know what to do with it. It therefore sends the data to the Prefetch unit, where it joins the line of pending data/instructions. The Prefetch unit scans the CPU's main memory for an instruction, wherein the software tells the Prefetch to send the code via the data bus to the instruction cache. There it is assigned an address for future use. The Prefetch unit then sends the address and the data code to the Decode unit. The Decode unit translates the data, the instruction with it, and its address into binary code. The translated information is then sent to the Control unit, which is told to store the data in the Data cache and wait for further instructions. Almost simultaneously, the letter A appears on your monitor (due to the built in instructions) via the Data bus. The process is repeated for the remaining seven letters of your password.

When you hit Enter, a new procedure begins. The BIOS (Basic Input/Output System) runs the command to read through the Data bus to the Prefetch unit. Checking the Instruction cache it finds none and goes on to main memory. The OS sends the Prefetch the collected data of eight letters and a command to compile (translate) and compare the data with a known stored value. The new instruction, addresses, and data are sent to the Decode unit where it again converted to binary code. All this is now passed on to the ALU for processing. The Control unit sends the first letter of the stored password (in the OS) as binary code along with the address of your first typed letter, which has been stored in the Data cache. The ALU runs a Boolean compare on each bit of the binary code for the first letter. The result, a TRUE or FALSE, is stored in a memory register until the comparison is complete for each character.

A parallel “computation” has found an instruction in the main memory to provide access if the information from the Prefetch unit, the Instruction cache, and the Decode unit all result in a final condition of TRUE from the ALU. Now you can proceed – or gracefully quit.

For a more detailed view of CPUs and their features, visit www.pcmag.com and scroll down to Comparison: CPU Road Map.

ANSI (American National Standards Institute) 8 bit (0 through 255 decimal) code is used for characters. The number zero (0) is assigned 48 10 , upper case A is 65 10 , lower case ‘a' is 97 10. The corresponding binary values are 0011 000, 0100 0001, and 0110 0001