Lesson 71 - Microprocessors

4004/6502/Z80/8008+ Microprocessors

The basis for any programmable computer (or even a calculator) is a microprocessor. Instead of covering each one in particular, we will cover the "similarities" between them and a little of their history. In doing so, you will gain a more firm grasp, and have the ability to either intelligently research them, repair their circuitry, or use them in a project you may be fabricating.

When computers were first starting out, they were nothing more than glorified calculators. As a matter of fact, the first true commercially manufactured "processor" chip was the 4004, which was manufactured by IBM for a Japanese calculator company, then a year later marketed as a computer processing "set". I say set, because the functions of a computer are so complex, that at the time they couldn't be done with just one single chip! It required a processor, a ROM, a RAM, and a shift register.

From IBM's standpoint, the growth of the processor began with the 4 bit 4004 processing chip, followed by the 8 bit 8008, then the 8080, the 16 bit 8086/8088, 80286, the 32 bit 80386, 80486, "Pentium"(586), 64 bit Itanium, etc. But what exactly is the bit about bits?

At this point, having some understanding of the basics behind computer languages is a little helpful. The programs you use (Open Office, Firefox, Angry Birds etc) are wonderful ways for you to use and interface with the computer - but in the end - the computer still only speaks 1's and 0's. A computer program may be written in "programming languages" like Java, C, or BASIC for that matter - but the computer STILL doesn't actually speak these languages. They have to either be interpreted, or compiled into 1's and 0's in order for the computer to understand what you want it to do. Computer Commands consist of a series of eight 1's and 0's, called a BYTE. Each individual digit (1 or 0) is called a BIT - short for Binary Digit. There are 8 BITs in one BYTE.

The IBM 4004 processor could only process 4 bits, or half a command, at a time. You would have to give it two 4 bit entries before it knew what to do. For instance, in order to write something into random access memory, you had to give the 4004 chip the command "1110 0000". Now memorizing all these one's and zeros in order is darn difficult. So we invented a computer language called ASSEMBLY language that eased the chore. Now they could say WRM which meant WRite to Memory. This three letter acronym was called a Mnemonic. Learning the Mnemonic Assembler commands allowed you to write directly to the processor. Later Interpreted languages and Compiled languages would create the Mnemonic program for you.

Of course, with the 4 bit computer, it would take 2 half-commands just to perform one command. If they could create an 8 bit computer, then the computer would be able to injest a whole command at a time, cutting the speed it would take to perform a task in half! In the hayday of computer growth - there were several competing "8 bit" processors that had a chance at conquering the world. The IBM 8000 series, The Motorola 6800 series, the 6502 (Apple II and Commodore), the Zilog (Tandy/Texas Instruments/Radio Shack) Z80.

So how is it that the IBM 8000 series won the processor wars? Well having seen it firsthand, and in my humble opinion, the overall effect was like this:
  • Ex-Motorola employes (6800 designers) leave the super company to create their own 6502 chip.
  • Commodore and Apple II have the superior chip the 6502 - with onboard color graphics and sound!
  • Quantum Link, a Commodore friendly bulliten board, file sharing and chat system, was nationwide. Later joined with the Internet, and renamed as America Online (AOL)
  • Commodore, who clearly holds the marketshare for the home computer makes a pricy move to build a dual core machine - that will work with both 6502 and the Z80, taking Radio Shack's share.
  • A clone for the IBM 8080 was designed, making the well designed chip "open source", and hence cheaper to obtain. Less expensive chips mean less expensive computers, and after all - IBM is established as a "Business Machine" platform.
  • Tandy has their secret own plan to ABANDON the Z80 because it is clearly an inferior chip, and begin building IBM clones.
  • Commodore, due to the Tandy move, and clone computers taking over the market place - goes belly up.
  • IBM clone chips win out, and even IBM is left in the dust.
IN short - Open Source eventually wins! The same situation held true for VHS over BETA (proprietary data will never have the public attention as long as the word FREE), and eventually will hold true for Linux - just look at what Android (a version of Linux) is doing with cell phones now. In short - why would anyone ever want to pay for something they can get for free? (And why are you still clinging to your Microsoft Operating System?) Try Linux today! It's FREE and SUPERIOR to Windows! The only thing that has held Linux back to date has been copyrights and litigation, not ability.

As stated before, the 4004 was only 1 of a family of chips that were required in order to build a computer. The complete list included:
  • 4001 ROM (read-only memory) with four lines of output
  • 4002 RAM (random access memory) with four lines of input/output
  • 4003 was a static shift register to be used chores like scanning/controlling outside devices (keyboard, monitor, printer, external storage)
  • 4004 Microprocessor
As such, the 4004 had to have the following things before it could work:
  • BIAS (VDD, VSS, and Ground)
  • Clock (Sync Out, Φ 1, and Φ 2)- timing so it would stay in sync with the other three chips
  • ROM input
  • 4 CM (memory) outputs and 4 Data Outputs.
The ROM (Read Only Memory) would hold all the necessary program mnemonics to boot up and run the computer. The computer would boot up, read the ROM, and start running. After it was going, you could access the RAM (Random Access Memory) during the normal operation of the program. Keep in mind that the MAJORITY of the Mnemonic commands in the original 4004 chipset were mathematical in nature, as it was designed as a calculator. (ADD - self explaniory, SUB - subtract, ADM - ADD with Carry, etc). These same commands were carried onto later versions of processors, and are still in use today!

The Z-80 was the foundation of the TRS-80 (Tandy-Radio Shack 80) computer. This computer sold for a bargain price of $2000, and that was a lot more money back in the late 70's than it is now. I can recall owning and using one of these. I now find it amazing that you can buy a fully usable laptop for $400 nowdays, and think you are paying too much!

The Zilog 80 (Z-80) chip, at best, was a cludge! Any engineer wanting to argue that can simply take a look at the pinouts of the chip compared to the pinouts of the 8008. They were randomly placed wherever it was convenient - not necessarily done with any plan or order. It was, however, a functional 8 bit processor and had a good command set. An adept programmer could make this computer do magic. It also had all the support that a multimillion dollar company could give it - unltil Tandy got wise to the fact it was inferior, and finally abandoned it.



Clearly the forerunner technologically in its day, the 6502 had built in color video (amazing at the time), and sound. Commodore Computers took advantage of this low priced chip and actively targeted the home computer user market. At only $600 for a whole usable computer (less than 1/2 the price of any of their competitors)- this naturally lent itself to a spawning generation of computer enthusiests, as they could not only balance their checkbook with it - they could play GAMES! If you haven't played it, you should try out ELITE- the first 3D first person shooter! Its graphics left a little to be desired by today's standards, but it certainly had a good plot, and would keep people glued to the computer for hours on end.


At this time Quanutm Link came about, and "linked" computers and people together across the entire American continent. It had access to university databases, chat rooms, News, online shopping - it was everything the internet would become, before the Internet knew it. At the time, the internet was a "secret" linking of university, government, and hospital computer systems. But then the internet joined forces with Q-Link, and something magical happened. The Internet BECAME what Q-Link was, and Q-Link changed its name... to America Online.

Note that Commodore wasn't the only computer to use the 6502 processor. It was just the best marketed and supported. The Apple II also used the 6502, and may have been the superior of the two computer designs, but it didn't sell nearly as many computers.



The 8008 was IBM's foray into the 8 bit scene. Having developed the 4004, the 8008 was simply an extension of technology for them. They were still wary of the "home computer" market, and didn't see it as really taking off. But when they saw the success of Commodore, they didn't want to be left behind - just in case. IBM created "The Personal Computer" the IBM PC

After the 8080, 8086, and 8088, they started working on 16 bit, 32 bit, and now even 64 bit computers, each doubling the possible processing speed of the computer. In the end, even the most complecated computer begins with the simplest of circuits - the transistor used as a NOT gate or inverted switch, and as such requires bias and ground to operate.

All microchips you ever run into will have the same testing procedures. Does it have Bias? Is the ground really grounded? Does it have a clock signal? Are all the inputs present? If all of the above is true, then it should have the correct outputs. If not, the chip is at fault. Most likely though, you'll find that the solution to your "chip related" problem has already presented itself by the time you asked the first three questions.
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