8 Bits and Pieces
Bits, Bytes, Nibbles, and Microprocessors
Maybe you heard the term "32 bit software" or "64 bit software" - did you ever wonder what it meant? The number of bits refers to how much information a microprocessor (also called a CPU) can process at a time. Modern microprocessors handle data in chunks of either 32 or 64 bits. Earlier CPU's were much smaller. The first IBM PC's used a 16 bit CPU (the Intel 8088) and most earlier home computers used 8 bit CPUs. If you think that's small, the first commercially available (the Intel 4004) was only 4 bits wide, and Motorola even made a 1 bit CPU - the MC14500. This lens is dedicated to these smaller CPUs. I hope you enjoy...
The Soul of a New Machine
by Tracy Kidder
If you're interested in early computers, you will love this book. It describes the design, prototyping, testing, and production of an early mini computer - the Data General Eclipse.
The Motorola MC14500B
Only 1 bit? You've got to be kidding...
You might not think that a 1 bit CPU would be useful for anything - but it is (or at least was). In fact, at one time a 1 bit CPU was so useful that Motorola made a lot of money manufacturing and selling them. The 1 bit Motorola MC14500B was perfect for industrial control applications, where it replaced hardwired logic built from relays. The programmability of the MC14500B allowed manufacturers to update their products without having to redesign the circuit board. Today, the simplicity of the MC14500B makes it perfect for studying if you want to learn how to design and build your own CPU.
The first commercially available CPU was a 4 bit processor...
The first commercially available 4 bit CPU was the Intel 4004. Most 4 bit CPUs were used in electronic calculators, but Intel marketed the 4004 as a general purpose CPU. Today the idea of a 4 bit processor sounds kind of quaint, but at the time the 4004 was revolutionary.
The Intel 8080
The first widely used CPU...
The first widely used 8 bit CPU was the 8080 by Intel. It was marketed as a single chip CPU but in reality 3 chips were needed to build a system. The 8080 was the CPU used in the first widely available personal computer, the Altair 8800. The 8080 had 7 8-bit registers (A, B, C, D, E, H, and L). The A register was the 8 bit accumulator and the rest could be used as 8 bit registers or combined into 16-bit register pairs (BC, DE, and HL). At a time when most 8 bit CPUs used memory mapped addressing to deal with input and output (IO), the 8080 had dedicated instructions for handling IO. At a time when memory was expensive and address space was limited, tyhis was a big advantage. The 8080 was eventually supplanted by the Z80 which was software compatible, but more powerful and simpler to use.
The Motorola MC6800 and MC6802
An easier to use 8 bit CPU...
The Motorola MC6800 was introduces shortly after Intel's 8080. Compared to the 8080, the MC6800 had a simpler instruction set and used memory mapped I/O - which meant that as far as the CPU was concerned, memory and I/O devices were exactly the same. Advocates of the MC6800 saw this as a huge advantage over the 8080 since it made writing programs easier. Advocates of the 8080 saw it as a huge disadvantage since it "stole" 256 bytes from the memory address range.