My Favorite 8 Bit CPUs
Fun With 8 Bit Microcomputers
It might be hard to believe, but not long ago most computers were controlled by a Central Processing Unit (CPU) that processed data 8 bits at a time, were limited to 64K bytes of RAM (for programs and video), and operated at clock speeds between 1 and 4 million cycles per second (MHz). In spite of these limitations, some excellent personal computers were built around these 8 bit CPUs, and they are still useful today. The thing I like best about the 8 bit CPUs is that you can buy some parts and actually put together your own computer from scratch. These 8 bit CPUs are easy to work with and easy to program, and building and programming your own computer from scratch is an excellent way to learn about digital electronics, electronic fabrication, and computer programming and interfacing. This lens is about some of my favorite 8 bit CPUs. Hope you enjoy...
The Soul of a New Machine - This is what got me started in electronics...
The Soul of a New Machine by Tracy Kidder is the book that got me started in electronics. Even though the book details the design of a mini computer (the Data General Eclipse), it is still a fascinating read for anyone interested in computers. The best part about this book is it details the human aspects of designing a computer instead of the technical issues. You might think such a book would be dry reading, but Mr. Kidder writes in a style that is not only readable, but very entertaining. If you have any interest at all in early computing history you will probably enjoy this book very, very much. If you live near a college that has an engineering library you can probably even check it out before you buy it.
The RCA COSMAC 1802
One of the most unique 8 bit CPUs ever...
My very first computer was a Quest Super Elf that was based on the RCA COSMAC 1802 CPU. The 1802 is one of the most unusual 8 bit CPUs ever devised. It has 16 general purpose registers (many more than most 8 bit CPUs), a "progam mode" that allows programs to be manually keyed in (so you don't need permanent memory to hold a control program), and it doesn't have an instruction for calling subroutines. The 1802's main claim to fame was its use in NASA's Galileo Spacecraft. Because the 1802 doesn't need permanent memory for program storage (programs can be keyed in by hand), it's the easiest 8 bit CPU to use if you're designing and building a computer from scratch.
1802 Links - Lots of good info for COSMAC fans...
cosmacelf.com is probably the best 1802 resource on the web. They have 1802 technical documents, software listings, and links to many other 1802 resources. If you want to build a computer around the COSMAC 8 bit CPU, you really need to check out this
- Elf 2000 Kit
Spare Time Gizmos has an awesome kit based on the 1802. The ELF 2000 has everything - 32K RAM, 32K ROM with built-in Operating System (and several programming languages), 1861 emulation, and even a solid state drive using SDRAM. If you want the ultim
- My COSMAC Elf page
My own COSMAC ELf page about my experiences with the 1802 CPU. Also includes parts sources for those who want to build their own Elf.
The MOS Technology 6502
The 6502 was one of the most popular 8 bit CPUs ever...
My second computer was an Ohio Scientific Superboard II that was based on the Mos Technology 6502. Other more famous computers based on the 6502 were the Apple II and various Commodore computers (PET, VIC-20, and C-64). The 6502 is interesting in that it sets aside the first 256 bytes of memory (Zero Page) for special use by the processor. The special instructions allowed by this CPU dedicated Zero Page make the 6502 a very powerful CPU, and allow it to execute programs as quickly as other CPUs with clock rates up to 4 times higher.
6502 Links - Learn more about the 6502 CPU...
6502.org is the absolute best 6502 site on the web. Tons of data, home built projects, programming information, historical data on the 6502...
Motorola's entry into the 8 but CPU field...
The 6802 microprocessor was based on Motorola's 6800 CPU, adding an on-chip clock and 128 bytes of RAM. Like the 6800, the 6802 had a 16 bit address bus, built-in support for Direct Memory Access (DMA) by peripheral chips, and 6 internal registers. The register set included a 6 bit status register, 16 bit program counter, stack pointer, and index register, and 2 8 bit accumulators. For certain operations the 2 accumulators could be combined to make a single 16 bit accumulator. The 6802 instruction set was as simple as the architecture with only 72 basic instructions. The 6802 was popular as an embedded processor in computer peripherals, point of sale (POS) terminals, etc. It never caught on as a processor for personal computers, but its simplicity makes it an excellent choice for learning. If you want to know more about the 6802, check out Wikipedia's excellent 6802 Page.
6802 Links - More information on the 6800/6802 CPUs...
- Wikipedia 6800/6802 Page
Excellent article on the Motorola 6800/6802 microprocessors and the 68xx family peripheral and support chips. Probably the best information on the 6800 family that you'll find on the web.
- 6802 Nano Computer
Very simple 6802 computer using only 3 chips - also includes 2 examples of using it to control traffic lights and play dice. Don't worry about the pop-ups for the asm files - they're just program listings for the 2 examples. Open in Wordpad (or simil
- Jameco Electronics
The 6802 microprocessor has been out of production for a long time and they're kind of hard to find. You can get them from Jameco Electronics but they've been pulled from working circuits (they're not "new"). If anyone knows of another source please
Intel 8051 (and Dallas Semiconductor, etc...)
Yes, a Harvard Architecture chip made the list...
I'm starting to like the 8051 more and more. I know it's been described as having a "retarded architecture" and at first I didn't like the fact the chip can't write to code memory (making self-modifying code, or expandable interpreters impossible to implement). The thing is though, for the tasks it's designed to do, it has a great architecture, and there are techniques to allow the 8051 to share code and data memory, so my FORTH interpreter is possible after all.
The most advanced 8 bit CPU...
Motorola's 6809 was the most advanced 8 bit CPU made before the industry moved to 16 bit (and larger) CPUs.
Bringing a Computer to Life