Which 6502?

The original 6502 was made in 1975, by MOS Technology. Throughout the 70s there were a number of transistor technologies that were used for microprocessors: PMOS, NMOS and CMOS.

Compared to NMOS and CMOS, PMOS had a number of disadvantages: it used more energy, as power was always running through it; PMOS often needed two positive voltages, a negative voltage and a ground; and PMOS required a strong clock circuit.

NMOS can be further broken down into enhancement-load and depletion-load. Ultimately, the move to depletion-load NMOS brought many improvements, including the ability to only need one power supply. Further improvements arrived when CMOS was used. The original 6502 had a maximum speed of 1 MHz. The latest iteration can work up to speeds of 14 MHz! WDC (Western Design Centre) makes the modern CMOS version and are known as 65C02, the C denoting the use of CMOS technology.

I will be using a modern 6502. There are a number of reasons for this:

  • It is ‘static’, the original is ‘dynamic’. With a static 6502 you can stop the clock and single-step it, allowing you to see what’s on the data bus. This cannot be done with a dynamic 6502 because its internal registers would lose their data!
  • It has the potential to be used up to 14MHz.
  • The original 6502 contained a number of bugs and undocumented instructions. These were fixed with the CMOS version.

BEWARE

There are many fakes out there! This is not a problem just with 6502s. Places like eBay are a great place to buy electronics but please be careful, there are many fake ICs out there! You can buy the 6502 from WDC, as well as from many other electronics sites, which will remove the chance of ordering a fake. If you have a device (such as a BBC Master) that you can pull one from, then you can safely use that. However, you wouldn’t be able to benefit from the ability to stop and single-step the clock, as well as having to run the computer at a much lower clock speed!