6502
The 6502 CPU is an 8-bit processor designed in the mid 70s. It has been used in various home computers and consoles at the time, amongst them the Apple II. The core (or a variation of it) is still in use in some Microcontrollers. Estimates are that more than 6 billion units have been built. Lowering the cost to only $25, it has been one of the most influential microprocessors ever designed, accelerating the trend of making computers affordable.
Contents
What is so great about the 6502?
The 6502 was the first affordable microprocessor. Developed in 1976, it was sold at a fraction of the price of the Intel 8080, helping to create and make "home" computers such as the Apple ][ affordable. Despite its simple design, it achieved an astonishing performance, with a 1MHz 6502 achieving similar performance as a 4 MHz Z80. All of that has been achieved with just about 3200 transistors.
Programming
The 6502 is a real 8-bit machine, with very few registers.
Registers
- PC - 16 bit Program counter
- S - 8 bit stack pointer. Stack is always located in the first page, so from 0x100 to 0x1FF
- ACC - 8 bit accumulator
- X - 8 bit index register
- Y - 8 bit index register
- Flags - 8 bit Flag register
Sweet 16
Some programs have been using Sweet 16, a pseudo processor implemented originally by Steve Wozniak https://en.wikipedia.org/wiki/Steve_Wozniak Sweet 16 was a pseudo 16-bit processor, with 16 16-bit registers. It required 32-bytes in the Zero page of the 6502, to hold the values of these registers and needed less instructions than the 6502 for most programs. Typical programs written in SWEET-16 needed about half the size and 5 to 10 times as much time as pure 6502 assembly code. The Sweet 16 executer was implemented in the ROM of the original Apple ][. 6502 assembler and Sweet 16 could be mixed easily: Sweet 16 code was inlined and simply preceeded by a function call to the executer, which was located at a known address (0xF689 or in Apple speech $F689) could easily be started by a subroutine call. Despite its simplicity, Sweet 16 achieved an amazing code density, which is still hard to beat today. For more information on SWEET-16, see http://www.6502.org/source/interpreters/sweet16.htm
More information on the web
An excellent article exists on Wikipedia: https://en.wikipedia.org/wiki/MOS_Technology_6502 For those really interested in hardware design of a CPU, the Javascript simulator might be interesting http://www.visual6502.org/JSSim/index.html Another really interesting site is Monster 6502: They build a complete, working transistor-scale replica of the classic MOS 6502 microprocessor. https://monster6502.com/ The main designer was Chuck Peddle, https://en.wikipedia.org/wiki/Chuck_Peddle The story of the team behind the chip that launched a revolution ... https://www.team6502.org
