Processor: 6502B (2 MHz) with bank switching and enchanced indirect addressing, double stack and zero pages

The Apple III central processing unit is based on a 6502B microprocessor with custom external circuitry that provides a number of enhancements to the normal 6502 instruction set. These enhancements include expanded addressing range, alternate stack and zero pages, and improved indirect addressing that is supported by a separate pointer page.

Although the technical information provided by Apple is somewhat vague, apparently the 6502B is run at 2 MHz during the video blanking intervals and at 1 MHz while the beam is writing information onto your monitor screen. This provides an average speed of about 1.4 MHz, but the screen can be turned off temporarily during program execution to allow the processor to run at its full 2-MHz speed, if desired.

While a normal 6502B can address a maximum of 64K bytes of memory, the Apple III uses bank switching to expand this range to a theoretical maximum of 512K bytes.

A 6502B is just a 6502 rated for 3Mhz.

It looks like they used external logic chips to logically split the 64K address space into three banked areas with some logic to detect zero page and stack access and redirect them to a different pages.

http://apple3.org/Documents/Magazines/AppleIIIExtendedAddressing.html

Whenever the 6502 uses a (ZP),Y addressing mode, and the zero page register ($FFD0) contains a number from $18 to $1F, the "switch box" uses the above-mentioned extra byte (called the "Xbyte") to perform "extended addressing."

The Apple III didn't fail because it has a 6502 in it. It failed because it was badly engineered and hellishly unreliable.

Although the III Plus boosted the installed user base to an estimated 120,000 units, Apple abruptly dropped the line in April 1984 and quietly removed it from the product list in September 1985[

The Apple III didn't fail because it has a 6502 in it. It failed because it was badly engineered and hellishly unreliable.