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Keyboard interaction on the Apple II is entirely CPU-dependent, memory-mapped, and works like this: when a key is pressed, the ASCII code of the character is put at $C000, with the high bit set (a consequence of this is that there is no way for the computer to know that you pushed the shift key, for instance, or to tell the difference between typing Return or typing Control-M). Your program is responsible for checking $C000 from time to time. When the high bit is set, you read the character and access memory location $C010, which clears the $C000 high bit. Interrupts are not used at all, despite the 6502 supporting them, probably because disk and cassette IO is similarly done under CPU control with tight timing tolerances, and getting an interrupt in the middle of writing data would wreak havoc.

Details on page 16 of http://www.classiccmp.org/cini/pdf/Apple/Apple%20II%20Refere...

An exception is the "Reset" key which is hard-wired to the reset pin of the 6502. On early Apple II models, pushing "Reset", which is located just above Return, had disastrous consequences. People used to put a washer under the Reset key to make it harder to push by accident. Magazines published a simple hardware mod that required Control to be held at the same time as Reset, and later versions of the Apple II came with the mod built-in.

The Apple IIe keyboard adds two "apple" keys on both sides of the space bar which are dealt with in a completely different way, and are mapped to the same locations as the two joystick/paddle buttons. This was useful for playing games which required repetitive button-mashing, as it is easier to type a key quickly than to push a joystick button quickly. Since I was used to controlling the joystick with one hand and pushing a key on the keyboard with another hand, I had no problem adapting to early Macintosh software that required shift-click, option-click and command-click to overcome the limitation of a one-button mouse.

"The Macintosh mouse has four buttons, it's just that three of them are on the keyboard."