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As described e.g. in the Apple IIe Technote #6: 'The Apple II Paddle Circuits' it doesn't work to call PREAD several times in immediate succession. However, so far the Apple II joystick driver did just that in order to read the two joystick axis. Therefore the driver now uses a custom routine that reads both paddles _at_the_same_time_. The code doing so requires nearly twice the cycles meaning that the overall time for a joy_read() stays roughly the same. However, twice the cycles in the read loop means half the resolution. But for the cc65 joystick driver use case that doesn't hurt at all as the driver is supposed to only detect neutral vs. left/right and up/down. CPU accelerators are supposed to detect access to $C070 and slow down for some time automatically. However, the IIgs rather comes with a modified ROM routine. Therefore it is necessary to manually slow down the IIgs when replacing the ROM routine.
cc65 is a complete cross development package for 65(C)02 systems, including a powerful macro assembler, a C compiler, linker, librarian and several other tools.
cc65 has C and runtime library support for many of the old 6502 machines, including
- the following Commodore machines:
- VIC20
- C16/C116 and Plus/4
- C64
- C128
- CBM 510 (aka P500)
- the 600/700 family
- newer PET machines (not 2001).
- the Apple ][+ and successors.
- the Atari 8-bit machines.
- the Atari 2600 console.
- the Atari 5200 console.
- GEOS for the C64, C128 and Apple //e.
- the Bit Corporation Gamate console.
- the NEC PC-Engine (aka TurboGrafx-16) console.
- the Nintendo Entertainment System (NES) console.
- the Watara Supervision console.
- the VTech Creativision console.
- the Oric Atmos.
- the Oric Telestrat.
- the Lynx console.
- the Ohio Scientific Challenger 1P.
The libraries are fairly portable, so creating a version for other 6502s shouldn't be too much work.