System Specification for C65
March 1, 1991
1.4 System Concerns
1.4.1 C64 Compabibility Issues
C64 software compatibility is an important goal. To this end, when the system is in C64 mode the processor will operate at average 1.02MHz speed and dummy "dead" cycles are emulated by the processor. The C64 ROM is the same except for patches to serial bus routines in the kernel (to interface buxlt-in drive), the removal of cassette code (there is no cassette port), and patches to the C64 initialization routines to boot C65 mode if there is no reason (e.g., cartridges) to stay in C64 mode.
Compatibility with C64 software that uses previously unimplemented 6502 opcodes (often associated with many copy-protection schemes) or that implements extremely timing dependent "fast loaders" is inherently impossible. Because the VIC-III timing is slightly different, programs that are extremely timing dependant may not work properly. Also because the VIC-III does not change display modes until the end of a character line, programs that change displays based strictly upon the raster position may not display things properly. The aspect ratio of the VIC-III display is slightly different than the VIC-II . The use of a 1541-II disk drive (optional) will improve compatibility. C64 BASIC 2.2 compatibility will be 100% (within hardware constraints). C128 BASIC 10 compatibility will be moderate (graphic commands are different, some command parameters different, and there are many new commands).
C64 hardware compatibility is limited. Serial bus and control port devices (mouse, joysticks, etc.) are fully supported. Some user port devices are supported such as the newer (4-DIP switch) 1670 modems, but there is no 9VAC so devices which require 9VAC won't function correctlv The expansion port has additional pins (50 total), and the pin spacing is closer than the C64 (it's like the PLUS/4). An adaptor ("WIDGET") will be necessary to utilize C64 cartridges and expansion port devices Furthermore, timing differences between some C64 and C65 expansion port signals will affect many C64 expansion devices (such as the 1764 ).
1.4.2 DOS Compatibility
The built-in C65 DOS is a subset of Commodore 1581 DOS. There is no track cache, index sensor, etc. To load and run existing 1541-based applications, the consumer must add a 1541 drive to the system. Many commercial applications cannot be easily ported from 1541/5.25" media to 1581/3.5" media, due to copy protection or "fast loaders". Most C64 applications that directly address DOS memory, specific disk tracks or sectors, or rely on DOS job queues and timing characteristics will not work with the built-in drive and new DOS.