Commodore 128 Programmer's Reference Manual page 15

INTRODUCTION
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SOUND
658/: SID Chip
INPUT/OUTPUT
6526: Joystick ports/keyboard scan/cassette
6526: User and serial ports
MEMORY MANAGEMENT
8921: PLA (C64 plus C128 mapping modes)
8922: MMU (Custom gate array)
For details on these and other hardware components see Chapter 16, Commodore
128 Hardware Specifications.
COMPATIBILITY WITH
COMMODORE 64
The Commodore 128 system is designed as an upgrade to the Commodore 64. Accord
ingly, one of the major features of the Commodore 128 design is hardware and software
compatibility with the Commodore 64 when operating in C64 Mode. This means that in
C64 Mode the Commodore 128 is capable of running Commodore 64 application
software. Also, the Commodore 128 in C64 Mode supports Commodore 64 peripherals
except the CP/M 2.2 cartridge. (NOTE: The Commodore 128's built-in CP/M 3.0
capability supersedes that provided by the external cartridge. This cartridge should not
be used with the Commodore 128 in any mode.)
The C128 Mode is designed as a compatible superset to the C64. Specifically, all
Kernal functions provided by the Commodore 64 are provided in the C128 Kernal.
These functions are also provided at the same locations in the jump table of the C128
Kernal to provide compatibility with existing programs. Zero page and other system
variables are maintained at the same addresses they occupy in C64 Mode. This simpli
fies interfacing for many programs.
Providing Commodore 64 compatibility means that the new features of the Com
modore 128 cannot be accessed in C64 Mode. For example, compatibility and memory
constraints preclude modifying the C64 Mode Kernal to support the 1571 fast serial disk
drive. As noted previously, C64 Mode sees this drive as a standard serial disk drive. For
the same reason, C64 Mode does not have an 80-column screen editor, and C64 Mode
BASIC 2.0 cannot use the second 64K bank of memory.