the venerable 6502, that was capable of addressing up to 1 megabyte of RAM via bank switching (however, no CBM-II model came with more than 256 kilobytes of RAM). The sound chip is the 6581 SID, the same one that was used in the popular C64 but with some limitations as it was over-clocked to 2 MHz. Additionally, the CBM-II has an industry-standard RS-232 serial interface and an IEEE-488 parallel bus (for use by disk drives and printers) just like the PET/CBM series. The CBM-II's built-in operating system uses an enhanced version of CBM BASIC version 4.0. An optional Intel 8088-based coprocessor board allows the CBM-II series to run CP/M-86 1.1 and MS-DOS 1.25; however, the computers were not IBM PC compatible and very little, if any, software taking advantage of this capability ever appeared. The coprocessor board only runs on high-profile machines due to power supply and mechanical spacing requirements. A Z80 card was also announced if the user wished to run CP/M-80, as well as a Zilog Z8001 board, but none are known to exist. The 8088 board never reached production apparently because of difficulties getting it to work with the system. The CBM-II line uses a complicated RAM banking scheme. Bank 0 contains the video RAM on P-series machines and is empty on B-series machines. Banks 1-4 contain the main system RAM, with Bank 15 containing the system ROMs, cartridge ROM, I/O registers, video RAM on B-series machines, and a small amount of RAM to store system variables. BASIC program text is stored in Bank 1. On 128 KB models, Bank 2 contains all BASIC variables, however on 256 KB models they're spread out among Banks 2–4. However, unlike the later released Commodore 128, the CBM-IIs do not have the system ROMs mapped into banks other than 15, which has very limited program RAM (1k free, plus a 4k expander board could be installed). This meant that software had to include its own OS routines to handle device I/O and interrupts, as it is not possible to "far call" code across different memory banks. In theory, if Banks 5–14 were filled, up to 1 MB of RAM could be installed, but in practice the computer is limited to 256 KB as the PLA chip does not have enough address lines for it. Passing data between different banks requires use of the register at $0 which selects the