page_title: Commodore CBM-II
section_title: Technical description
hierarchical_section_title: Commodore CBM-II / Technical description
caption_attribution_description: Commodore CBM 610
context_page_description: The Commodore CBM-II series is a short-lived range of 8-bit personal computers from Commodore Business Machines, released in 1982 and intended as a follow-on to the Commodore PET series.
context_section_description: The CBM-II has two incarnations, the P series (P = personal, or, home use) and the B series (B = business use). The B series was available with a built-in monochrome monitor (hi-profile) with detached keyboard, and also as a single unit with built-in keyboard but no monitor (lo-profile). These machines are known as the "Porsche PETs" for their unique styling.
  The P-series uses the VIC-II 40-column color video chip like the Commodore 64 (C64). It also includes two standard Atari-style joystick ports. The 6509 CPU runs at 1 MHz in the P-series due to the use of the VIC-II chip.
  Both B and P-series machines have a 6581 SID chip, although the B-series' 2 MHz clock speed makes it impossible to read any of the SID's registers.
  The B-series uses a 6545 CRTC video chip to give an 80-column "green screen" monochrome output more suitable for word processing and other business use than the VIC-II's 40-column display. Most models have the Motorola 68B45 installed which is a pin-compatible variant rather than the MOS 6545A1 2 MHz part. On the B-series the 6509 CPU runs at 2 MHz. The joystick ports are not present on the B-series, but the connector is still on the motherboard.
  CBM-IIs are the only Commodore 8-bit machines with an RS-232 port instead of the standard user port. The I/O registers for the user port are still present (as they're an internal function of the 6522 chips) but there is no connector for it on the motherboard.
  A cartridge slot is also included on the machines, however no known cartridge software was developed.
  The B-series retained the IEEE-488 interface from the PET instead of the IEC serial interface on the VIC-20 and C64.  The small amount of software Commodore developed for the B-series was distributed on 500k 8050 format disks rather than the 170k 4040/1541 format.
  Features common to both the P and B-series included an MOS Technology 6509 CPU, an enhanced version of 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 256k as the PLA chip does not have
caption: A Commodore CBM 610, the European version of a Commodore B128
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