Editing Commodore CBM-II (section)

==Technical 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 often referred to as the "[[Porsche]] PETs" due to incorrect rumors that the case was designed by Porsche. Though Commodore did initially consult Porsche for a case design, it proved too expensive to produce, so Commodore enlisted designer Ira Velinski to create one based on the original PET prototype.<ref>{{Cite web|url=https://dfarq.homeip.net/commodore-256-the-8-bit-that-never-was/|title = Commodore 256: The 8-bit that never was|date = 21 July 2017}}</ref><ref>{{Cite web|url=http://www.6502.org/users/sjgray/computer/cbm2/index.html|title=The Commodore CBM-II Page}}</ref><ref>{{Cite web|url=http://www.floodgap.com/retrobits/ckb/secret/b128.html|title=Secret Weapons of Commodore: The 6509s: The P500, B128/256, B500, 600, 700, BX128/256}}</ref><ref>[http://www.old-computers.com/MUSEUM/computer.asp?c=746&st=1 Commodore PET 200 at old-computers.com] {{Webarchive|url=https://web.archive.org/web/20101121035210/http://www.old-computers.com/museum/computer.asp?st=1&c=746 |date=2010-11-21 }}, accessed 25 August 2014</ref>

The P-series uses the [[MOS Technology VIC-II|VIC-II]] 40-column color video chip like the [[Commodore 64]] (C64). It also includes two standard [[Atari]]-style joystick ports. The [[MOS Technology 6509|6509 CPU]] runs at 1&nbsp;MHz in the P-series due to the use of the VIC-II chip.

Both B and P-series machines have a [[MOS Technology 6581|6581 SID]] chip, although the B-series' 2&nbsp;MHz clock speed makes it impossible to read any of the SID's registers.

The B-series uses a [[Motorola 6845#Differences from the 6545|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 6845|Motorola 68B45]] installed which is a pin-compatible variant rather than the MOS 6545A1 2&nbsp;MHz part. On the B-series the 6509 CPU runs at 2&nbsp;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 [[MOS Technology 6522|6522]] chips) but there is no connector for it on the motherboard.

A [[ROM cartridge|cartridge]] slot is also included on the machines.  JCL Software released both a BASIC extender and an assembler/editor in cartridge format.<ref>[http://6502.org/documents/publications/cbug_escape/cbug_escape_issue_04_summer_1986_part_2.pdf The CBUG Escape, issue 4, Summer 1986, part 2], accessed 16 February 2025</ref>

The B-series retained the [[IEEE-488]] interface from the PET instead of the [[Commodore bus|IEC serial]] interface on the [[VIC-20]] and C64.  The small amount of software Commodore developed for the B-series was distributed on 500k [[Commodore 8050|8050]] format disks rather than the 170k [[Commodore 4040|4040]]/[[Commodore 1541|1541]] format.

Features common to both the P and B-series included a [[MOS Technology 6509]] CPU, an enhanced version of the venerable [[MOS Technology 6502|6502]], that was capable of addressing up to 1 [[megabyte]] of [[random access memory|RAM]] via [[bank switching]] (however, no CBM-II model came with more than 256 [[kilobyte]]s 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&nbsp;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 [[Commodore BASIC|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 [[Zilog Z80|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 [[bank switching|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&nbsp;KB models, Bank 2 contains all BASIC variables, however on 256&nbsp;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 [[interrupt]]s, as it is not possible to "far call" code across different memory banks. In theory, if Banks 5–14 were filled, up to 1&nbsp;MB of RAM could be installed, but in practice the computer is limited to 256&nbsp;KB as the [[Programmable logic device#PLA|PLA chip]] does not have enough [[address line]]s for it. Passing data between different banks requires use of the register at $1 which selects the banks to be read from and written to via LDA (zero page),Y and STA (zero page),Y CPU [[opcode]]s. The register at $0 selects the bank where the CPU fetches instructions from, which will default to 15 ($F) on power up.

The BASIC included with the CBM-II series is known as BASIC 4.0+. It contains the enhanced BASIC 4.0 disk commands as well as a few other added features for structured programming and error trapping.

Commodore had plans for a wide variety of RAM configurations as well as slimline and "high profile" models. The slimline machines were all-in-one units while the high-profile models had a separate keyboard and attached swivel-mount monitor. Both slimline and high-profile models have a connector for internal floppy drives and coprocessor boards, although only the latter have physical room in the case and a sufficient power supply for them. No production machines had internal floppy drives, however these likely would have been a half-height variant of the 8050. Most of the planned CBM-II models never made it past the prototype stage. Originally, there would be four models in each series, with memory increasing in 64&nbsp;KB increments. 64k and 192k models were dropped early in development and no such models were produced.

In the end, the only CBM-II models to enter production and be sold to end users were the B128/600, B256/700, CBM-128/710, and [[PAL]] model P500s. The B128 and P500 were slimline models with no internal disk drives or attached monitor while the CBM 128 and B256 were high-profile models. The B128 was the biggest selling of the three.

[[File:Commodore CBM 610 (Thomas Conté) A.jpg|thumb|A Commodore CBM 610, the European version of a Commodore B128]]
[[File:CBM610 back (filtered).jpg|thumb|Connectors on the back of a CBM 610<br>[[Reset button|reset]], [[RS-232|RS-232C]], [[Commodore Datasette|datasette]], cartridge 44-pin, [[Ground (electricity)|GND]], audio, [[Mains electricity|mains]], [[IEEE-488]], power switch, [[Fuse (electrical)|fuse]] ]]
[[File:1-1111 CBM610 04.JPG|thumb|Mainboard and power supply of a CBM 610]]
The production naming within the [[United States]] and [[Canada]] was the B128/B256 and CBM128-80/CBM 256-80 while in [[Europe]] they were known as the '''600''' and '''700''' series respectively (no "B" in front of the model number). The P machine was known worldwide as the '''500''' series.  There are prototype models though such as the B500 (earlier B128 design) and B700 (earlier CBM 128-80/CBM 256-80 design) known to exist.

{| class=wikitable
|+ Model variants<ref>{{cite web|title=Under the Hood of the CBM 610 |date=2012-11-09 |accessdate=2016-04-28 |url=http://www.von-bassewitz.de/uz/oldcomputers/cbm610/}}</ref>
! Model !! RAM !! co-processor
|-
| 610 || 128 KB ||
|-
| 620 || 256 KB ||
|-
| 630 || 256 KB || co-processor card
|-
| 710 || 128 KB ||
|-
| 720 || 256 KB ||
|-
| 730 || 256 KB || co-processor card
|}