Commodore Plus/4

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Commodore Plus/4
Commodore Plus/4
Type: Home Computer
Producer: Commodore
Price: 299 US$
Released: 1984
Discontinued: 1986
Processor: MOS 7501/8501 @ 0,885/1,76 MHz
Memory: 64 KByte, upgradeable
OS: BASIC 3.5
Info: Include office software
part of the Commodore-264 series

The Plus/4 was a Commodore home computer sold from 1984 to 1986. It featured a compact plastic casing and a fully-fledged keyboard. The cursor keys and function keys were made of rubber. The Plus/4 was similar to the C16 and C116, and it belonged to the Commodore 264 series, which were all software compatible. The original price was about $300.

Commodore produced about 827,000 units worldwide, with about 286,500 sold in Germany. It was also available with a modified Plus/4 logo (+4=) for the Canadian market and with additional Hungarian characters for the Hungarian market.

Compared to the VIC-20 and C64, the operating system of the Plus/4 had an advanced BASIC (version 3.5), which meant that the software for the Plus/4 was not compatible with the C64. Also incompatible were the peripheral devices, including the datasette 1531, disk drive 1551, and joysticks. However, a simple adapter was available for the datasette and joysticks.


Built-in Software[edit | edit source]

Start screen of a Commodore Plus/4.

The Plus/4 was one of the first home computers to feature integrated user software in ROM. This was one of the first office suites.

  • Word processor: Start with F1 +RETURN  or SYS1525 (plus <RETURN>). In the Plus/4 software area you can change by using the key combination C= +C  and the following input of tw (plus <RETURN>; shortcut for "to Word" ) in the command mode to get back to the word processor.
  • Spreadsheet: Start with F1 +RETURN  or SYS1525 (plus <RETURN>), then press C= +C  and enter tc (plus <RETURN>; shortcut for "to the Calculator" ) in command mode.
  • Additional graphics option for the spreadsheet, i.e. creation of block- or scatter diagrams with data from the tables.
  • Database: Start with F1 +RETURN  or SYS1525 (plus <RETURN>), then press C= +C  and enter tf (plus <RETURN>; shortcut for "to File Manager") in the command mode.
    Important: Before the first use of the database a special file for the database management must be created.

The built-in machine code monitor TEDMON is part of the BASIC V3.5. It can be called with SYS 0 or MONITOR.

Constructon[edit | edit source]

Right side[edit | edit source]

  • Reset button
  • On/Off switch

Rear view[edit | edit source]

Plus/4 rear view
  • Power connector (power; 4 pins, square) for the power supply unit. PSU basically compatible to C64, but uses different (square) connector.
  • Serial bus resp. Serial Bus (IEEE-488, 6 pin, I/O) to connect a disk drive 1541/70/71/81 or printer MPS-801/802/803, VIC-1515/1525/1526
  • Cassette connector (cassette port round Mini-DIN, 7-pins; E/A) to connect a cassette recorder (Datasette) 1531
  • User Port (RS-232; 24-pins; I/O) used to connect expansions, not compatible with the user port of the C 64. The used MOS 6551 ACIA of the Plus/4 supports data transfer up to 19.200 baud in hardware. The MOS 6529 provides 8 port lines
  • Memory expansion; (50 pins; I/O) to connect cartridges or the disk drive 1551
  • Two control ports (Joy0 and Joy1, round mini-DIN, 8 pins, I) to connect a Joystick
  • Audio-/Video (8 pins; O) to connect a monitor and/or a stereo

Left side[edit | edit source]

  • TV connector, (HF modulator output; O) to connect a television (in the versions NTSC or PAL)


Mainboard[edit | edit source]

Plus/4 box
  • CPU: MOS 7501 or 8501 with 0.885/1.78MHz clock frequency (controlled by TED) - average approx. 1.1 MHz.
  • Memory: 64 KByte RAM, with 60671 bytes free usable for BASIC
  • ROM: 2×32 KByte in total, divided to:
    • CBM BASIC V3.5 (16 KByte) ($8000-$BFFF; 32768-49151)
    • KERNAL (14 KByte) ($D800-$FFF9; 55296-65529)
    • Character set (2 KByte) ($D000-$D7FF; 53248-55295)
    • ROM-Chip with integrated office software
  • Graphics: TED 8360
    • Display in text mode (either uppercase characters & block graphics or upper- & lowercase characters) 40 (rows) × 25 (columns) characters or in graphic mode 320×200 Pixel or 160×200 pixel. Also display graphics with 5 rows of text at 320*160 or 160×160 pixel, additional Shapes in BASIC
    • Colors: 121 (15 colors in 8 shades and black)
  • Sound: TED 8360 is used as well for sound generation and can create 2 independent voices with tones or sound effects covering 4 octaves (approx. 28 tones without intermediate tones).
  • Keyboard: 67 keys (QWERTY or QWERTZ), with multi-assignments, function key are already predefined.
  • Others:
    • Dimensions: 335mm × 202mm × 65mm (13.19in × 7.95in × 2.56in)
    • Weight: approx. 1 kg (2.20 lbs)
    • Power supply: 9 V AC and 5 V DC via power supply unit

Block diagram[edit | edit source]

Plus4 Blockschaltbild.png


ROM listing[edit | edit source]

RAM chip[edit | edit source]

 Memory addresses          | Description
Hexadecimal | Decimal     |
$0000-$07FF | 00000-02047 | Zero page, system RAM
$0800-$0BFF | 02048-03071 | Color RAM
$0C00-$0FFF | 03072-04095 | Video RAM
$1000-$FD00 | 04096-64768 | BASIC RAM begin (with active graphics this area will be moved up!)
$1800-$1BFF | 06144-07167 | Luminance (with active graphics)
$1C00-$1FFF | 07168-08191 | Color table (with active graphics)
$2000-$3FFF | 08192-16383 | Video ram for graphics (with active graphics)
$4000-$FD00 | 16384-64768 | BASIC RAM begin (with active graphics)!
$FD01-$FFFF | 64769-65535 | free

ROM-Chip[edit | edit source]

 Memory addresses          | Description
Hexadecimal | Decimal     |
$8000-$BFFF | 32768-49151 | BASIC V3.5
$C000-$CFFF | 49152-53247 | free for BASIC extensions
$D000-$D7FF | 53248-55295 | Char set
$D800-$FBFF | 55296-64511 | KERNAL
$FC00-$FCFF | 64512-64767 | ROM banking routines
$FD00-$FEFF | 64768-65279 | I/O addresses
$FF81-$FFF9 | 65409-65529 | KERNAL
$FFFA-$FFFB | 65530-65531 | NMI vector
$FFFC-$FFFD | 65532-65533 | RES vector
$FFFE-$FFFF | 65534-65535 | IRQ vector

Deviations color table and control characters[edit | edit source]

The commodore color table deviates on the TED beginning with the 9th color (brown):

Color       | Key com-   | Color value  | In program:  | Or by PRINT "control character"
            | bination   | for POKE     | PRINT CHR$(X) | (in quotes!)  
yellow-green| <C=>+<3>   | 090          | 150           | inverse graphic X  
Pink        | <C=>+<4>   | 107          | 152           | inverse ring 
Blue-green  | <C=>+<5>   | 092          | 151           | inverse clubs
Light blue  | <C=>+<6>   | 109          | 153           | inverse vline right    
Dark blue   | <C=>+<7>   | 046          | 154           | inverse check  
Light green | <C=>+<8>   | 095          | 155           | inverse clubs

Hint: There are some new control characters on a Plus/4:

PRINTCHR$(027) Escape key
PRINTCHR$(130) Text flashing on
PRINTCHR$(131) Text flashing off
PRINTCHR$(140) Help key


Function Keys[edit | edit source]

The Plus/4 function keys are usually:

  • F1  SYS 1525: 3-PLUS-1 (starts the built-in 3-Plus-1-Office-Software)
  • F2  DLOAD+CHR$(34)
  • F3  DIRECTORY+CHR$(13) (the key RETURN  is hit)
  • SHIFT +F4  SCNCLR+CHR$(13)
  • SHIFT +F5  DSAVE+CHR$(34)
  • SHIFT +F6  RUN+CHR$(13)
  • SHIFT +F7  LIST+CHR$(13)
  • HELP  or F8  HELP+CHR$(13)

To overview or modify function keys can be accomplished through the BASIC-command KEY in direct mode or in programs. After a reset or a reboot are the standard function key configuration active.

Bundles and purchased parts package[edit | edit source]

In Germany the Plus/4 was available as single package and later it was sold in a bundle.

The purchased parts of a Plus/4 package are (1984):

  • Computer (Plus/4)
  • Power Supply
  • Manual Commodore Plus/4 (ring binder)
  • Manual 3+1 Software (ring binder)
  • Aerial cable
  • Commodore warranty card
  • Commodore extra guarantee
  • Interference free certificate
  • Cardboard package
  • Styrofoam shells top and bottom

In Germany the additional content of the bundle Commodore Plus/4 Computer-Lernkurs BASIC-Programmiersprache for approx. DM 250 was (1986):


The additional purchased parts package of the bundle Commodore Plus/4 Plus Pack" outside Germany (Commodore Plus/4 Presentation Pack or Plus/4 Liquidation Package is (1985) for £ 99.95 (Great Britain pounds):

  • Cardboard package of the bundle
  • Datasette 1531
  • Joystick Commodore T1341
  • 10 cassettes with software: Icicle Works, Harbour Attack, Fire Ant, Exorcist, Crazy Golf, Mayhem, Number Builder, Number Chaser, Space Sweep, Space Invaders and Treasure Island

The additional purchased parts package of the bundle Commodore Plus/4 Das Komplett-Paket mit Floppy-Disk 1551 for approx. DM 500 is (1986/87):

  • Cardboard package of the bundle
  • Serial cable
  • Commodore warranty card for the Floppy 1551
  • Interference free certificate for the floppy 1551
  • Styrofoam shells top and bottom for the floppy 1551

Links[edit | edit source]

WP-W11.png Wikipedia: Commodore_Plus/4