Editing BBC Micro Model A/B

{{Infobox computer
| name         = BBC Micro Model A/B
| image        = [[File:BBC_Micro_Front_Restored.jpg|250px]]
| caption      = BBC Micro Model B
| manufacturer = Acorn Computers Ltd
| type         = Home Computer
| release date = 1 December 1981
| discontinued = 1986
| price        = Model A: £235, Model B: £335 (1981, inc. VAT)
| cpu          = MOS Technology 6502A @ 2 MHz
| memory       = Model A: 16 KB RAM, Model B: 32 KB RAM; 32 KB ROM (BASIC, MOS, DFS)
| storage      = Cassette tape, optional 5.25" floppy disk (DFS), ROM cartridge
| display      = Teletext, 640×256 (monochrome), 320×256 (4 colours), 160×256 (8 colours), 80×25 text
| sound        = Texas Instruments SN76489: 3 sound channels + 1 noise channel
| dimensions   = 40 cm × 34 cm × 8 cm
| weight       = 4.5 kg
| os           = Acorn MOS (Machine Operating System) in ROM
| predecessor  = [[Acorn Atom]]
| successor    = [[BBC Micro Model B+]]
| codename     = Proton
| model        = Model A, Model B, Model B+
| logo         = [[File:BBC_Micro_Logo.svg|250px]]
}}

The '''BBC Microcomputer System''' (commonly known as the '''BBC Micro''') was a series of microcomputers designed and built by Acorn Computers Ltd for the BBC Computer Literacy Project. Launched in late 1981, it became a defining educational and home computer in the UK, renowned for its robust design, expandability, and role in British computing history.

== Architecture and Processor ==
At its heart, the BBC Micro employed the '''MOS Technology 6502A''' 8-bit microprocessor, clocked at 2 MHz. The system architecture was designed for flexibility and performance, with a fast system bus and a highly modular layout supporting a wide range of expansion options.

Key features:
* '''CPU''': MOS 6502A @ 2 MHz
* '''System bus''': 8-bit data, 16-bit address (64 KB address space)
* '''ROM''': 32 KB (split between BASIC II, Acorn MOS, and optionally DFS or other utilities)
* '''Custom ULA''': Unified Logic Array for video, keyboard, and I/O timing

== Memory and Storage ==
The BBC Micro was available in two main variants:
* '''Model A''': 16 KB RAM
* '''Model B''': 32 KB RAM (most popular)

Both models included 32 KB of ROM, containing:
* '''Acorn MOS''' (operating system)
* '''BBC BASIC II'''
* Optional: Disc Filing System (DFS) or other ROMs in cartridge slots

'''Storage options:'''
* '''Cassette tape interface''' (standard, 1200 baud)
* '''5.25" floppy disk drive''' (optional, via DFS)
* '''ROM cartridge slots''' for instant software loading

== Display and Graphics ==
Graphics and video output were handled by a custom ULA and Motorola 6845 CRTC, supporting a range of text and graphics modes:

{| class="wikitable styled-table" style="width:70%; text-align:center;"
! Mode !! Resolution !! Colours !! Text Columns × Rows
|-
| 0 || 640×256 || 2 (monochrome) || 80 × 25
|-
| 1 || 320×256 || 4 || 40 × 25
|-
| 2 || 160×256 || 8 || 20 × 25
|-
| 3 || 640×256 || 2 (monochrome) || 80 × 25 (text only)
|-
| 4 || 320×256 || 2 || 40 × 25
|-
| 5 || 160×256 || 4 || 20 × 25
|-
| 6 || 320×256 || 2 || 40 × 25 (reduced memory)
|-
| 7 || Teletext (Mode 7) || 8 (Teletext) || 40 × 25
|}

* '''Teletext Mode 7''': Hardware support for the UK's teletext broadcast standard, enabling high-quality text and simple graphics.
* '''Hardware scrolling''', cursor, and palette controls.

== Sound Capabilities ==
Audio was provided by the '''Texas Instruments SN76489''' programmable sound generator, offering:
* '''3 tone channels''' (independent pitch and volume)
* '''1 noise channel'''
* 7-octave range
* Internal speaker (mono), with line-level output

The BBC Micro's sound system was widely used in educational software, games, and music applications.

== Input/Output and Expansion ==
The BBC Micro was designed for expandability, with a comprehensive set of I/O ports and internal connectors:

* '''User port''' (8-bit parallel, TTL-level)
* '''Printer port''' (Centronics parallel)
* '''RS-423 serial port''' (for modems, printers)
* '''Analogue port''' (joysticks, ADC)
* '''RGB, composite, and UHF video outputs'''
* '''Cassette interface'''
* '''Econet network port''' (optional)
* '''Tube interface''' (for second processor expansion, e.g., 6502, Z80, 32016, ARM)
* '''1 MHz bus''' (for peripherals such as ADC, IEEE-488, etc.)
* '''ROM cartridge sockets''' (up to 4 sideways ROMs)

The Model B included all ports as standard; Model A required upgrades for some interfaces.

== PCB Schematics & Service Manual ==
The BBC Micro featured a robust motherboard design, with separate logic and power supply boards in most models.

{| class="wikitable styled-table" style="width:80%; text-align:center;"
! Logic Board
! Logic Board Schematic
|-
| [[File:BBC Micro Motherboard.jpg|center|thumb|321x321px|BBC Micro Model B Logic Board]]
| [[File:BBC Micro Schematic.png|center|thumb|350x350px|BBC Micro Model B Schematic]]
|}
Full service manuals and schematics are available at [[BBC Microcomputer Service Manual October 1985]]

== Gallery ==
<gallery mode="packed" heights="200">
File:BBC Micro Front Restored.jpg|BBC Micro Model B (front view)
File:BBC Micro Model B rear ports.jpg|Rear panel with I/O ports
File:BBC Micro Motherboard.jpg|BBC Micro Model B motherboard
</gallery>

== Related Pages ==
* [[BBC Micro Model A/B General Maintenance]]
* [[BBC Micro Model A/B Troubleshooting Guide]]
* [[BBC Micro Keyboard Repair Guide]]
* [[BBC Micro Model A/B Capacitor Replacement Guide]]
* [[BBC Micro ROM Upgrades]]
* [[BBC Micro Tube Expansion Guide]]

[[Category:Acorn Computers]]
[[Category:BBC Micro]]

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