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Image:BBC Master.jpeg
BBC Master Series
Type	8-bit Microcomputer
Released	Early 1986
Discontinued	1994
Processor	MOS Technology 65C12, optional Intel 80186
Memory	128 kB - 512 kB
OS	Acorn MOS, optional DOS Plus

The BBC Master was a home computer released by Acorn Computers in early 1986. It was designed and built for the British Broadcasting Corporation (BBC) and was the successor to the BBC Micro Model B.

The Master featured several improvements on its predecessor. The systems had 128 KiB RAM as standard, alleviating the shortage of available RAM which inter alia discouraged use of the best graphics modes in the original design, and had two cartridge slots mounted above the numerical keypad. Rather than the MOS Technology 6502 microprocessor used by the Model B it ran on the slightly improved 65C102: the cost of this CPU compatibility with the Model B was that the address bus was still only 16 bits, meaning that only 64 KiB could be directly addressed at any one time and the remaining memory had to be paged in as required. However the 65C102's extra instructions allowed a little more to be shoehorned into the OS and BBC BASIC ROMs, limited by the memory architecture to 16 KiB each.

Although the Master was intended to be compatible with "legally written" software for the older models, there were some problems running older programs, particularly games. Conversely, although few programs were ever targeted specifically at Master series machines (except the Master 512), many later BBC games (and Master versions of earlier classics such as Elite) included enhanced features which took advantage of the extra memory.

The BBC Computer Literacy Project Owl appeared on the bottom left of the keyboard on both standard and Master Compact cases.

The Master was available in several different models.

This was the standard issue computer. The 128 in the name referred to its 128 KiB of RAM, though it also featured 128 KiB ROM.

This was a Master with 4 MHz 65C02 second processor card (which could be either bought with the machine or added to an existing Master 128).

The ET (Econet Terminal) system was designed for use in a network and as such had no interfaces except RGB and Composite video, plus an Econet interface module and ANFS fitted as standard (it was usually an option). It used the same main circuit board as the Master 128, but the components for missing interfaces were simply not fitted (though there was nothing stopping them being added later by someone with appropriate soldering skills). The internal ROM also contained much less software than that of the Master 128.

This system boasted a second processor card with a 10 MHz Intel 80186 and 512 KiB memory. It also had the ability to run DOS+ and the GEM graphical user interface.

This model separated the keyboard from another unit which could be placed under the monitor. Only the ADFS filing system was supplied as standard, though it is possible to load a 1770 DFS ROM into sideways RAM, or to insert a ROM or EPROM containing it. The Master Compact also utilised a limited re-burn EEPROM, instead of the battery backed CMOS memory found in the other models.

The unit under the monitor housed a 3½” floppy disk drive and the system power supply. The remainder of the system was housed in the same unit as the keyboard, much like a conventional Master 128. The cartridge and cassette ports were removed as a space saving measure. The loss of the latter was a move Acorn later came to regret. Software for the Compact became very expensive (typically £20 for a game) due to the small user base.

The Master Compact GUI

The Compact included a copy of Acorn's first public GUI interface. Little commercial software or utilities, others than those included on the Welcome disk were ever made available for the system.

The BBC Master Turbo was the basic hardware platform required to run the 1986 BBC Domesday Project. Also required were a SCSI interface and videodisc player, both of which were designed specifically for this application.

• 2 MHz Rockwell R65C102 processor
• 128 KiB ROM, consisting of 16 KiB MOS (Machine Operating System), always accessible, and seven 16 KiB ROMs, any one of which could be paged into memory at a time:
  • 16 KiB BBC BASIC
  • 16 KiB Disk Filing System
  • 16 KiB Advanced Disk Filing System
  • 16 KiB Acornsoft View (word processor)
  • 16 KiB Acornsoft ViewSheet (spreadsheet)
  • 16 KiB EDIT (editor)
  • 16 KiB MOS extras (such as the cassette filing system)
• 128 KiB RAM, comprising:
  • 32 KiB main user program/data storage
  • 20 KiB video memory (paged over main user RAM)
  • 12 KiB OS workspace (paged over main user RAM)
  • 64 KiB workspace accessible to user machine code applications (in four 16 KiB blocks any one of which could be paged into memory if BBC BASIC and the other paged ROMs were not required)
• Full-travel keyboard with a top row of ten red-orange function keys  f₀ − f₉ and AT-style numeric keypad
• Highly configurable graphics display based on the Motorola 6845. Unlike on the original BBC Micro, separate video RAM was used so that choosing a high-resolution mode did not reduce the amount of available user RAM. (However, user RAM could still be used as the video buffer if required, in order to allow effects such as double buffering.) Eight graphics modes were provided by the system ROM:
  • Modes 0 to 6 could display a choice of colours from a logical palette of sixteen, though only eight physical colours could really be generated by the hardware; the eight RGB colours (black, red, green, yellow, blue, magenta, cyan, white) and said colours in a flashing state;
  • Mode 7's Teletext capability was provided by a Mullard SAA5050 Teletext chip.

• Four independent sound channels (one noise and 3 melodic) using the Texas Instruments SN76489 sound chip
• Built-in hardware support included:
  • pluggable ROMs, directly or via cartridge slots
  • floppy disk drives (both DFS and the newer ADFS supported) with WD1770 disk controller
  • tape interface (with motor control), using a variation of the Kansas City standard data encoding scheme
  • Centronics parallel printer
  • serial communication (using RS-423, a superset of RS-232)
  • display output for TV, RGB or 1v p-p video monitor
  • A 15 pin 'D shaped' port with four analogue inputs (suitable for two joysticks, four digital/contact ports (for buttons) and a special Light pen input, and
  • proprietary "Tube" interface for internal or external second CPU (in the Master 512 model, an 80186 was used; other options included a 3 MHz extra 6502, a Zilog Z80 for e.g. CP/M, an NS32016, an ARM1, and others)
  • a 16 pin IDC style "user port" consisting of 8 general purpose digital I/O pins (and two special ones) mapped directly into the 6502 via, and
  • generic expansion through the "1 MHz bus".
  • Econet interface, installed by adding a module board and the ANFS ROM.

Several of the inputs were directly wired to specific registers in order to allow the hardware to do some of the heavy lifting. For example the light-pen input would directly halt a counter which was started by the start of the vertical sweep of each display refresh, making calculation of where the lightpen was touching the screen little more than a simple divide/remainder operation. Likewise, the motor control relay for the audio cassette tape was controlled by a simple command and could be readily used in numerous applications.