BBC Master

Infobox computer
Name = BBC Master Series
Photo =

Type = 8-bit Microcomputer
Released = Early 1986
Discontinued = 1994
Processor = MOS Technology 65SC12, optional Intel 80186 or 65C102 depending on model
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 KB RAM as standard, alleviating the shortage of available RAM which had amongst other things discouraged use of the best graphics modes in the original design, and had two cartridge slots mounted above the new numerical keypad. Rather than the MOS Technology 6502 microprocessor used by the Model B it ran on the slightly improved 65SC12 [As shown on pictures of the Master 128 motherboard http://www.bbcmicro.net/old-8bs/insides.htm#m128] : the cost of this CPU compatibility with the Model B was that the address bus was still only 16 bits, meaning that only 64 KB could be directly addressed at any one time and the remaining memory had to be paged in as required. However the 65SC12's extra instructions allowed a little more to be shoehorned into the OS and BBC BASIC ROMs, limited by the memory architecture to 16 KB 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.

Models

The Master was available in several different models.

Master 128

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

Master Turbo

This was a Master with 4 MHz 65C102 second processor card (which could be either bought with the machine or added to an existing Master 128).

Master AIV

The Master AIV (Advanced Interactive Videodisc) was essentially a Master Turbo model with a SCSI interface and a VFS (Videodisc Filing System) ROM added, and formed the basis of the BBC Domesday System. Although normally supplied as part of a Domesday System, with LaserVision player, Domesday videodiscs, monitor and trackerball included, an upgrade kit was also available to turn a normal BBC Master into a Domesday System.

Master ET

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.

Master 512

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

Master Compact

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 Compact also utilised a limited re-burn EEPROM, instead of the battery backed clock plus CMOS memory found in the other models, and hence had no real time clock of its own (the time could be fetched via Econet where available).

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, and RS-232 hardware not populated on the circuit board as standard - the loss of the latter was a move Acorn later came to regret. A multifunction mouse and joystick port was provided as a 9 pin D type with its function configured in software.

Software for the Compact became very expensive (typically £20 for a game) due to the much lower demand for the 3½” disk format (5¼" was the de facto standard for the Master and earlier BBC Micro).

The Compact included Acorn's first public GUI. Little commercial software, beyond that included on the Welcome disk, was ever made available for the system.

Notable applications

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.

Specifications

*2 MHz Rockwell R65SC12 processor
*128 KB ROM, consisting of 16 KB MOS (Machine Operating System), always accessible, and seven 16 KB ROMs, any one of which could be paged into memory at a time:
**16 KB BBC BASIC
**16 KB Disk Filing System and Sideways RAM utilities
**16 KB Advanced Disk Filing System
**16 KB Acornsoft View (word processor)
**16 KB ViewSheet (spreadsheet)
**16 KB Acorn Screen Editor AKA Edit (text/BBC BASIC editor)
**16 KB Terminal emulator and MOS extras (such as the cassette filing system)
*128 KB RAM, comprising:
**32 KB main user program/data storage
**20 KB "shadow" video memory (paged over main user RAM)
**12 KB OS workspace (paged over main user RAM)
**64 KB workspace accessible to user machine code applications (in four 16 KB "Sideways" 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 $f\_0-f\_9$ and AT-style numeric keypad. The 'BREAK' reset key could be disabled by rotating a small plastic cam, particularly useful in educational environments
*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;
** Modes 3 and 6 were special software (framebuffer) text modes. To save RAM, the count of lines was reduced from 32 to 25. As this would reduce the height of the frame, filler rows were created between each line of text when the frame was output, where no pixels were read from the framebuffer. This creates characteristic black lines between the rows of text when a different background colour is set, and a blank gap at the bottom of the display with the left-over pixels. The screen mode is otherwise held in memory as a regular graphics mode.
**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
**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 handshaking ones) mapped directly into the 6522 VIA
**generic expansion through the "1 MHz bus", and
**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 control applications.

References