Acorn Computers Limited BBC A, Hardware Manual, Page: 8 / 47

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The Computer has eight different formats for the screen layout these are called screen modes and are
numbered 0 to 7, each giving different resolutions and colour availability. The screen display is stored
within the machines RAM, the amount required being dependant upon which particular screen mode is
selected. The highest resolution modes require 20K of RAM. Mode 7 is unique, in that it uses a special
teletext character generator IC. The advantages of using this method of character generation are that
high quality text and easy to use graphics are available using only 1K of memory.
The two most common mass storage mediums used with the microcomputer are an audiocassette
system, (models A and B), or a floppy disc system (with the relevant upgrade). The cassette interface is
very tolerant with regard to the quality of the signal and the cassette storage system. Although the
cassette interface used within the BBC Microcomputer is very reliable it still only represents a good
compromise for a data storage and retrieval system. If you intend to use the machine for any serious
work then you will probably find the cassette system rather inadequate. A floppy disc or "Econet" system
will provide a much more suitable solution.
A single floppy disc can store between 100 and 300 Kilobytes of information with a 100-fold decrease in
access time over a cassette based system.
The BBC Microcomputer has an on-board sound generator which provides four independent sound
channels. Three of these channels can be used for music synthesis each one having full envelope
control, attack, sustain, decay and release. The fourth channel is a white noise generator and uses the
same envelope control features as the other three channels.
The machine also has the capability to interface to a great variety of other equipment, through its
numerous Ports, the connectors for which are mainly situated on the underside of the case. These are as
follows: -
Floppy Disc Interface (5 ¼ and 8 inch).
A Centronics type parallel printer interface.
1 MHz Bus Extension I/O.
The Acorn "Tube" second processor port.
An 8-bit parallel "User Port".
The Cartridge ROM system.
The Acorn "Econet" local area network system.
The analogue-to-digital converter.
RS423 (RS232C compatible) serial interface.