Note this only appplies to the Models AandB as the Master had configuration set in NVRam.

One of the BBC's initial listed requirements for their computer to Acorn was a default of Mode 4 :


A set of internal links will define the input, output and storage channels, and the screen display option, to be used on machine start-up. Default options will be as follows:

Console input - Keyboard.

Console output - VDU display in mode 4.

File input - Cassette recorder, high speed.

File output - Cassette recorder, high speed "

Source ( as well as other sites )


I do realise the tape control speed was moved to an OS command and the links on production machines control the startup mode and disc drive timings.

The question here is simply -why- was the Mode 4 requirement dropped ?

You could say default available memory vs Mode 7 however any program worth it's salt would start by selecting a Mode. A standard Model A does support this mode although memory would be a bit sparse for Basic unless it was changed.

Also if you look around at various keyboard pics which is where the option links or dip switches would live, it seems very few of any at all early models had these fitted. Back then I saw numerous new machines and I do not recall seeing any with the dip switch fitted and none with a default mode other than 7 :)

  • 2
    What the links actually do in production machines, for the curious: beebmaster.co.uk/BeebHelp/ConfigBBCB.html — in short, three for screen mode, one for whether to try to boot from the filing system automatically, two for disk drive stepping rate, one for network FS versus disk FS by default. So possibly the first home machine that could be set to net boot?
    – Tommy
    Commented Jan 14, 2020 at 13:29
  • Yes I forgot to mention one controls the shift break operation too.
    – AndyF
    Commented Jan 14, 2020 at 14:23
  • This link is a bit more readable, as it's a scan of the original document: favorite texts BBC Microcomputer System Technical Description. Another spec intentional document said that BBC BASIC had to be fully compatible with Microsoft MBASIC, and we know how that ended …
    – scruss
    Commented Jan 14, 2020 at 14:41
  • 1
    Probably not a consideration, but who knows: if you're going to have DIP switches, which may or may not be present, then either Mode 0 or Mode 7 is most natural default. Otherwise you're going to have to say something like "these three switches select the default graphics mode, ummm, XOR 4".
    – Tommy
    Commented Jan 14, 2020 at 16:39
  • Trouble is a 16K machine cannot use modes 0 to 3 as they eat too much ram :) Forcing such a Mode results in a "Bad Mode" error message hehe. To be fair to both of us a cursory look shows that machines still with only 16K are quite rare as it appeared to be one of the most popular (and easy to do to!) Model A upgrades.
    – AndyF
    Commented Jan 14, 2020 at 16:45

1 Answer 1


I expect the default screen mode was changed to 7 precisely because it used less memory. For text display purposes, it has the same number of rows and columns as well as the ability to handle colour, but uses only a tenth of the RAM - on the Model A, that would have been a serious concern.

A consequence of this is that a program of any size can be loaded using Shift-BREAK, without having to potentially make room for it first by changing screen mode. This would have been an annoying and potentially confusing extra step for many of the machine's intended users, which included young children.

  • MODE 7 also has clearer text (higher resolution: 10×18, as compared with the 8×8 in all other modes), so it's significantly easier to read.  Plus it allows 8 colours.  Can't do arbitrary graphics, but as a default mode for managing the system and entering commands and code, it seems the best choice.
    – gidds
    Commented yesterday

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