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Fixed Store
Extracodes
The Atlas basic instruction set was quite rich. On top of this basic set was a set of extracodes. They were distinguished from the basic instructions by having a 1 in the first postion of the function code rather than a zero. This initiated an automatic entry to one of many built-in subroutines, the choice being determined by the remaining three octal digits of the function number. As their name implied, the extracodes extended the basic order-code, providing additional complicated operations which on earlier machines had been obtained by the use of library subroutines. Functions like sine and square root on Atlas were extracodes.
Figure 1: 34 bits of the Atlas Fixed Store
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On Atlas the Supervisor occupied most of a special store called the fixed store which was a read only store in multiples of 4096 words where binary ones and zeros were represented by ferrite and copper slugs in a wire mesh. As well as the Supervisor, the fixed store also held the extra-codes. Figure 1 shows a fixed store typical module often called a brush. Figure 2 shows the relative size versus a UK pound coin.
Figure 2: 34 bits of the Atlas Fixed Store
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Each brush consisted of 17 columns of two bits which were all bits from different Atlas words. An Atlas word consisted of 1 bit from 48 brushes. In consequence, making changes to the Fixed Store was done lightly as almost certainly any change would result in many brushes having to be be re-engineered.
