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A single Atlas word can be used to represent any of the following:
A 48-bit floating or fixed point number, with the 8 most-significant bits representing the exponent and the other 40 the mantissa.
Two 24-bit half-word numbers. These are taken usually as 21-bit signed integers in digits 0-20, with an octal fraction in digits 21-23.
Eight 6-bit characters.
An instruction, specifying a function F (most-significant 10 bits), two index registers Ba and Bm (7 bits each) and an address N (least-significant 24 bits).
The 23-bit main store address consists of
The bits 12-20 define the address of an instruction within a 512-word page while the block address uniquely defines the address within the 1-level storage system. Blocks are moved from drum to store as required under the control of the Supervisor.