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I'm reading about the Atlas operating system and I came across this:

The Atlas system used a British computer with 48-bit words. Addresses were 24 bits but were encoded in decimal, which allowed 1 million words to be addressed.

Why is it 1 million here if it's encoded in decimal?

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    – D.W.
    Mar 4 at 0:22

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It looks like by "encoded in decimal" what they mean is binary coded decimal (BCD), which is a format which uses 4 bits to encode the numbers 0-9. Obviously this wastes a little, as those 4 bits could also encode 10-15, but that is sacrificed to make a 1:1 mapping between groups of bits and digits of a number in base 10

24-bits divided by 4 bits per base 10 digit yields 6 base 10 digits. 6 digits is enough to count from 0 to 999,999, hence a million adddresses.

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