2.9 Page Description Register

The page description register defines:

(a)

the size of the lower part of the page. (The number stored is actually the number of 32 word blocks less one);

(b)

a bit which is set when the upper part is the valid part. (Also known as the “expansion direction” bit);

(c)

access mode bits defining “no access” or “read only access” or “read/write access”.

Note that if the valid part is null, this fact must be shown by setting the access bits to “no access”.