Ignoring The LSBs: Late Select On Carry
The formulation above checks the entire result of an add. However, in a CPU cache decoder, the entire result of the add is a byte address, and the cache is usually indexed with a larger address, in our example, that of an 8-byte block. It is preferable to ignore a few of the LSBs of the address. However, the LSBs of the two addends can't be ignored because they may produce a carry-out which would change the doubleword addressed.
If R and O are added to get some index I, then the actual address Addr is equal to either I, or I + 1, depending on whether R+O generates a carry-out. Both I and I+1 can be fetched if there are two banks of SRAM, one with even addresses and one with odd. The even bank holds addresses 000xxx, 010xxx, 100xxx, 110xxx, etc., and the odd bank holds addresses 001xxx, 011xxx, 101xxx, 111xxx, etc. The carry-out from R+O can then be used to select the even or odd doubleword fetched later.
Note that fetching from two half-size banks of SRAM will dissipate more power than fetching from one full-size bank, since we are switching more sense amps and data steering logic.
Read more about this topic: Sum Addressed Decoder
Famous quotes containing the words ignoring the, ignoring, late, select and/or carry:
“We had won. Pimps got out of their polished cars and walked the streets of San Francisco only a little uneasy at the unusual exercise. Gamblers, ignoring their sensitive fingers, shook hands with shoeshine boys.... Beauticians spoke to the shipyard workers, who in turn spoke to the easy ladies.... I thought if war did not include killing, I’d like to see one every year. Something like a festival.”
—Maya Angelou (b. 1928)
“Dirty old men, ignoring society, continue to follow nature.”
—Mason Cooley (b. 1927)
“This was the Eastham famous of late years for its camp- meetings, held in a grove near by, to which thousands flock from all parts of the Bay. We conjectured that the reason for the perhaps unusual, if not unhealthful development of the religious sentiment here, was the fact that a large portion of the population are women whose husbands and sons are either abroad on the sea, or else drowned, and there is nobody but they and the ministers left behind.”
—Henry David Thoreau (1817–1862)
“Here lies the body of Sir John Guise.
Nobody laughs, and nobody cries;
Where his soul is and how it fares
Nobody knows and nobody cares.”
—Anonymous. From Frobisher’s New Select Collection of Epitaphs (c. 1791)
“I herewith commission you to carry out all preparations with regard to ... a total solution of the Jewish question in those territories of Europe which are under German influence.... I furthermore charge you to submit to me as soon as possible a draft showing the ... measures already taken for the execution of the intended final solution of the Jewish question.”
—Hermann Goering (1893–1946)