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:
“I read here and there in books, enjoying the examples and ignoring the argument.”
—Mason Cooley (b. 1927)
“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)
“I realized how for all of us who came of age in the late sixties and early seventies the war was a defining experience. You went or you didn’t, but the fact of it and the decisions it forced us to make marked us for the rest of our lives, just as the depression and World War II had marked my parents.”
—Linda Grant (b. 1949)
“Cry cry what shall I cry?
The first thing to do is to form the committees:
The consultative councils, the standing committees, select committees and sub-committees.
One secretary will do for several committees.”
—T.S. (Thomas Stearns)
“Steam was till the other day the devil which we dreaded. Every pot made by any human potter or brazier had a hole in its cover, to let off the enemy, lest he should lift pot and roof and carry the house away.”
—Ralph Waldo Emerson (1803–1882)