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)
“There is no philosophy without the art of ignoring objections.”
—Joseph De Maistre (1753–1821)
“The truth is, I do indulge myself a little the more in pleasure, knowing that this is the proper age of my life to do it; and, out of my observation that most men that do thrive in the world do forget to take pleasure during the time that they are getting their estate, but reserve that till they have got one, and then it is too late for them to enjoy it.”
—Samuel Pepys (1633–1703)
“The director is simply the audience. So the terrible burden of the director is to take the place of that yawning vacuum, to be the audience and to select from what happens during the day which movement shall be a disaster and which a gala night. His job is to preside over accidents.”
—Orson Welles (1915–1984)
“In progress of time, when my mind was, as it were, strongly impregnated with the Johnsonian æther, I could, with much more facility and exactness, carry in my memory and commit to paper the exuberant variety of his wit and wisdom.”
—James Boswell (1740–1795)