gwyn@smoke.BRL.MIL (Doug Gwyn ) (11/29/88)
In article <8811251004.aa29471@SMOKE.BRL.MIL> CS656@OUACCVMB.BITNET writes: >>This is nonsense. The 6502 transfers 8 bits of data to/from memory in >>parallel. There is no "first" bit, and which is "leftmost" I suppose >>depends on whether you're looking into the top of your Apple II from >>the left side of the case or the right. >Do the terms ASL and LSR mean anything to you? Sure they do. They're mnemonics that refer to instructions commonly described with reference to a particular way of drawing the little rectangular boxes that represent words made up of bits in hardware manuals. Notice also that the same people who come up with these diagrams often document bit/byte order incorrectly in those manuals, and they can't seem to agree which bit is number 0, let alone whether byte addresses should increase within a word in the same direction as bit numbers. I don't think such documentation should be taken as a model of how to do things! I assure you that the logic circuitry does not actually check against the diagrams to perform its operations correctly. The bit that is wired up in the ALU to have most arithmetic significance is also the one that "falls off the end" on a so-called left shift instruction. That is the CONVENTIONAL DEFINITION of what a "left" shift means. If one did not know that, there would be no rational way of deducing it from the word "left", unlike "most significant" which is unambiguous.