> > > I had read somewhere that a 32 bit processor can > > read/write only and only 32 bits in any one > cycle. > > That's not strictly true, as most 32-bit processors > these days are > multiple-issue, but let's carry on. This conflicts with your later statement of a processor reading a whole word. I presume that the word is 32 bits on a 32 bit processor. Or, is it that they can read more than 32 bits. I was referring to reading atleast 32 bits. > > > 2. When a processor gives an instruction to read > only > > 8 bits, then is it manipulating the 32 bits by > itself? > > No. Is it not manipulating them in the barrel register after reading a word? > > > 3. If the address of my 'char' is 0x5, then do we > only > > read the memory location 0x5, or from 0x4 to 0x7, > and > > then manipulate it to get the value at 0x5 in the > > lowest 8 bits of the 32 bit register. > > OK, this is how it usually works: > > The processor reads a whole word. > > The word is loaded into a barrel shifter (see > http://en.wikipedia.org/wiki/Barrel_shifter) and the > byte we want is > shifted to the correct position in the word. > > The high-order bits in the word are masked to zero > (or the byte is > sign extended). > > Writing a single byte is harder: we either have to > do a > read/modify/write cycle or use byte select lines to > the memory > devices. This is where caches come into play: it is > very likely that > the memory word we want to write is already in the > cache, so even if > we do want to do a read/modify/write, that doesn't > involve a read > access to external memory. > Essentially you are also saying the same thing. When I reuqest for one byte, the processor reads one word (32 bits), and manipulates those bits (in barrel register) to get one byte in the lowest 8 bits of the register. So, this confirms that a 32 bit (1 word = 32 bits) processor can read only and only 32 bits (1 word) in one cycle. It may read more in one cycle but not less. Please let me know if I have not understood something correctly. -- __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com
