by Wallboy
 P: 4 I'm currently learning the MIPS architecture and am implementing my own version of the ISA in a logic simulator. I'm sure I understand the differences between Byte and Word addressing, but I don't understand why we need Byte Addressing. How I currently have my memory system implemented, is I have a 32x32 sized RAM. When I address my memory, I leave the lower two bits disconnected from the address input so all my addresses end in 00, or a value divisible by 4. This way all the addresses between a a number divisible 4 select the same data. Example of the PC addressing some instruction: PC Address Data Physical Address 0x00000000 0x1234ABCD 0x00000000 0x00000001 0x1234ABCD 0x00000000 0x00000002 0x1234ABCD 0x00000000 0x00000003 0x1234ABCD 0x00000000 0x00000004 0x567890EF 0x00000001 0x00000005 0x567890EF 0x00000001 ... Now for implementing load byte instructions, I use a 4:1 MUX where each of the inputs is 8 bits that make up each Byte of the 32 bit word of data. I then use the offset value within the load byte instructions as the select to the MUX to select which byte I want. Then sign/zero extend the value to be written back to the register file. This is where I don't see the use of using byte addressing when we can just use word addressing and use some simple logic like the MUX to select individual bytes within the 32 bit word.
 Mentor P: 21,284 It seems to me that you answered your own question. If you can directly access an indivisual byte within a word, doesn't that seem more efficient (i.e., fewer instructions, therefore faster) than doing all of the stuff you described? Although it doesn't seem to be done much any more, in times past, programmers would optimize loop bodies that executed many times by minimizing the number of instructions that had to be performed each loop iteration. Sometimes the goal was to reduce the size of the code when the amount of memory was limited, and sometimes the goal was to speed up the processing time. If the code was redrawing the image on the screen, you would want to be able to address each byte.
 Engineering Sci Advisor HW Helper Thanks P: 7,160 The hard part is not just doing byte operations, but doing them efficiently. For example, suppose you want to store several bytes from the same word in different registers, to do some fast processing, and then recombine them back into memory (and for extra credit, do the same with multiple processors!) You won't make many friends with compiler-writers, if you design a machine like that! FWIW the first versions of Unix on Cray supercomputers (back in the 1980s) were totally crippled because of this issue - the Cray 1 was a (64-bit) word addressible machine only, and dealinig with Unix data structures that were a mix of 8, 16, and 32 bit quantities was S---L---O---W.