knudsen@ihwpt.UUCP (mike knudsen) (12/03/86)
To: ihnp4!cuae2!clyde!rutgers!mit-eddie!ll-xn!cit-vax!cit-vlsi!wen-king Subject: Re: Byte ordering, and pros/cons References: <1271@ihwpt.UUCP>, <1239@cit-vax.Caltech.Edu> Well, about mis-information -- Motorola byte ordering DOES slow down arithmetic memory-to-register operations if the memory bus is narrower than the register. This is becuase the MSB is added first, then the LSB, then an extra cycle is needed to fix the carry. You can see this in the instruction timings for the 6809 for the "double" operations. Yes, the PC could have skipped ahead by 2 and jerked back -1 to add in "proper" order, but that would have probably slowed the chip down even more. Maybe you rich guys who started out with 68000s never noticed the above 8-bit micro problem -- but how about adding a 32-bit int in RAM to one in a 68K register? OK, this uses so many 68K micro-cycles that you never noticed the carry adjust operation in the timings, but I'll bet it's there. Then again, maybe I should have been more explicit. As for Intel and graphics, it is definitely true that if the video hardware maps consecutive bytes (not words) onto the screen, then if you read two of these bytes as a word into an 8086 and do a left or right shift on the WORD (to move a "sprite" right or left on the screen), and just store the WORD back into video RAM, you will get the WRONG effect. Granted, the video hardware could be designed by words to fix this -- or would it? Before people flame about mis-info, maybe they should ask for clarification first. And give some specifics about their "correct" programming techniques to make the problems I raised go away. BTW, the "endian" dispute occured a year or so ago, but was mostly hot air & religion. I thought this time I'd stick in a couple of facts. Yes, facts. I would not have posted this reply but it seemed my intelligence was being insulted in public. --mike k -- Mike J Knudsen ...ihnp4!ihwpt!knudsen Bell Labs (AT&T) (312)-979-4132 (work) You think AT&T cares about CoCos, music, or Star Trek? No? Then, these opinions must be all mine!