what does upward/downward code compatible mean?

[Dan]

If processor x is upward compatible with y that means all of x's
instructions work on y's and not the other way around or is that downward
compat?
(couldn't find the answer on google or jeeves).
Dan

 

[Tim Dicus]

If processor x is upward compatible with y that means all of x's
instructions work on y's and not the other way around or is that downward
compat?
(couldn't find the answer on google or jeeves).
Dan

The way I understand it is:

Upward compatibility refers to the ability of future product releases being capable of running code generated by a compiler for the
"upward compatible" processor.

Downward compatibility refers to the ability of previous product releases being capable of running code generated by a compiler for
the "downward compatible" processor.

Tim

 

[John Woodgate]

I read in sci.electronics.design that Dan <[email protected]

wrote (in <[email protected]>) about 'what

does upward/downward code compatible mean?', on Mon, 26 Jan 2004:

If processor x is upward compatible with y that means all of x's
instructions work on y's

I believe so, but, as you imply, the terminology is a bit confusing and
incorrect statements may be made.

and not the other way around

'upward' doesn't *necessarily* imply 'but not downward'. However, if y
is an 'improvement' of x, and has more instructions, there is a high
probability that at least some of those won't work with x: y is not
downward compatible with x.