In short word
there isn't a specific SEGMENT:OFFSET pairs can be used because depending on system and microprocessor.
for example
in windows before access a specific phisical address must select a page of memory and after you can addressing a specific location.
that is calculated from MMU (memory menagement unit)
for other system i don't know
for example in the pic16f..
you can select the address of external RAM and after you can address a specific location on external RAM.
But in the main RAM fist 20byte is used for pheriferical and external ram.
i hope that help you
sorry for my poor english