<i>"but in this case you need about 15 minutes to write the code using C and about 1 hour to do the same using Assembler."</i>

That depends on your relative proficiencies in 'C' and assembler! ;-)

Actually, using a lookup table, I don't think there's really be that much difference between the two?

<i>"I mean hex to dec conversion and 7-seg.LED driver. Isn't it?"</i>

Might even be able to go straight from the register value to 7-seg?