Why does an assembler need two passes to compile a program W

Why does an assembler need two passes to compile a program? What would be a more efficient way?

Solution

It requires two passes to compile programm because i.e one pass is for to compute offsets and another pass is to produce a code obviously .Anyway two pass or three pass is based on the transformations that a code produce avaliable from source code to final output.

Two pass is more efficient way because it intends to compute the every expression as it faces into two passes known as \"First\" and Second\" passes.At the time of first pass any unspecified symbols are found it replaces a dummy value.it caluclates a value for expression.