Explain the pass of compiler -

One complete scan of a source program is called pass
Pass include reading an input file and writing to the output file
In a single pass compiler analysis of source statement is immediately followed by synthesis of equivalent target statement
It is difficult to compile the source program into single pass due to:
Forward reference: a forward reference of a program entity is a reference to the entity which precedes its definition in the program
This problem can be solved by postponing the generation of target code until more information concerning the entity becomes available
It leads to multi pass model of compilation
In Pass I: Perform analysis of the source program and note relevant information
In Pass II: Generate target code using information noted in pass I

It is desirable to have a few passes, because it takes time to read and write intermediate file
On the other hand if we group several phases into one pass we may be forced to keep the entire program in the memory. Therefore memory requirement may be large.