I heard many times that the assembler and linker need to traverse its input file at least 2 times, is this really necessary? Why cannot it been done in one pass?
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I heard many times that the assembler and linker need to traverse its input file at least 2 times, is this really necessary? Why cannot it been done in one pass?
The assembler translates a symbolic assembler language into a binary representation.
In the input language (assembler), labels are symbolic too.
In the binary output language they are typically a distance in bytes, relative to the current position or some other fixed point (e.g. jump so many bytes ahead or back).
The first pass just determines the offset from the start of the code or some other fixed point of all assembler instructions to fixate the position of the labels.
This allows to calculate the correct jump distances from branch instructions in the second pass.
One pass assembler would be possible, but you would only be able to jump to labels you already had declared (“bacK”) not forward.