SYMBOLS CHART
Every one of the parts on code line in assembler is known as token, for instance on the code line:
MOV AX, Var
We comprise three tokens, the MOV instruction, the AX operator, and the VAR operator. What the assembler does to produce the OBJ code is to read each one of the tokens and appear for it on an internal "equivalence" chart identified as the reserved words chart, which is where all the mnemonic meanings we use as instructions are establish.
Follow this process, the assembler reads MOV, seems for it on its chart and recognizes it as a processor instruction. similarly it reads AX and identifies it as a register of the processor, but when it seems for the Var token on the reserved words chart, it does not get it, so then it seems for it on the symbols chart which is a table where the names of the constants, variables and labels used in the program where their addresses on memory are incorporated and the sort of data it contains, are found.
at times the assembler comes on a token which is not distinct on the program, consequently what it does in these cased is to pass a second time by the source program to confirm all references to that symbol and put it on the symbols chart. There are symbols which the assembler will not find given that they do not belong to that segment and the program does not know in what part of the memory it will get that segment, and at this time the linker comes into action, which will make the structure necessary for the loader so that the segment and the token be distinct when the program is loaded and previous to it is executed.