First pass

Edit online

During the first pass, the assembler checks to see if the instructions are legal in the current assembly mode.

On the first pass, the assembler performs the following tasks:

  • Checks to see if the instructions are legal in the current assembly mode.
  • Allocates space for instructions and storage areas you request.
  • Fills in the values of constants, where possible.
  • Builds a symbol table, also called a cross-reference table, and makes an entry in this table for every symbol it encounters in the label field of a statement.

The assembler reads one line of the source file at a time. If this source statement has a valid symbol in the label field, the assembler ensures that the symbol has not already been used as a label. If this is the first time the symbol has been used as a label, the assembler adds the label to the symbol table and assigns the value of the current location counter to the symbol. If the symbol has already been used as a label, the assembler returns the error message Redefinition of symbol and reassigns the symbol value.

Next, the assembler examines the instruction's mnemonic. If the mnemonic is for a machine instruction that is legal for the current assembly mode, the assembler determines the format of the instruction (for example, XO format). The assembler then allocates the number of bytes necessary to hold the machine code for the instruction. The contents of the location counter are incremented by this number of bytes.

When the assembler encounters a comment (preceded by a # (pound sign)) or an end-of-line character, the assembler starts scanning the next instruction statement. The assembler keeps scanning statements and building its symbol table until there are no more statements to read.

At the end of the first pass, all the necessary space has been allocated and each symbol defined in the program has been associated with a location counter value in the symbol table. When there are no more source statements to read, the second pass starts at the beginning of the program.

Note: If an error is found in the first pass, the assembly process terminates and does not continue to the second pass. If this occurs, the assembler listing only contains errors and warnings generated during the first pass of the assembler.