Translate assembly language programs to object programs or machine
code is called an Assembler.

• One-pass assemblers are used when
• it is necessary or desirable to avoid a second pass over the source program
• the external storage for the intermediate file between two passes is slow or is inconvenient to use
• Main problem: forward references to both data and instructions
• One simple way to eliminate this problem: require that all areas be defined before they are referenced.
• It is possible, although inconvenient, to do so for data items.
• Forward jump to instruction items cannot be easily eliminated.

Data structures for assembler:
Op code table
Looked up for the translation of mnemonic code

• key: mnemonic code
• result: bits

Hashing is usually used

• once prepared, the table is not changed
• efficient lookup is desired
• since mnemonic code is predefined, the hashing function can be tuned a priori

The table may have the instruction format and length

• to decide where to put op code bits, operands bits, offset bits
• for variable instruction size
• used to calculate the address

Symbol table
Stored and looked up to assign address to labels

• efficient insertion and retrieval is needed
• deletion does not occur

Difficulties in hashing

• non random keys

Problem

• the size varies widely

pass 1: loop until the end of the program
1. Read in a line of assembly code
2. Assign an address to this line

• increment N (word addressing or byte addressing)

3. Save address values assigned to labels

• in symbol tables

4. Process assembler directives

• constant declaration
• space reservation

Algorithm for Pass 1 assembler:

begin
            if starting address is given
                        LOCCTR = starting address;
            else
                        LOCCTR = 0;
            while OPCODE != END do                 ;; or EOF
                        begin
                        read a line from the code
                        if there is a label
                                    if this label is in SYMTAB, then error
                                    else insert (label, LOCCTR) into SYMTAB
                        search OPTAB for the op code
                        if found
                                    LOCCTR += N           ;; N is the length of this instruction (4 for MIPS)
                        else if this is an assembly directive
                                    update LOCCTR as directed
                        else error
                        write line to intermediate file
                        end
            program size =  LOCCTR - starting address;
end
           
Load-and-go assembler

• Load-and-go assembler generates their object code in memory for immediate execution.
• No object program is written out, no loader is needed.
• It is useful in a system oriented toward program development and testing such that the efficiency of the assembly process is an important consideration

  
       Forward Reference:

•     Load-and-go assembler
• Omits the operand address if the symbol has not yet been defined
• Enters this undefined symbol into SYMTAB and indicates that it is undefined
• Adds the address of this operand address to a list of forward references associated with the SYMTAB entry
• Scans the reference list and inserts the address when the definition for the symbol is encountered.
• Reports the error if there are still SYMTAB entries indicated undefined symbols at the end of the program
• Search SYMTAB for the symbol named in the END statement and jumps to this location to begin execution if there is no error