ASSEMBLER

Assembler

General Design Procedure:

1. Specify the Problem

2. Specify Data Structure

3. Define Format of Data Structure

4. Specify Algorithm

5. Look for modularity

6. Repeat 1 through 5 on modules

Statement of problem

Pass 1 purpose: Define symbols and literals

Determine length of machine instructions

(MOTGET1)

Keep track of location counter (LC)

Remember values of symbols until pass 2

(STSTO)

Process some pseudo ops, e.g. EQU, DS

(POTGET1)

Remember literals (LITSTO)

Pass 2 purpose: Generate object program

Look up value of symbols

(STGET)

Generate instructions (MOTGET2)

Generate data (for DS, DC and

literals)

Process pseudo ops(POTGET2)

Data Structure Pass 1 data bases:

1. Input source program

2. A Location Counter (LC), used to keep track of each

instruction’s location.

3. A table, the Machine Operation Table (MOT), that

indicates the symbolic mnemonics for each instruction

and its length

4. A table, the Pseudo Operation Table (POT), that

indicates the symbolic mnemonics and action to be

taken for each pseudo-op in pass 1

Data Structure

5. A table, the Symbol Table (ST), that is used to

store each label and its corresponding value.

6. A table, the Literal Table (LT), that is used to

store each literal encountered and its

corresponding assigned locations.

7. A copy of the input to be used later by pass

2. this may be stored in a secondary storage

device.

Data Structure

Pass 2 data bases:

1. Copy of source program input to pass 1

2. Location Counter (LC)

3. A table, the Machine Operation Table (MOT), that

indicates for each instruction: i) symbolic mnemonics

ii)length iii)binary machine op code and iv) format

4. A table, the Pseudo Operation Table (POT), that

indicates for each pseudo op the symbolic

mnemonics and the action to be taken in pass 2

Data Structure

5. The Symbol Table (ST), prepared by pass 1, containing

each label and its corresponding value.

6. A table, the Base Table (BT), that indicates which registers

are currently specified as base registers by USING pseudo-

ops and what are the specified contents of these registers.

7. A work space, INST, that is used to hold each instruction

as its various parts are being assembled together.

Data Structure

8. A workspace, PRINT LINE, used to

produce a printing listing

9. A workspace, PUNCH CARD, used prior to

actual outputting for converting the

assembled instructions into the format

needed by the loader.

10. An output deck of assembled instructions

in the format needed by the loader.

Format of Data Bases

Format of Data Bases

Machine-Op Table (MOT) for pass 1 and pass 2

Format of Data Bases

Pseudo-Op Table (POT OR OPTAB) for pass1 and pass 2

Format of Data Bases

Symbol Table (ST OR SYMTAB) for pass 1 and pass 2

Pass 1

Pass 2

Look for ModularitySome functions that may be isolated in Pass 1:

Look for modularitySome functions that may be isolated in pass 2

1. READ22. POTGET23. MOTGET24. EVAL5. PUNCH6. PRINT7. DCGEN8. DLENGTH9. BTSTO10. BTDROP11. BTGET12. LTGEN