One-Pass Assemblers

Two methods can be used:

- Eliminating forward references

Either all labels used in forward references are defined in the source program before they are referenced, or forward references to data items are prohibited.

- Generating the object code in memory

No object program is written out and no loader is needed. The program needs to be re-assembled every time.

Multi-Pass Assemblers

Make as many passes as needed to process the definitions of symbols.

Example 3:

A EQU B

B EQU C

C DS 1

3 passes are required to find the address for A.

Such references can also be solved in two passes: entering symbol definitions that involve forward references in the symbol table. Symbol table also indicates which symbols are dependent on the values of others.

Example 4:

A EQU B

B EQU D

C EQU D

D DS 1

At the end of Pass1:

Symbol Table

A	&1	B
B	&1	D			A
C	&1	D
D					B			C

After evaluating dependencies:

Symbol Table

Date: 2015-01-11; view: 1148