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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

A  
B  
C  
D  

 


Date: 2015-01-11; view: 597


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