CS270 Colorado State University

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LC3 assembler
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Ch 7

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Canonical Starting Example

        .ORIG   x3000
        LEA     R0, MYSTR
        PUTS
        HALT
       
MYSTR   .FILL   x48
        .FILL   x69
        .FILL   x20
        .STRINGZ "World!\n"
        .END
        
    What does the above assembly program do?
    
        
    
    
    
    How can we convert it to LC3 machine code?
    
        

        

            


        
        
            
            
        
        
            
           
    
    
    
    
    
    
    
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Overview of PA4

     
        
    
    Part 1 - Implement sym_table_lookup() function.
    
    Part 2 - Implement pass_two of assembler, which figures out offsets
             and pass_three, which constructs machine code.
             
             
    
    Use the available documentation.
        
        
        
        
    Example READMEs
        
        
        
        
        
        
        
    

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Symbol Table

    A symbol table is a kind of dictionary.
        Key, Value pairs
        Key is label.
        Value is address.


    Binary tree is one way to implement a dictionary.
        Each node holds a key, value pair.
        
        A node's left subtree contains only keys that are less than the
        key stored at the node.

        A node's right subtree contains only keys that are more than the
        key stored at the node.


    Labels are stored as C strings
        
         
         
         char a;
            

            

    String comparison
        
        
        
        
        
                
        
    int strcmp ( const char * str1, const char * str2 );
    
        
    
        
        
        #include <string.h>
        #include <stdio.h>
         
        int main() 
        {
            char mystr = "hi";
            char other = "bye";
         
            printf("strcmp(mystr, other) = %d\n", strcmp(mystr, other));
            printf("strcmp(other, other) = %d\n", strcmp(other, other));
            printf("strcmp(other, mystr) = %d\n", strcmp(other, mystr));
         
            return 0;
        }
        
    
    
    Insertion into a binary tree
        struct sym_node_struct {
            char* symbol;
            int address;
            sym_node_t* left;
            sym_node_t* right;
        };
        
    
        
          
          
        
        
        
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Pointers and Pointer to Pointers in C


    C uses pass-by-value semantics.
    


    Examples of when we need a pointer?
    
    
    
    Examples of when we need a pointer to a pointer?
    
    
    
    
    
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mstrout@cs.colostate.edu, 10/15/08