Lecture 9

 

Lecture 9

 

• Character Strings
  
  • Storage techniques
    
    • 1.1
      
      • Counting – not used
        
      • Count number of characters in a string, one by one until string is fully read
        
    • 1.2
      
      • Sentinel - used
        
        • Adds a null character '\0' to the end of the string
          
  • DATA Segment
    
    • A way to put data into main memory
      
    • Open with assembler directive '.data' pus string into main memory
      
    • Msg: .ascii "the sum is: "
      
      • The directive ascii puts the string into memory without null termination
        
    • CRLF: .asciiz "\n"
      
      • The directive asciiz puts string into memory with null termination
        
    • System services strings
      
      • Used for input / output
        
      • Very primitive
        
      • Unique to system (not the same on all platforms)
        
      • Spim on unix
        
      • See text appendix A 48 and the lesson nmber two
        
      • Invoked using 'syscall'
        
    • Memory access by bytes
      
      • Byte load and save
        
        • Load byte
          
          • Lb ch, 0(aa)
            
            • Aa is he base
              
            • 0 is the offset
              
            • Ch is the absolute address of register to load to
              
            • Lb is 'load byte' to register from main memory
              
            • Actual address = base + offset
              
        • String copy
          
          • Sub $sp, $sp, 4
            
          • Sw $s, 0($sp)
            
          • Save the saved register on stack to start
            
        • Void strcopy (char x[], char y[])
          
        • {int i = 0;
          
        • While((x[i] = y[i]) !=0_
          
        • i = i+1