Slide 11.5: AND instructions
Slide 11.7: XOR instruction
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OR Instruction

The OR instruction performs a boolean OR operation between each pair of matching bits in two operands and places the result in the destination.

OR — Inclusive Logical OR

Usage: OR dest, src
Flag O D I S Z A P C
Result 0     * * ? * 0

Logical inclusive OR of the two operands returning the result in the destination. Any bit set in either operand will be set in the destination.
Clocks
Operands 286 386 486 Size Bytes
reg, reg 2 2 1 2
mem, reg 7 7 3 2-4
reg, mem 7 6 2 2-4
reg, immed 3 2 1 3-4
mem8, immed8 7 7 3 3-6
mem16, immed16 7 7 3 3-6
accum, immed 3 2 1 2-3

The operands can be 8, 16, or 32 bits, and they must be the same size. The OR instruction is often used to set selected bits and preserve others. We can also use OR a number with itself (or zero) to obtain certain information about its value. Therefore, you can use OR instruction instead of CMP instruction for finding whether or not the operand is zero or negative.

 Zero Input Test   Negative Input Test  
 INCLUDE Irvine32.inc
 .data
 prompt1 BYTE "Zero.", 0
 prompt2 BYTE "Not zero.", 0
 .code
 main PROC
       call  ReadInt
       or    eax, eax

     

      
        
      
      

    
    
    

 
    

      		
     

    
       mov   edx, OFFSET prompt2
       call  WriteString
       exit
 ZERO: mov   edx, OFFSET prompt1
       call  WriteString
       exit
 main ENDP
 END main
 
 INCLUDE Irvine32.inc
 .data
 prompt1 BYTE "Negative.", 0
 prompt2 BYTE "Not negative.", 0
 .code
 main PROC
       call  ReadInt
       or    eax, eax

     

      
        
      
      
    
    
    

 
    

      		
     

    
       mov   edx, OFFSET prompt2
       call  WriteString
       exit
 NEGA: mov   edx, OFFSET prompt1
       call  WriteString
       exit
 main ENDP
 END main