Assembly Language “CMP”

Question

I am working on my first project for my Machine Organization class. The program flips the screen from left to right. This works fine and I have no issues with this. However, in addition to flipping the screen, my professor has also asked us to display all uppercase C's in red on grey. I have attempted to do this within the loopRow LOOP. There are no errors when assembling, however, it does not work. I have also tried comparing to the ASCII code value but this did not work either. Any suggestions?

MyCode SEGMENT
        ASSUME CS:MyCode, DS:MyData   

MainProg  PROC                

    MOV     AX, MyData             
    MOV     DS, AX                 
    MOV     AX, 0B800h         
    MOV     ES, AX

    MOV BX, (25 * 160)


    loop25: 

      SUB BX, 160
      CALL flipRow

      CMP BX, 0
    JNE loop25                ;if not equal to 0, numLoops - 1 and repeat

    MOV     AH, 4Ch                
    INT     21h                   

MainProg ENDP  

flipRow  PROC 

    MOV DI, BX
    ADD DI, 158
    MOV SI, BX

 loopRow:

    MOV AX, ES: [DI]
    MOV CX, ES: [SI]
    MOV ES: [DI], CX
  CMP CX, 'C'                                 ;compare CX to 'C'
  JNE next                                    ;if != C go to next
    MOV ES: [DI + 1], BYTE PTR 01111100b
  next:
    MOV ES: [SI], AX
  CMP AX, 'C'                                 ;compare AC to 'C'
  JNE next2                                   ;if != C go to next2
    MOV ES: [SI + 1], BYTE PTR 01111100b
  next2:
    DEC DI
    DEC DI
    INC SI
    INC SI

  CMP SI, DI
  JL loopRow 
    RET
flipRow ENDP                 

MyCode ENDS         

Answer 1

The usual advice applies: learn to use a debugger.

That said, your problem is that you are comparing CX and AX , which contain the character and the attribute as well. You really want to compare just the character, so use CMP CL, 'C' and CMP AL, 'C' respectively.