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Inline assembler bug? (Very simple question)

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  • Inline assembler bug? (Very simple question)

    The following code is a test code which should simply read the first
    byte of an integer and of a string, using direct memory access.
    The assembler part should perform exactly the same operation as the
    DEF SEG / PEEK lines, but it does not!
    Could anybody tell me what is wrong with this code, or if there is a
    bug in the inline assembler?
    This is the code:

    DEFINT A-Z
    CLS

    test$ = "This is a test string."
    test2 = &H5564

    segm1 = STRSEG(test$): pointer1 = STRPTR(test$)
    segm2 = VARSEG(test2): pointer2 = VARPTR(test2)
    scrptr = 0

    DEF SEG = segm1
    peek1? = PEEK(pointer1)
    DEF SEG
    DEF SEG = segm2
    peek2? = PEEK(pointer2)
    DEF SEG

    ! PUSHF
    ! PUSH AX
    ! PUSH DS
    ! PUSH SI

    ! MOV AX, segm1
    ! MOV DS, AX
    ! MOV SI, pointer1
    ! MOV AX, DS:[SI]
    ! MOV asm1?, AL

    ! MOV AX, segm2
    ! MOV DS, AX
    ! MOV SI, pointer2
    ! MOV AX, DS:[SI]
    ! MOV asm2?, AL

    ! POP SI
    ! POP DS
    ! POP AX
    ! POPF

    PRINT peek1?, asm1?
    PRINT peek2?, asm2?
    END

    The expected result would be:
    84 84 (ASCII code for "T")
    100 100 (= &H64)
    But it was:
    84 0
    100 0
    When tracing the program line by line, I found that after the lines
    MOV SI, pointer1, MOV DS, AX (second time), and MOV SI, pointer2
    the values in DS resp. SI were zero.
    Can anybody help?

    Hans Ruegg


  • #2
    Hans,
    the DS register is used to locate the data (pointer1 and pointer2) within the data segment but you have overwritten DS before the data is looked at so the wrong segment value is used and the wrong pointer value is looked up. Replace all occurrances of DS with ES in your code and it'll work as expected.

    Paul.

    Comment


    • #3
      When accessing PB vars using ASM, DS must be restored prior to accessing it.
      You should use ES: DI, instead of DS: SI.
      Code:
      DEFINT A-Z
      CLS
      
      test$ =  "This is a test string."
      test2 = &H5564
      
      segm1 = STRSEG(test$): pointer1 = STRPTR(test$)
      segm2 = VARSEG(test2): pointer2 = VARPTR(test2)
      scrptr = 0
      
        DEF SEG = segm1
          peek1? = PEEK(pointer1)
        DEF SEG
        DEF SEG = segm2
          peek2? = PEEK(pointer2)
        DEF SEG
      
      ! PUSHF
      ! PUSH AX
      ! PUSH ES
      ! PUSH DI
      
      ! MOV AX, segm1
      ! MOV ES, AX
      ! MOV DI, pointer1
      ! MOV AX, ES:[DI]
      ! MOV asm1?, AL
      
      ! MOV AX, segm2
      ! MOV ES, AX
      ! MOV DI, pointer2
      ! MOV AX, ES:[DI]
      ! MOV asm2?, AL
      
      ! POP DI
      ! POP ES
      ! POP AX
      ! POPF
      
      PRINT peek1?, asm1?
      PRINT peek2?, asm2?
      END
      ------------------
      Peter.
      mailto[email protected][email protected]</A>

      [This message has been edited by Peter Lameijn (edited January 03, 2001).]
      Regards,
      Peter

      Comment


      • #4
        Thank you very much... now as I see the answer, it is really obvious!
        The reason I used DS was that the original program would then use
        REP MOVSB in order to copy the string to a different memory location,
        so DS:SI must necessarily point to the string's content when it comes
        to that step. I suppose I can follow your suggestion and then safely
        copy ES to DS when I do not longer need to access any PB variables
        directly?
        (Sorry, I do not have the PB compiler and internet access at the same
        place, so I can't test it just now.)

        Hans Ruegg.

        Comment

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