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Thread: Adding hex segments and offsets

  1. Sep 6th, 2002, 04:36 PM #1
    markman
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    Adding hex segments and offsets

    Code:
    MOV   AX, 0     ; AX = 0
MOV   AL, 0     ; AL = 0
MOV   AH, 0     ; AH = 0

MOV   AL, FFh   ; AL = FFh
  ; AX = 00FFh
  ; AH = 00h
;I understand up to here
INC   AX        ; AX = AX + 1
  ; AX = 0100h
  ; AH = 01h
  ; AL = 00h
;but not this. I thought that:
;   since AL (FFh) + 1  = 100
;   and AH (00h) +1 = 01h
;   that AH|AL would be 01|100, but it should be 0100h. What am I missing?
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  2. Sep 10th, 2002, 09:48 AM #2
    CornedBee
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    Code:
    Since AH and AL form the AX register together you have
; start
mov ax, 0
; AX: 00 00h
;     AH AL
mov al, 0
mov ah, 0
; those were both redundant, since they are already 0

mov al, FFh
; AX: 00 FFh
;     AH AL

inc ax
; AX: 01 00h
;     AH AL

; AL cannot hold more than FFh
    It's a little bit confusing, but AX is not an independent register but only the merger of AH and AL.
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  3. Jun 27th, 2003, 12:44 PM #3
    Darkwraith
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    Has this been resolved?
    "Can't" and "shouldn't" are two totally separate things.

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  4. Jun 27th, 2003, 01:41 PM #4
    CornedBee
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    I think it has, but I never heard from the poster again.
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    CornedBee

    "Writing specifications is like writing a novel. Writing code is like writing poetry."
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