Q111474: PRB: Usage of an ORG Directive in a STRUCT Declaration

Article: Q111474
Product(s): Microsoft Macro Assembler
Version(s): 6.0,6.0a,6.0b,6.1,6.11,6.1a
Operating System(s): 
Keyword(s): 
Last Modified: 06-MAY-2001

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The information in this article applies to:

- Microsoft Macro Assembler (MASM), versions 6.0, 6.0a, 6.0b, 6.1, 6.1a, 6.11 
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SUMMARY
=======

Beginning with MASM version 6.0, an ORG directive is permitted in a structure
declaration. This enables the generation of a template that can ease references
to the internals of data structures that are predefined, for example by the
operating system, the operating platform, or a high- level language compiler. A
consequence of declaring an ORG in a STRUCT is that an instance is not intended
to be defined in the source file, and in fact will generate the following
error:

  error: A2159: structure cannot be instanced

This is the expected behavior.

MORE INFORMATION
================

For additional information about the usage of an ORG directive inside a
structure declaration, please see the following article(s) in the Microsoft
Knowledge Base:

  Q111473 BUG: ORG in STRUCT Affects Current Segment Size with -Sf

The following code sample illustrates the usage of an ORG in a structure
declaration. The sample declares a structure that is used as a template to refer
to video memory for the last two lines of a text mode display. Possible usage
includes easing the display of status information of an application running in
text mode. This is not necessarily the best method to accomplish this task, but
is presented to demonstrate a point.

This code sample assumes a color video adapter running in the 80X25 text display
mode with the processor running in real mode, and must be modified for another
configuration.

Sample Code
-----------

  ; Assemble options needed: none
  .MODEL SMALL,C
  .386
   
  VIDEOMEM     EQU 0B800h   ; segment address of video memory
  NUMLINES     EQU 25
  NUMCOLS      EQU 80
  BLACKONWHITE EQU  70h     ; display attributes
  YELLOWONBLUE EQU  1Eh
   
  status STRUCT
      org (NUMLINES - 2) * NUMCOLS * 2
      line1   db ?
      attrib1 db ?
      org (NUMLINES - 1) * NUMCOLS * 2
      line2   db ?
      attrib2 db ?
  status ENDS
   
  .STACK
   
  .DATA
      msg1    db "This is the first status line, "
              db "with yellow characters on a blue background."
      lenmsg1 dw $-msg1

      msg2    db "This is the second status line, "
              db "with black characters on a white background."
      lenmsg2 dw $-msg2
   
  .CODE
  .STARTUP
   
      mov ax, VIDEOMEM
      mov es, ax
   
      mov cx, NUMCOLS
      xor di, di
  @@:
      mov es:[di].status.attrib1, YELLOWONBLUE
      mov es:[di].status.attrib2, BLACKONWHITE
      inc di
      inc di
      loop @B

      xor si, si
      xor di, di
      mov cx, lenmsg1
  @@:
      mov al, msg1[si]
      mov es:[di].status.line1, al
      inc si
      inc di
      inc di
      loop @B

      xor si, si
      xor di, di
      mov cx, lenmsg2
  @@:
      mov al, msg2[si]
      mov es:[di].status.line2, al
      inc si
      inc di
      inc di
      loop @B

  .EXIT
   
  END

Additional query words: 6.00 6.00a 6.00b 6.10 6.10a 6.1x

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Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM600 kbMASM610 kbMASM611 kbMASM610a kbMASM600a kbMASM600b
Version           : :6.0,6.0a,6.0b,6.1,6.11,6.1a

=============================================================================

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