type
  PMyStruct = ^TMyStruct;
  TMyStruct = packed record
    A: LongWord;
    B: LongWord;
  end;

procedure WriteMapData;
var
  MapFile: LongWord;
  MapData: PMyStruct;
begin
  MapFile := OpenFileMapping(FILE_MAP_ALL_ACCESS, False, 'MyMapFileName');

  if (MapFile <> 0) then
  begin
    MapData := MapViewOfFile(MapFile, FILE_MAP_ALL_ACCESS, 0, 0, 0);

    if Assigned(MapData) then
    begin
      MapData^.B := 123;

      UnmapViewOfFile(MapData);
    end;

    CloseHandle(MapFile);
  end;
end;    

struct MyStruct
  A dd ?
  B dd ?
ends

MapFile dd 0
MapData MyStruct ; ???

invoke  OpenFileMapping, FILE_MAP_ALL_ACCESS,FALSE,'MyMapFileName'
mov     [MapFile],eax

invoke  MapViewOfFile, [MapFile],FILE_MAP_ALL_ACCESS,0,0,0
mov     [MapData],eax

;mov     [MapData^.B],123
mov     [MapData.B],123 ; ???

invoke  UnmapViewOfFile, [MapData]
invoke  CloseHandle, [MapFile]
    

procedure CreateMapFile;
var
  MapFile: LongWord;
  MapData: PMyStruct;
begin
  MapFile := CreateFileMapping(INVALID_HANDLE_VALUE, nil,
    PAGE_READWRITE, 0, SizeOf(LongWord), 'MyMapFileName');

  if (MapFile <> 0) then
  begin
    MapData := MapViewOfFile(MapFile, FILE_MAP_ALL_ACCESS, 0, 0, 0);

    if Assigned(MapData) then
    begin
      WriteMapData;

      ShowMessage(IntToStr(MapData^.B));

      UnmapViewOfFile(MapData);
    end;

    CloseHandle(MapFile);
  end;
end;    

MapData DD ? ;this is a pointer
...
mov [eax+MyStruct.B],123    

var
  A: PLongWord;
begin
  A^ := 123;
end;
    

A dd ?

mov eax,[A]
mov [eax+4],123 
    

A dd ? 

mov eax,[A] 
mov [eax],123
    

MapFile dd 0
MapData dd 0

invoke  MapViewOfFile, [MapFile],FILE_MAP_ALL_ACCESS,0,0,0
mov     [MapData],eax
test    eax,eax
jz      error

mov     eax,[MapData]
mov     eax,123
    

mov     [eax],123