How to Read Text Mode Screen of MS-DOS Session in Windows

MORE INFORMATION

In a VxD, by using VM Control Block (handle) for the virtual machine (VM) that contains the MS-DOS session, you can obtain the base linear address of the VM from the CB_High_Linear member of the control block. CB_High_Linear will contain the linear address of "virtual 0000:0000" for the MS-DOS session.

Use this linear address to offset to [CB_High_Linear + 0400h] in the VM to poke around in the BIOS DATA area. Information such as what video mode is active, how many screen columns are active, and so on can be found in the BIOS DATA area. You can use this data to determine the address and the size of the text screen in the MS-DOS session. Then you can use the address and size to read the whole screen or a portion of it.

By using MEMORY.LST included with Ralf Brown's Interrupt List compilation, or by using a good BIOS DATA reference found in many books, you can get this information:

===========================================================================
Format of BIOS Data Segment at segment 40h:
                {items in curly braces not documented by IBM}
Offset  Size    Description
  . . .
 49h    BYTE    Video current mode
 4Ah    WORD    Video columns on screen
 4Ch    WORD    Video page (regen buffer) size in bytes
 4Eh    WORD    Video current page start address in regen buffer
 50h 16 BYTEs   Video cursor position (col, row) for eight pages,
                0 based
 60h    WORD    Video cursor type, 6845 compatible, hi=startline,
                lo=endline
 62h    BYTE    Video current page number
 63h    WORD    Video CRT controller base address: color=03D4h,
                mono=03B4h
 65h    BYTE    Video current setting of mode select register
                03D8h/03B8h
 66h    BYTE    Video current setting of CGA palette register 03D9h
===========================================================================
				

By reading at [CB_High_Linear + 449h], for example, you can determine the screen mode active in the MS-DOS session. If it is 7, it is text mode on an MDA adapter, and the text screen buffer is at [CB_High_Linear + 0B000h]. Otherwise, if it is a text video mode number, a color text mode is in use in the MS-DOS session, and the text screen buffer is at [CB_High_Linear + 0B800h].

If you determine that a color text mode is in use, the adapter could be CGA, EGA, or VGA, so the MS-DOS session could be in either 25-, 43-, or 50-line mode.

The size of the screen regenerate buffer is stored at [CB_High_Linear + 044Ch]. Based on the number of bytes the screen takes and the number of screen columns, you can determine the number of screen rows.

The screen regenerate buffer will typically contain one of the following values: The number of columns on the screen is stored at [CB_High_Linear + 044Ah]. By dividing the buffer size by the number of columns, then dividing by two (unless screen attributes need to be read, too), you can determine the number of rows on the screen:

  Buffer size   Number of columns   Skip Attribs   Result
  -------------------------------------------------------
  2048          / 40                / 2             25.6
  4096          / 80                / 2             25.6
  7136          / 80                / 2             44.6
  8256          / 80                / 2             51.6
				

NOTE: the results are not exact. Therefore, if the regenerate buffer is greater than 4096 bytes or if the number of screen columns is not equal to 25, the result must be decremented by 1 to get the real number of screen lines.

Once you determine the screen buffer offset, number of columns, and size of text screen, you can read the screen by dereferencing to the screen buffer and reading the characters out of the screen buffer memory.