Q120565: How to Read Text Mode Screen of MS-DOS Session in Windows
Article: Q120565
Product(s): Microsoft Windows Device Driver Kit
Version(s): 3.1
Operating System(s):
Keyword(s):
Last Modified: 06-NOV-1999
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The information in this article applies to:
- Microsoft Windows Device Development Kit (DDK) for Windows, version 3.1
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SUMMARY
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Sometimes it is necessary to read the text from the screen of an MS-DOS session
running under Windows version 3.1 in 386 Enhanced mode. Use a simple VxD to read
the screen in an MS-DOS session. You don't need to access the MS-DOS session
screen through the VDD or GRABBER to read it.
MORE INFORMATION
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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:
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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:
On CGA (25 lines, 40 columns), the buffer size will be 0800h (2048d)
On CGA (25 lines, 80 columns), the buffer size will be 1000h (4096d)
On EGA (43 lines, 80 columns), the buffer size will be 1BE0h (7136d)
On VGA (50 lines, 80 columns), the buffer size will be 2040h (8256d)
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.
REFERENCES
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Windows 3.1 Device Driver Kit "Virtual Device Adaptation Guide" Microsoft
Developers Network "Developers Library" CD
Additional query words: 3.10 VxD VDD DISPLAY
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Keywords :
Technology : kbAudDeveloper kbWin3xSearch kbWinDDKSearch kbWinDDK310
Version : :3.1
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