[fpc-pascal]Reading the color attributes from the screen

[Thomas Schatzl]

From: James_Wilson at i2.com
Subject: [fpc-pascal]Reading the color attributes from the screen

Hi,

>The following bit of code is something I use in my go32v2 (DOS)
>programs to save/restore the contents of the screen to/from memory.
> [...]
>the array after I've saved the screen? Or perhaps there's another
> mem[] region I can grab and modify the colors their instead?
>
>//  for screens up to 132x60
>const
> BUF_SIZE = 7920;

Note that this value for the buffer size only covers half of the textmode
screen memory, see below why.

>// array to hold screen contents
>var
> ScreenBuffer : array [1..BUF_SIZE] of byte;
> [...]

(VGA-)Textmode has the following memory layout:
Each cell of the screen is represented by two bytes, the first is the color
attribute the second is the ascii character value. The cells are stored one
after another in a linear fashion from left to right, line by line from top
to bottom.
E.g. a procedure which sets the attribute and ascii character value of a
cell could look like the following:

procedure PutChar(x, y : DWord; attr : Byte; character : Char);
begin
    ScreenBuffer[(y * WIDTH + x ) * 2] := attr;
    ScreenBuffer[(y * WIDTH + x ) * 2 + 1] := Ord(character);
end;

where WIDTH is the screen's physical number of cells per line. (Note that
the x/y coords are interpreted as zero based).

Use the mem[] operator with the same offsets into the screen buffer area as
used in the previous example to change cell character and ascii value
directly on the screen.
E.g.

procedure PutChar(x, y : DWord; attr : Byte; character : Char);
begin
    mem[$B8000 + (y * WIDTH + x ) * 2] := attr;
    mem[$B8000 + (y * WIDTH + x ) * 2 + 1] := Ord(character);
end;

Should do the trick (everything untested).

Regards,
    Thomas