Hi there!

Is anyone able to tell me why the following code plots the red pixel but not the blue one, and why the locking function does not work?

----8<----
Program Test;
Uses
  CRT,GO32;
Var
  Screen:Array[0..199,0..319] Of Byte Absolute $A0000;
Begin
  WriteLn(lock_linear_region($A0000,320*200));
  ReadKey;
  Asm
    mov eax,13h
    int 10h
  End;
  Screen[0,0]:=1; { blue pixel }
  Mem[$A0001]:=12; { red pixel }
  ReadKey;
  Asm
    mov eax,03h
    int 10h
  End;
  WriteLn(unlock_linear_region($A0000,320*200));
End.
----8<----

Thanks in advance!

absolute uses always addressing via FS, see below.

and why the locking function does not work?

You don't have to lock memory, if you don't use interrupt handlers.

Use Mem[$a000:0001] instead.
FPC uses some tricks to access dos memory (it is done via the fs selector).

Florian Klaempfl wrote

absolute uses always addressing via FS, see below. FPC uses some tricks to access dos memory (it is done via the fs selector).

If "absolute" uses the FS selector as you say, and DOS memory is accessed via FS, then why am I not able to use the Screen array to access DOS memory?

Probably because you declare it as
   Var
  Screen:Array[0..199,0..319] Of Byte Absolute $A0000;
Try to redeclare it as
Var
  Screen:Array[0..199,0..319] Of Byte Absolute $A000:0;

This should work better!