freepascal.compiler/docs/go32ex/rmpmint.pp

uses
	crt,
	go32;

var
	r : trealregs;
	axreg : Word;

	oldint21h : tseginfo;
	newint21h : tseginfo;
procedure int21h_handler; assembler;
asm
	cmpw $0x3001, %ax
	jne .LCallOld
	movw $0x3112, %ax
	iret

.LCallOld:
	ljmp %cs:oldint21h
end;

procedure resume;
begin
	Writeln;
	Write('-- press any key to resume --'); readkey;
	gotoxy(1, wherey); clreol;
end;

begin
	clrscr;
	Writeln('Executing real mode interrupt');
	resume;
	r.ah := $30; r.al := $01;  realintr($21, r);
	Writeln('DOS v', r.al,'.',r.ah, ' detected');
	resume;
	Writeln('Executing protected mode interrupt without our own',
		' handler');
	Writeln;
	asm
		movb $0x30, %ah
		movb $0x01, %al
		int $0x21
		movw %ax, axreg
	end;
	Writeln('DOS v', r.al,'.',r.ah, ' detected');
	resume;
	Writeln('As you can see the DPMI hosts default protected mode',
		'handler');
	Writeln('simply redirects it to the real mode handler');
	resume;
	Writeln('Now exchanging the protected mode interrupt with our ',
		'own handler');
	resume;

	newint21h.offset := @int21h_handler;
	newint21h.segment := get_cs;
	get_pm_interrupt($21, oldint21h);
	set_pm_interrupt($21, newint21h);

	Writeln('Executing real mode interrupt again');
	resume;
	r.ah := $30; r.al := $01; realintr($21, r);
	Writeln('DOS v', r.al,'.',r.ah, ' detected');
	Writeln;
	Writeln('See, it didn''t change in any way.');
	resume;
	Writeln('Now calling protected mode interrupt');
	resume;
	asm
		movb $0x30, %ah
		movb $0x01, %al
		int $0x21
		movw %ax, axreg
	end;
	Writeln('DOS v', lo(axreg),'.',hi(axreg), ' detected');
	Writeln;
	Writeln('Now you can see that there''s a distinction between ',
		'the two ways of calling interrupts...');
	set_pm_interrupt($21, oldint21h);
end.