freepascal.compiler/docs/go32ex/keyclick.pp

{$ASMMODE ATT}
{$MODE FPC}

uses
	crt,
	go32;

const
	kbdint = $9;

var
	oldint9_handler : tseginfo;
	newint9_handler : tseginfo;

	clickproc : pointer;
	backupDS : Word; external name '___v2prt0_ds_alias';

procedure int9_handler; assembler;
asm
	cli
	pushl %ds
	pushl %es
	pushl %fs
	pushl %gs
	pushal
	movw %cs:backupDS, %ax
	movw %ax, %ds
	movw %ax, %es
	movw dosmemselector, %ax
	movw %ax, %fs
	call *clickproc
	popal
	popl %gs
	popl %fs
	popl %es
	popl %ds
	ljmp %cs:oldint9_handler
end;
procedure int9_dummy; begin end;

procedure clicker;
begin
	sound(500); delay(10); nosound;
end;
procedure clicker_dummy; begin end;

procedure install_click;
begin
	clickproc := @clicker;
	lock_data(clickproc, sizeof(clickproc));
	lock_data(dosmemselector, sizeof(dosmemselector));

	lock_code(@clicker,
		longint(@clicker_dummy) - longint(@clicker));
	lock_code(@int9_handler,
		longint(@int9_dummy)-longint(@int9_handler));
	newint9_handler.offset := @int9_handler;
	newint9_handler.segment := get_cs;
	get_pm_interrupt(kbdint, oldint9_handler);
	set_pm_interrupt(kbdint, newint9_handler);
end;

procedure remove_click;
begin
	set_pm_interrupt(kbdint, oldint9_handler);
	unlock_data(dosmemselector, sizeof(dosmemselector));
	unlock_data(clickproc, sizeof(clickproc));

	unlock_code(@clicker,
		longint(@clicker_dummy)-longint(@clicker));
	unlock_code(@int9_handler,
		longint(@int9_dummy)-longint(@int9_handler));
end;

var
	ch : char;

begin
	install_click;
	Writeln('Enter any message. Press return when finished');
	while (ch <> #13) do begin
		ch := readkey; write(ch);
	end;
	remove_click;
end.