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Sluiceboxes and SetPorts

Today I got a delightful (and long) email from sluicebox, of the ScummVM SCI team. He wrote about a lot of things but one thing stood out and he’s right, I should write about it.

Remember when I fixed the imitation AGI windows in Space Quest 4? There’s something very strange going there that sluicebox pointed out in the email.

If you’ll remember:

(method (open &tmp port temp1)
  ; temp0 was unused so we're taking it for proper SetPorting.
  (= color gColor)
  (= back gBack)
  ; Set our type to ONLY wCustom, not wCustom|wNoSave, and open.
  (= type 128)
  (super open:)
  ; Nothing will have appeared because wCustom don't draw anything, but a port has been set up!
  ; Switch to drawing on the whole screen but also *save the window's port*.
  (= port (SetPort 0))
 
  (= temp1 1)
  ; ...
  (Graph grUPDATE_BOX lsTop lsLeft lsBottom lsRight 1)
 
  ; Reset to the window's port.
  (SetPort port)
)

But if you look at this GitHub commit from ScummVM you’ll see the interesting description

SSCI doesn’t return zero; it doesn’t return anything. This shouldn’t affect any games since no scripts should depend on a non-existent return value, but this discrepancy came up while investigating a fan script that accidentally relies on this.

So I checked the leaked source code that I made SCI11+ from.

global KERNEL(SetPort)
{
	if (argCount >= 6)
	{
		picWind->port.portRect.top = arg(1);
		picWind->port.portRect.left = arg(2);
		picWind->port.portRect.bottom = arg(3);
		picWind->port.portRect.right = arg(4);
		picWind->port.origin.v = arg(5);
		picWind->port.origin.h = arg(6);
		if (argCount >= 7)
			InitPicture();
	}
	else
	{
		if (arg(1))
		{
			if ((arg(1)) == -1)
				RSetPort(menuPort);
			else
				RSetPort((RGrafPort*)Native(arg(1)));
		}
		else
		{
			RSetPort((RGrafPort*)RGetWmgrPort());
		}
	}
}

No return value, which is obvious really because the KERNEL define expands to a void function. Return values are instead handled by setting the acc global variable. So lets dig a little deeper.

RSetPort proc	pPtr:word
	mov	ax, pPtr
	mov	rThePort, ax
	ret
RSetPort endp

Nothing. It sets the rThePort global and that’s all. There’s an RGetPort function right above that does the opposite, but nothing in the kernel function calls that.

Looking back at my description of BorderWindows, there’s an important difference:

(= oldPort (GetPort))
(SetPort 0)
(Graph grUPDATE_BOX lsTop lsLeft lsBottom lsRight VISUAL)
(SetPort oldPort)

It’s very interesting indeed how this happened to Just Work. Even so, I should probably go back and correct that SQ4 script.

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