From: seebs@... (Peter Seebach)
Date: 2007-07-28T04:44:36+09:00
Subject: Re: How to pass arguments by reference in a function

In message <026a01c7d085$106a05c0$6442a8c0@musicbox>, "Bill Kelly" writes:
>Incidentally, I do recall one interesting use of this XOR swap
>technique.

>I believe this is how GUI menus were drawn on the screen on the
>Amiga.  When you'd click the mouse to get a menu, the system
>would allocate off-screen bitmap memory, and render the menu
>into the off-screen bitmap.  Then it would XOR-swap the off-screen
>rendered menu, with the location on the screen where the menu
>would be displayed.  Thus preserving the pixels from the screen
>that would be obscured by the menu, so that the pixels could
>be restored when the menu was dismissed.

Interesting.  There were a few different ways; you could request
automatic backing store, or you could do it yourself and get notifications,
but it was mostly transparent.

>Pretty neat trick, since it avoided allocating an extra buffer
>to store the obscured screen pixels.  (And it was fast, because
>the Amiga bitmap blitter supported logical raster op's including
>XOR, etc.)

Except it does allocate a buffer, really.  I don't recall the details
anymore.

-s