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A number of third-party developers have announced large-screen display
peripherals for Macintosh. One of them, Radius Inc., has issued a set of guidelines for
developers who wish to remain compatible with their Radius FPD; unfortunately, one of
their recommendations can cause system crashes. This note suggests a more
correct approach.
[Nov 01 1986]
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Introduction
On the first page of the appendix
to their guidelines, "How to be FPD Aware," Radius recommends
the following:
"First, to detect the presence of a Radius FPD, you should check address
$C00008..."
Unfortunately, this assumes that you're running on a Macintosh or Macintosh
Plus; this test will not work on Macintosh XL, nor on a Macintosh II. Since
these displays weren't designed to work with systems other than Macintosh and
Macintosh Plus, you should make sure you're running on one of these systems
before addressing I/O locations (such as those for an add-on display).
Before testing for the presence of any large-screen display, you should first
check the machine ID; it's the byte located at (ROMBASE)+8 (that is, take
the long integer at the low-memory location ROMBASE [$2AE],
and add 8 to get the address of the machine ID byte. On a Macintosh or
Macintosh Plus, this address will work out to be $400008;
however, use the low-memory location, to be compatible with
future systems that may have the ROM at a
different address!).
The machine ID byte will be $00 for all current Macintosh systems. If
the value isn't $00, you can assume that no large-screen display is present,
but don't forget to follow M.OV. Compatibility's guidelines for
screen size independence!
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Note:
If you are a developer of an add-on large-screen display, we'd be
happy to review your guidelines for developers in advance of distribution;
please send them to us at the address for comments in Technical Note #0. Future
versions of this note may recommend general guidelines for dealing with add-on
large-screen displays.
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References
M.OV.Compatibility
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