Also, used as a prefix in RISC OS to indicate an application program.
As in NeXTStep and Mac OS X, application programs in RISC OS
exist in the file system as directories ('folders', if you must) rather
than simple binary files (as in Windows or classic Mac OS). These
directories contain not only the application image and it's components,
but ancillary files connected with the application such as window
templates, localisation information, documentation, sometimes OS
extension modules.
This make applications easy for the user to organise since they can be
moved to any location in the filesystem as a single self-consistent unit.
The '!' prefix in Risc OS differentiates an application
directory from any other directory, and affects how the OS treats
it. Instead of showing the user a standard light blue directory icon
in directory displays, the filer (RISC OS's equivalent of Finder,
Explorer etc) will show an Application icon (traditionally, the Archimedes 'A' logo). Instead of displaying the directory's
content when the user tries to open it, it starts the application. This is
more or less equivalent to Mac OS X's '.app' directory name extension.
Of course, this behaviour can be bypassed by opening the application;
by holding down the Shift key when opening the directory, the user
can display the contents. The Mac OS X equivalent being to select 'Show Package Contents' from the context menu (Ctrl-click) for the application; thanks Brontosaurus, Lila, wamckee and SharQ...)
The use of the '!' character as a name prefix is similar to UNIX's use
of the '.' character as a prefix to indicate hidden files. Though the
choice of that character might seem fairly arbitraty, there is in fact
a fairly sound reason, which seems obvious once you know it: The ASCII
code for '!', 33, is the second printable character code (the first
being a plain space ' '). Since a space isn't a valid character in
a RISC OS filename, applications will always appear as the first items
in a directory window or listing, if it's sorted (as is usual) by ASCII
collation order (or simply 'alphabetical order').
The '!' also generally denotes files within an application which have a
special purpose.
- !Run
-
When the user tries to invoke the application directory (using the filer
or with one of the many variants of 'exec' that RISC OS provides), the
file which actually gets invoked is the !Run file within the application
directory.
Normally in RISC OS, this is an Obey file (shell script, if you will)
which will set up any shared environment variables, set initial memory
allocation for the program, and start executing the actual program
image.
- !Boot
-
In RISC OS, the mapping of file types to applications which understand
them is done dynamically, and a degree of control is given to the user.
When an application is first 'seen' by the filer, that is it's parent
directory is opened, the !Boot file of any applications in the parent
directory are executed. This conventionally declares any file types the
application may be able to handle, and loads any icons associated with the
files or application.
Since this can cause any arbitrary code to be executed merely by opening
a directory viewer, it was quite readily exploiatable, and indeed quite
frequently exploited, by viruses and other rogue pieces of software. To
help get round this, and the filer was quickly given the ability to
bypass the !Boot file if a user was suspicious about it; by holding
down the Ctrl key when opening a directory, the user could be sure
that no !Sprites were loaded or !Boot filess processed.
-
!RunImage
Not actually special to the OS itself, but by convention this is the
actual program executable, started by the !Run file of the application.
-
!Sprites
A Spritefile which holds icons for the application (to show in filer and
the icon bar) and any documents it may handle. The !Sprites file is only
loaded if there's no !Boot file either: hence any !Boot file has to load
the !Sprites file itself.
- !Sprites22
-
Very similar to the !Sprites file, but containing icons whose pixels
are a different aspect ratio from those in the !Sprites file.
The icons in !Sprite are expected to be 16-colour icons, with dimensions
34 pixel wide by 17 pixels high. This gives a square icon on displays with
pixel aspect ratio of 0.5: for historical reasons, early RISC OS
sytems were designed for a display resolution of 640x256, giving tall
rectangular pixels. !Sprites22 was retrofitted to accomodate standard
'square' pixel displays.
- !Help
-
Documentation, usually a text file but sometimes an executable of some
sort, which RISC OS will show to the user as a menu option for that
application.
Also, in later versions of BBC Basic, ! could also be used as an infix operator,
which adds two arguments to form an address which then became an lvalue.
DEF FNcons(car%, cdr%)
cons%=FNalloc(8) : REM asuming we have some allocator function...
cons%!0 = car%
cons%!4 = cdr%
:= cons%
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