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SECTION 4
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INTERFACE
4.1 I N T R O D U C T I O N
The Message-Based Interface has a simple and flexible software architecture that allows
users to load and execute their own application code in a variety of ways. Additionally,
the Message Based Interface provides several system services at the level of C-language
function calls. M a n y Message-Based Interface system configuration parameters are
programmable giving the user the ability to custom-tailor the Message-Based Interface for
a particular application.
Users may load many individually named blocks of application-specific code, and later
invoke selected blocks by name. The user may also load an application-specific command
parser which may also invoke these blocks, and Message-Based Interface system services.
It is advantageous t o users t o load and execute their own code f o r speed-critical
applications. B y doing so, the user's application can execute at full speed of the Message-
Based Interface's on-board Motorola 68000 microprocessor.
4.2 T H E MESSAGE-BASED INTERFACE SOFTWARE ARCHITECTURE
4.2.1 Overview
The Message-Based Interface has a single-threaded control-flow architecture. A l l
Message-Based Interface firmware runs in the supervisor mode, and utilizes the supervisor
stack pointer. After the initial power-up sequence, the Message-Based Interface command
parser gains control. I t loops endlessly, examining the character input stream for
Message-Based Interface commands.
Named blocks of executable application code may be loaded into a reserved memory area
of the Message-Based Interface address space. A t run time, these blocks may be invoked
by name. Syntactically, these invocations look like C-language function calls. Also, an
application-specific command parser may be loaded and invoked in two ways. First,
whenever the Message-Based Interface is waiting for input, it will make a function call
to the application parser rather than "busy wait" on input. When the machine is idle, the
application parser is repeatedly called. Second, the user may explicitly command the
Message-Based Interface to redirect the subsequent input stream to the application-specific
parser (the exact details of this maneuver are described in the section on Message-Based
Interface command syntax).
4-1
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