Racal Instruments 7064, Instruction Manual

Page: 55 / 179

This section is a partial summary of the system services provided by the Message-Based
Interface. A t the time of this writing, the number of services provided is growing. The
complete list is to be found in the C include file "vtl _rou.h". This section also describes
how users can compile and link to Message-Based Interface system services.
4.7.2 L i n k i n g - How The Jump Table Works
The file "jumptab1.68k" is a 68000 assembly source file defining the organization and
absolute address of the Message-Based Interface jump table. When this file is assembled
and linked into the user's application, it binds all references to Message-Based Interface
system services. When a user's application calls a system service, like "rstbitO", the call
is bound to an entry in the jump table. Each entry in the jump table contains a single
JMP instruction that jumps to the selected service function.
4.8 P U T T I N G I T A L L TOGETHER
Refer to Figure 1. T h i s is a block-diagram of a Message:Based Interface system into
which an application command parser and a suite o f application functions has been
loaded. The boxes represent major functions. The arrows show control flow, e.g "A-->B"
is read as function A calls function B. The Message-Based Interface system services are
all enclosed inside a dotted border. T h e jump table is shown and illustrates how user
applications are linked to Message-Based Interface system services. Each line of control
flow is labeled. The individual flows are discussed below.
Flow 1:
The Message-Based Interface command parser directly calls the system
services. ASCII command strings like "RSTBIT" are mapped directly into
function calls.
Flow 2: T h e Message-Based Interface command can call a named block of code
that's been downloaded.
Flow 3: T h e Message-Based Interface command parser w i l l call an application
specific parser when (a) it is idling, waiting for new input, or (b), when it
receives an explicit command to do so.
Flow 4: T h e application specific parser can also make direct calls to other blocks
of application code.
Flow 5: A p p l i c a t i o n code invokes a Message-Based Interface system service. I t
really calls an entry in the system jump table.
Flow 6: A JMP to the absolute address of the selected service.
4-26