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Chapter 4: Viewing Code and Data
Using Simulated I/O
How Simulated I/O Works
Communication between your program running in the simulated system and
the SIMIO process takes place through contiguous single-byte length memory
locations. The first memory location is called the Control Address (CA). The
Control Address and the memory locations that follow it are called the CA
buffer.
Control Address buffers are less than or equal to 260 bytes in size. A
maximum of 256 bytes of information can be transferred between the debugger
and the host system at one time. Some simulated I/O commands require four
additional bytes for command parameters.
Communication between a program and the simulated I/O process is a series
of requests by the program and responses by the SIMIO process:
In the debugger/simulator, the debugger stops executing the program while a
SIMIO command is being processed. This causes no timing related problems
because all timing is relative to the system simulator. Once the SIMIO request
has finished processing, the program continues executing. This behavior has
the effect of producing an I/O system that takes 0 cycles to complete. When
simulation stops, the simulated cycle counting also stops. When simulation
resumes, the I/O request is complete and the time reference of cycle counting
continues.
Simulated I/O Connections
The SIMIO system supports three types of I/O connections. These are:
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The program places a SIMIO command in the CA buffer and then waits
for a return code to be placed in the first byte of the CA.
The SIMIO process polls the CA buffer memory. When it finds a
command, the SIMIO process executes the command. When the SIMIO
process completes the command, the first byte of the CA buffer is changed
to the command return code.
Keyboard and display.
UNIX files.
UNIX processes.
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