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<h1 class="chapter"> 4. Display Coprocesses </h1>
<p>This chapter documents the protocol that simulavr uses to pass register
and memory information to a display coprocess.
</p>
<p>A display coprocess is a separate program started by simulavr for the
sole purpose of displaying register and memory information while an AVR
program is running in the simulator. Using a separate program and a
standardized communication protocol, keeps the simulavr code simpler and
allows for a variety of display programs to be used.
</p>
<p>When the user asks simulavr to display register and memory information
during execution, simulavr will start a coprocess to perform the display
work. A pipe will be opened in simulavr into which the data will be
written using the following commands:
</p>
<table>
<tr><td><p> ‘<samp>q</samp>’
</p></td><td><p> Quit.
</p>
</td></tr>
<tr><td><p> ‘<samp>r<reg>:<val></samp>’
</p></td><td><p> Set register to val.
</p>
</td></tr>
<tr><td><p> ‘<samp>p<val></samp>’
</p></td><td><p> Set program counter to val.
</p>
</td></tr>
<tr><td><p> ‘<samp>i<reg>:<val></samp>’
</p></td><td><p> Set io register to val.
</p>
</td></tr>
<tr><td><p> ‘<samp>I<reg>:<name></samp>’
</p></td><td><p> Set io register name.
</p>
</td></tr>
<tr><td><p> ‘<samp>s<addr>,<len>:XX</samp>’
</p></td><td><p> Set sram addrs to values (one XX pair per addr).
</p>
</td></tr>
<tr><td><p> ‘<samp>e<addr>,<len>:XX</samp>’
</p></td><td><p> Set eeprom addrs to values (one XX pair per addr).
</p>
</td></tr>
<tr><td><p> ‘<samp>f<addr>,<len>:XXXX</samp>’
</p></td><td><p> Set flash addrs to values (one XXXX quad per addr).
</p>
</td></tr>
<tr><td><p> ‘<samp>n<clock_ticks></samp>’
</p></td><td><p> Update the number of clock ticks.
</p>
</td></tr>
</table>
<p>All values are hexidecimal numbers, except for <name> which is a string.
</p>
<p>In order for the display process to know which pipe to read the
information, it must handle either the ‘<samp>--pfd <fd></samp>’ option or check
the <code>SIM_PIPE_FD</code> enviroment variable. The value passed using
either method will be the file descriptor number of the pipe from which
the display prgram will read the informtion.
</p>
<p>Simulavr will start all display programs like so (sizes are decimal
numbers of bytes and sram_start is just the decimal address of the
first byte of sram, usually 0x60 [96] or 0x100 [256]):
</p>
<p>‘<samp><prog> --pfd <fd> <flash_size> <sram_size> <sram_start> <eeprom_size></samp>’
</p>
<p>The user can specify the display program to use via the
‘<samp>--disp-prog</samp>’ option to simulavr or using the <code>SIM_DISP_PROG</code>
environment variable. If both are not specified, then no display will be
used.
</p>
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