printf.htm

mips架构的bootloader,99左右的版本 但源代码现在没人更新了

	<title>PMON - printf Call Tree</title>
	<h1 align=center>printf Call Tree</h1>

<!--INDEX "printf Call Tree" -->

This page describes what happens when PMON calls printf, or a client
program calls printf via the vector table.<p>


<b>Call Tree</b>
<pre>
   fprintf --\    putchar ------ putc --\
             |                          |
    printf --+-- vfprintf --+-- fputs --+-- write
                            |           |
                     puts --/  fwrite --/
</pre>


<dl>
<dt><b>
printf()</b><dd>
printf() calls vfprintf() in order to convert the arguments into a list
of arguments for use by the macros in varargs.h. It also uses this call
to convert the printf(fmt) into fprintf(stdout,fmt).<p>

<dt><b>
vfprintf()</b><dd>
vfprintf() calls the common routine vsprintf() which is used for printf(),
sprintf(), and fprintf(). This routine places the converted string into a
buffer. The buffer is then passed to fputs() for printing.<p>

<dt><b>
fputs()</b><dd>
Since PMON does not implement buffered output, fputs() calls write()
directly.<p>

<dt><b>
write()</b><dd>
write() calls _write(). This indirection is necessary to permit the library
to be used for both building PMON and building clients.<p>

<dt><b>
_write()</b><dd>
Because PMON does not use interrupt driven I/O, it is necessary for
PMON to poll the devices frequently looking for newly arrived
characters. This is done by calling scandevs(). Having done this _write()
then calls _chwrite().<p>

<dt><b>
scandevs()</b><dd>
This function scans all devices looking for newly-arrived input.  It
does this by calling the driver with OP_RXRDY. If the return status
indicates that a character is ready, it calls with OP_RX to retreive
that actual character. The characters are placed in the input queue via
Qput(). Characters that need echoing are sent to the device via
_chwrite().<p>

<dt><b>
_chwrite()</b><dd>
This function writes a single character to the designated device. It
does this by repeatedly calling the driver with OP_TXRDY until ready.
It then calls the driver with OP_TX to actually transmit the
character.<p>

</dl>
</dl><p><hr>
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