Editing SmartBox OS (section)

=== OS_READBYTE and OS_SENDBYTE: Serial Port ===

The serial port on the controller is currently based on the 6850
UART, this need not bother you as the OS deals with the thing entirely by
itself.  The chip is setup by the OS to always have a word format of 8n1 and
a baud rate of 9600, this translates to 960cps.  The 6850 has two divide
rates for the baud rate, the OS only uses one, using the other one is beyond
the scope of this document and shouldn't be neccessary to use.

Only receive interupts are used, transmission of characters is done
on a polled basis.

As said, there are two calls OS_READBYTE and OS_SENDBYTE. 

==== OS_READBYTE ====

OS_READBYTE tests the serial input buffer and returns the character (if any
in the A register), the c flag states whether there was a character
returned.

{|class="calltable"
|Entry||
A, X, Y, c, z = undefined
|-
|Exit||
* X, Y preserved
* z = undefined
* if c = 1 (no character received)
*: A is preserved
* if c = 0 (character received)
*: A = character received
|}

==== OS_SENDBYTE ====

OS_SENDBYTE sends the character in the A register out of the serial
port, taking into consideration flow control etc.

{|class="calltable"
|Entry||
* A = character to send
* X, Y, c, z = undefined
|-
|Exit||
* A, X, Y, c is preserved
* z = undefined
|}