Embedded Systems/IO Programming
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An embedded system is useless if it cannot communicate with the outside world. To this effect, embedded systems need to employ I/O mechanisms to both receive outside data, and transmit commands back to the outside world. Few Computer Science courses will even mention I/O programming, although it is a central feature of embedded systems programming. This chapter then will serve as a crash course on I/O programming, both for those with a background in C, and also for those without it.
The Dos.h header file commonly included in many C distributions, especially on DOS and Windows systems. This file contains information on a number of different routines, but most importantly it contains prototypes for the inp( ) and outp( ) functions that can be used to provide port output directly from a C program. Many embedded systems however, will not have a Dos.h header file in their library, nor will they have any precompiled C routines to handle port input and output. The purpose of this chapter then, is to teach the reader how to "brew their own" input and output routines.
The <iohw.h> interface
Some C compiler distributions include the <iohw.h> interface. It allows relatively portable hardware device driver code to be written. It is used to implement the standard C++ <hardware> interface. 
x86 Output Routines
The x86 instruction set contains 2 instructions: in and out both functions take 2 arguments, a port number, and then another parameter to receive the data from or to send the data to (depending on which command you use).
we can define 2 functions in assembly, using the CDECL calling convention, that we can link with our C programs, and call from our C programms to handle port output and input.
Synchronous and Asynchronous
Data can be transmitted either synchronously or asynchronously. synchronous transmissions are transmissions that are sent with a clock signal. This way the receiver knows exactly where each bit begins and ends. This way, there is less susceptibility to noise and jitter. Also, synchronous transmissions frequently require extensive hand-shakeing between the transmitter and receiver, to ensure that all timing mechanisms are synchronized together. Conversely, asynchronous transmissions are sent without a clock signal, and often without much hand-shaking.
- "Technical Report on C++ Performance" by Dave Abrahams et. al. 2003