Computers as Components, 2nd Edition

96 CHAPTER 3 CPUs

Example 3.3 illustrates a combination of input and output.

Example 3.3

Copying characters from input to output using busy-wait I/O

We want to repeatedly read a character from the input device and write it to the output device.

First, we need to define the addresses for the device registers:

#define IN_DATA 0x1000

#define IN_STATUS 0x1001

#define OUT_DATA 0x1100

#define OUT_STATUS 0x1101

The input device sets its status register to 1 when a new character has been read; we must

set the status register back to 0 after the character has been read so that the device is ready

to read another character. When writing, we must set the output status ...