The last part of the Hermes Computer that needs a description is the Input/Output section. Input/Output from the CRT console is done using the special memory locations for the keyboard and CRT screen. When a user presses a key on the keyboard, the keyboard sends the appropriate ASCII code to represent the key, this is deposited in the 1 byte keyboard buffer, where a program can access it. To put characters on the CRT display a program just has to place the ASCII codes in the Console Character Display Buffer (CCDB) at the right location. The CRT constantly scans the CCDB and translates the ASCII codes it finds into character representations on the CRT screen.
The Hermes Computer also supports disks, tapes, printers and other
devices.
This kind of IO is not described here,
but the method is different from the keyboard and Console CRT.
Other devices are accessed using a special set of CPU instructions
that direct a special Input/Output Unit (IOU) to read commands
which will tell the unit how and where to transfer data to and
from the devices.
The IOU is often called a channel,
the commands are another stored program similar to CPU programs,
they are called channel commands.
The channel works like a specialized CPU that is concerned with moving
data rather than transforming it.