# Input / Output Subsystem
Handles devices that allow the computer system to:

* Communicate and interact with the outside world
	- Screen, keyboard, printer

* Store information

![[io-subsystem.png]]

# The Control unit
Program stored in memory. (machine language in binary)

Task of control unit is to execute programs by:
* Fetch: from memory the next instruction
* Decode: instruction to determine what to do
* Execute: issuing signals to ALU, memory and IO systems.

Repeats until HALT instruction.

# Typical Machine Instructions
MOV: move data from a to b
ADD: add numbers
SUB: subtract numbers
JMP: transfers program control flow to the indicated instruction

![[example-machine-lang.png]]

# CPU Time vs IO Time
Total time required to run program is: CPU Time + IO Time

Typical time to read from IO: 20ms
Typical time to run instruction: 20ns
CPU can execute up to 200 million instructions while a block is read from a hard disk.

Conclusion: While I/O is executed, CPU is idling.

![[cpu-vs-io.png]]