131 Week 6 - IO System

Question 1

Function of the input output system

Answer 1

Allows input and output devices to be attached to the processor

Question 2

Speed gap challenge

Answer 2

Because I/O devices are often mechanical, they run orders of magnitude slower than the CPU. This means we need to make sure the CPU is not slowed down when it interacts with an I/O device e.g., fetching data from a hard disk.

Question 3

Device diversity challenge

Answer 3

Devices are very diverse and we need to make sure all types of devices can be connected to the processor

Question 4

Diversity of data access modes

Answer 4

Some devices are read-only or write-only or read-write.
Some devices access by the individual byte whereas some access by the block.
Some devices access data randomly but some access data sequentially.

Question 5

Device specific operations

Answer 5

Some devices will have operations which can only happen to that device.
E.g., change resolution for screens, set time for clocks or focus for cameras.

Question 6

I/O protocols

Answer 6

Data transfer errors can happen e.g., electric noise or wireless transmission can cause errors.
Devices can be synchronous or asynchronous

Question 7

Device drivers

Answer 7

Software plugins inside the OS which abstract over device diversity by grouping sets of devices that “look the same”, allowing communication between the device and processor.

Question 8

Functions of device drivers

Answer 8

Registering device with OS and initialising it.
Initiating data transfers to/from a device.
Monitoring status events from a device.
Managing device/system shutdown – ensure OS doesn’t stop until all unwritten data is stored and the device is left in a safe state.

Question 9

What are the 2 device types

Answer 9

Character and block devices.

Question 10

Character devices

Answer 10

Sends and receives 1 byte at a time, e.g., keyboard.

Question 11

Block devices

Answer 11

Sends and receives a multi-byte block, e.g., hard disk.

Question 12

Hard disk

Answer 12

Hard disks are block oriented devices.
The smallest unit of disk storage on a hard disk is called a sector - commonly 512 bytes in size.
Hard disks have multiple heads inside them. Each head is split up into tracks and each track contains multiple sectors.
Data is addressed at the device level as <head, cylinder, sector>.

Question 13

What are the 2 types of processor support for I/O

Answer 13

Isolated I/O and memory-mapped I/O

Question 14

Isolated I/O

Answer 14

I/O devices are assigned a separate address space, distinct from the memory address space.
Special instructions are used by the CPU to interact with the I/O devices.

Question 15

Pros and cons of isolated I/O

Answer 15

+ Suited to simple devices
- Having fixed I/O instruction set

Question 16

Memory mapped I/O

Answer 16

The I/O devices share the same address space as the system memory.
Regular memory instructions are used for both memory and I/O operations.

Question 17

Pros and cons of memory mapped I/O

Answer 17

+ simply, flexible programming model
- adds complexity to devices - need to understand larger addresses and work at memory speeds.