ECM 1413 I/O devices

Question 1

What does an I/O device do?

Answer 1

I/O devices enable computers to receive or output information to humans or devices.

Question 2

Types of I/O devices (+ examples for each)

Answer 2

human interface devices, such as screens or keyboards
storage devices, such as disks
transmission devices, such as network cards

Question 3

What is communication between I/O devices performed over?

Answer 3

Buses

Question 4

What does a controller do in the context of I/O devices?

Answer 4

The I/O devices are operated using controllers (embedded processors)

Question 5

The processor communicates with the controller by reading and writing to the controller’s registers.

What are the registers in question?

Answer 5

data-in register — data coming from device
data-out register — data going to device
status register — indicating status of device
control register — commands to device

Question 6

How can controller registers be accessed?

Answer 6

special instructions (port-based I/O):
the CPU uses special I/O instructions for transferring a small number of bytes to an I/O bus address

standard memory instructions (memory-mapped I/O):
the CPU treats the device “as memory”; control registers are mapped to specific memory addresses

Question 7

Differences between Port-mapped I/O (PMIO) and Memory-mapped I/O (MMIO)

Answer 7

PMIO uses a separate IO address space; this is treated as part of memory space in MMIO.

MMIO only requires standard memory instructions to access registers as they are a part of main memory; PMIO needs special instructions to access the IO address space.

MMIO is more common than PMIO

Question 8

The 2 approaches to controlling IO devices

Answer 8

1 polling
2 interrupts

Question 9

Polling

Answer 9

The CPU repeatedly checks the controller’s status register to see whether the controller is busy
When the controller is ready and the CPU wants to give new instructions, the CPU writes to the data-out register and signals that is has done so through the control register

Question 10

Interrupts

Answer 10

Interrupts The CPU regularly senses an interrupt-request line
When the CPU gets an interrupt signal through the interrupt-request line, it stops the current process and initiates a response

Question 11

Differences between polling and interrupts

Answer 11

Interrupts can monitor the status of several devices at the same time, and serve them based on priority
Polling needs to check each device individually in a round-robin fashion
Interrupts are much more commonly used since this frees the CPU from polling devices that do not need service

Question 12

Polling may be efficient if

Answer 12

the controller and the device are fast
the I/O rate is high
some I/O data can be ignored
the CPU has nothing better to do

Question 13

Large Data Transfers

Answer 13

Some devices (e.g., disk drives) will often do large data transfers
The transfer will be inefficient if the CPU has to feed data, byte by byte, to the controller’s registers
A better approach is to offload this work to a special-purpose processor (a direct memory access (DMA) controller)

Question 14

How does Direct Memory Access work

Answer 14

The CPU writes a command block into memory, specifying the source and destination of the transfer
The DMA controller can then perform multiple transfers via a single command
When the transfer is complete, the CPU receives an interrupt from the DMA controller
This enables the CPU to spend more resources on other tasks

Question 15

What does a device driver do

Answer 15

A device driver hides differences between various device controllers by defining an interface between the OS and I/O devices for a specific class of I/O devices.

Question 16

Instructions are executed either in user mode (application programs; plain instructions) or kernel mode (system functions; privileged instructions).

A system call is a request of a kernel service.

system calls for

Answer 16

1 character I/O
2 block I/O
3 network I/O

Question 17

character I/O

Answer 17

Examples: keyboards, computer mice, microphones, speakers

A character device transfers bytes one by one

Characters must be processed in order that they arrive in the stream

The interface includes get operation, to return the next character in the stream put operation, to add character to stream libraries for line-by-line access, with integrated editing services (e.g., use backspace to remove the preceding character from the stream)

Question 18

block I/O

Answer 18

Block devices (typically non-volatile mass storage devices) are used to transfer blocks of data

The interface includes read operation, for reading blocks of data write operation, for writing blocks of data block devices are high volume devices

Question 19

network I/O

Answer 19

A network socket interface has system calls for:

creating and connecting sockets

sending and receiving packets over the connection

select function, for determining the status of one or more sockets (whether the socket is ready for reading or writing)

The main difference between network I/O devices and other I/O devices is that with network I/O devices things routinely go wrong (missing packets, etc.)

Therefore, there need to be system functions for checking whether a transfer was successful recovering gracefully from unsuccessful transfers