I/O

Question 1

What are some ways a computer can do I/O?

Answer 1

• Interrupt-driven I/O
• Memory-mapped I/O
• Port I/O
• System I/O

Only system I/O is available because our ARM servers are running a Linux OS

Question 2

How do user-level programs communicate with external devices?

Answer 2

through OS to prevent malicious or accidental interactions

Question 3

OS/program communication diagram

Answer 3

check notes

Question 4

How does a program running in EL0 do I/O and describe the process

Answer 4

Does a system call
- generates an exception using svc instruction
- like a subroutine call:
- control transferred to a predefined system function. address for this is stored in the OS
- system code executes @ EL1 (able to interact directly w/I/O devices)

Question 5

How is the type of system call determined?

Answer 5

By the number put into x8 before svc

56: openat
57: close
63: read
64: write

Question 6

Where do system call args go?

Answer 6

x0-x5 and return values in x0

Question 7

In UNIX, how are all peripheral devices represented?

Answer 7

as files

Question 8

What is the typical I/O pattern?

Answer 8

1. open file (connect to device, get fd)
2. read/write to file
3. close file (disconnect from device)

Question 9

Difference btw file & standard I/O

Answer 9

File: interact w/ secondary mem
Standard: keyboard & screen devices

Question 10

How do you ensure that the fd is relative to the cwd?

Answer 10

AT_FDCWD (the value -100)

Question 11

What are flags?

Answer 11

A combination of constants (using |) indicating what will be done to the files

Question 12

Read-only flag
Write-only flag
read/write flag

Answer 12

O_RDONLY 00
O_WRONLY 01
O_RDWR 02

Question 13

Optional flags:
Create file
Fail if file exists
Truncate file
Append access

Answer 13

O_CREAT: 0100
O_EXCL: 0200
O_TRUNC: 01000
O_APPEND: 02000

Question 14

What is the mode?

Answer 14

An optional argument that specifies UNIX file permissions

• only required when creating a file
• specified in octal
• eg: 0700 specifies r/w/execute permission for ONLY the file owner

Question 15

What does the fd return on error?

Answer 15

-1

Question 16

What is the buffer?

Answer 16

Usually a local variable where bytes read will be stored

Question 17

What is standard I/O

Answer 17

keyboard/screen I/O
- I/O streams predefined by OS
- Always available (not open or closed)

Question 18

What is standard input (stdin) and its file descriptor?

Answer 18

input from the keyboard with fd 0

Question 19

What is standard output (stdout) and its file descriptor?

Answer 19

output to the monitor screen with fd 1

Question 20

What is standard error (stderr) and its file descriptor?

Answer 20

Also represents output to monitor screen with file descriptor 2

Question 21

Why is the svc instruction used?

Answer 21

To request a service or resource from the OS or higher privileged mode