Input and Output

Question 1

A computer may do input and output in a variety of ways:

Answer 1

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

Question 2

Since our ARM server are running a Linux-OS, only system I/O is available

User-level programs communicate with external devices through the OS

What does this do?

Answer 2

This prevents malicious or accidental interactions that may damage the device

Question 3

A program running exception level 0 does I/O by making a system call

What does it generate?

Answer 3

It generates an exception using the svc instruction like a subroutine call

• Control is transferred to a predefined system function (The address for this is stored in a table in the OS)
• The system code executes at EL1 (Is able to interact directly with I/O Devices)
• Once finished, control returns to the calling code (Changes back to EL0)

Question 4

The type of system call is determined by the…

Answer 4

…number put into x8 before invoking svc

Question 5

What are the I/O service requests?

Answer 5

[look at notes]

Question 6

Where are the arguments for system calls put?

Answer 6

In registers x0 - x5
Any return value in x0
In UNIX (Linux), all peripheral devise are represented as files

Question 7

What does it provide and what is the pattern?

Answer 7

Provides a uniform interface for I/O
Typical Pattern:
- Open the file (connect to the device, get its file descriptor)
- Read from or write to the file (do device I/O, transferring bytes)
- Close the file

Question 8

File I/O involves interacting with ____________

Answer 8

secondary memory (usually a dis or tape device)

Question 9

Standard I/O involves

Answer 9

The keyboard and screen devices

Question 10

In File I/O Opening a file…

Answer 10

[check notes]

dirfd: directory file descriptor
- is used only if pathname is a relative path
- Can be set to AT_FDCWD (the value -100) to indicate that the pathname is relative to the program’s current working directory

pathname: absolute or relative pathname of a file
flags: combination of constants (using 1) indicating what will be done to the file’s data

mode: optional argument that specifies UNIX file permissions
- Required only when creating a new file (using O_CREAT)
- Specified in octal

fd: the returned file descriptor
- Is -1 on error

Question 11

Flags

Answer 11

O_RDONLY 00 Read-only access
O_WRONLY 01 Write-only access
O_RDWR 02 Read/Write access

Question 12

Optional flags

Answer 12

O_CREAT 0100 Create a file if it doesn’t exist
O_EXCL 0200 Fail if file exists (with O_CREAT)
O_TRUNC 01000 Truncate an existing file
O_APPEND 02000 Append Access

Question 13

Reading a File code

Answer 13

[check notes]
fd: file descriptor
 - Previously set by openat( ), or 0 for stdin
buf: buffer where the bytes read will be stored
 - Usually a local variable
n: number of bytes to read
 - Must be <= buffer size
n_read: number of bytes actually read
 - -1 is returned on error

Question 14

Writing to a File code

Answer 14

fd: file descriptor
- Previously set by openat( ), or -1 for stdin
buf: buffer where the bytes read will be stored
- Usually a local variable
n: number of bytes to write
- Must be <= buffer size
n_read: number of bytes actually written
- -1 is returned on error

Question 15

Closing a File code

Answer 15

fd: file descriptor
status: 0 if successful, -1 otherwise

Question 16

Standard I/O

Answer 16

Are input and output streams predefined by the OS
- Are always available (are not opened or closed)

Standard input (stdin)

• Represents input coming from the keyboard
• File descriptor: 0

Standard output (stout)

• Represents output to the monitor screen
• File descriptor: 1

Standard error (stderr)

• Represents output to the monitor screen (Normally used to output error messages)
• File descriptor: 2

[Example in notes]