Operating Systems 2

Direct Memory Access (DMA)

1) CPU programs the DMA Controller to:

• transfer data OUT of memory

• transfer data INTO memory

2) UNFINISHED

Device-Dependent Software

Device Drivers

Device-Independent Software

You can use the exact same system call regardless of what type of device you’re reading from (shared across devices). Often user-level.

ex) read system call, write system call

Types of Devices

• Disk

• Terminal

• Clock

• Network

• etc.

Device Driver Components of a Simple Serial Line-Based Terminal Device

At the bottom tier of an OS:

1) Upper Half

2) Shared Buffer

3) Lower-half

Upper Half of a Device

Code that gets called when a request for the device is made

Lower Half of a Device

Code that gets called whenever you get an interrupt from a device (interrupt handler)

Shared Buffer of a Device

Data shared between the upper and lower half of a device. Data shared depends on the device and whether you need read/write access

Simple Terminal I/O Putcher() and Getcher() Problem

Two device drivers producing and receiving code (producer/consumer problem).

• wait() in upper half - ok!!

• wait() in lower half - bad!! cannot have an interrupt handler in lower half. Solution: if() - won’t block (good), else() ignore new data if full

File Systems

• Abstract entity provided by the OS

• Named collection of permanent data

File Naming

• old: ms dos - 8.3 (8 characters, 3 char extension)

• new: much more relaxed, sometimes extension aren’t even necessary

Directories (Folders)

file whose content is a list of files and their respective inodes

Aliases

Allows you to have multiple file names for the same file contents (two pointers to same object sorta)

Contents not removed until all aliases are removed.

ex) dot = current directory alias in linux, dotdot parent directory alias in linux

Soft Link/Symbolic Link/Indirect File/Shortcut

similar to aliases, difference is if one is deleted the contents are now inaccessible

File System Organization

What to store (Original Unix File System):

1) file attributes - stored in inode

2) file contents - stored in disk blocks

3) file name - stored in directory of file

• FAT - old but still common

• NTFS - windows modern

What is Stored in an inode?

File attributes such as:

• file ownership

• file type (ex. regular, directory, special(devices))

• file access

• previous access/modification time

• number of links (aliases)

• size of file in bytes

• pointers to data blocks

Unix+Linux retains 15 data block pointers…

• First 12 point directly at disk blocks

• Last 3 are single, double, triple indirection