Week 31

CST334 Week 7

This week was our seventh week in CST-334, also known as Operating Systems

Operating Systems: Three Easy Pieces

Chapter 36: I/O Devices

Chapter 36 discusses the basics of I/O devices.

Key Takeaways:

  • Interrupts are used to tell the CPU when a I/O request is complete.
  • A Direct Memory Access (DMA) engine is a specialized device for handling I/O requests.
  • Explicit I/O instructions and memory-mapped I/O are two ways to interact with I/O devices.
  • Device Drivers is a piece of software that knows how a device works, and abstracts that device to the system.
  • A basic protocol for an I/O device might be:
    • Wait for drive to be ready.
    • Write parameters to command registers.
    • Start the I/O.
    • Data transfer (for writes).
    • Handle interrupts.
    • Error handling. 

Chapter 37: Hard Disk Drives

Chapter 37 discusses hard disk drives.

Key Takeaways:

  • Data in HDDs is partitioned in blocks. 
  • A platter is a circular, hard surface within a HDD that stores data. Each platter has two sides, and HDDs may have more than one platter.
  • "Rows" on a platter are called tracks.
  • "Columns" on a platter are called sectors.
  • The head is a mechanism that reads/writes data to and from the platter.
  • Seeking is when the head moves to a new position.
  • Shortest Seek Time First (SSTF) prioritizes the requests for the closest block. It can starve requests for far-away blocks.
  • Elevator (SCAN or C-SCAN) scheduling moves up and down the platter repeatedly, avoiding starvation.

The following formulas are used for calculating average request time:

  • T-total = (# reads) * T-access + T-transfer
  • T-access = T-seek + T-rotation
  • T-rotation  = 1-minute/X-rotations * 60-seconds/1-minute * 1000-ms/1-second * 1-revolution/2
  • T-transfer = (number of reads * size of reads)-KB/1 * 1-MB/1024-KB * 1-second/(transfer rate)-MB * 1000-ms/1-second 

Chapter 39: Files and Directories

Chapter 39 discusses files and directories.

Key Takeaways:

  • Files have a low-level name, called an inode number.
  • Directories are special files that contain (user-readable name, inode numeber) pairs.
  • The open() function is used to open and/or create new files. It returns a file descriptor.
  • A file descriptor is a pointer to a file. They are tracked by the OS on a per process basis.
  • The functions read() and write() are used to read and write to files.
  • The function lseek() updates the offset used when accessing a file.
  • The function fsync() is used to immediately schedule data to be written to the disk.
  • The function unlink() is used to remove a reference to a file. The file is deleted when its last reference is removed.
  • The function mkdir() creates a new directory.
  • The function rmdir() removes a directory. A directory must be empty for it to be removed.
  • The function link() creates a new hard link to a file.
  • Soft links are files that link to another file.

Chapter 40: File System Implementation

Chapter 40 discusses how file systems are implemented.

Key Takeaways:

  • Some of a file structure is reserved for metadata.
  • An inode table stores inode data.
  • Data and inode bitmaps store whether an inode or piece of data is free.
  • The superblock stores extra information about the file system.
  • Direct pointers inside of inodes point to blocks contained within a file.
  • Indirect pointers point to a block that contains additional direct or indirect pointers. 

 The following formulas can be used to calculate inode location:

  • Address = Start of inode blocks + inode number * inode size
  • Block Number = Address // block size
  • Location in Block = Address % block size
  • Which inode in block = Location in Block // inode size

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