Disk Formatting

Figure 9: Left: Low-level Disk Formatting; A disk sector, Right: An illustration of cylinder skew.

• A hard disk consist of a stack of aliminum, aaloy, or glass platters 5.25 inch or 3.5 inch in diameter. On each platter is deposited a thin magnetizable metal oxide
• Before the disk can be used, each platter must recieve a low-level format , or physical formatting ; divide disk into sectors that disk controller can read and write (see Fig. 9 left)
• To use disk to hold files, OS needs to record own data structures on disk
  • partition disk into $\geq$ 1 groups of cylinders logical formatting or ``making a file system''
• Boot block to start up system
  • bootstrap code in ROM
  • bootstrap loader program minimum in ROM
• When reading sequential blocks, the seek time can result in missing block 0 in the next track
• Disk can be formatted using a cylinder skew to avoid this (see Fig. 9 right)
• Issue: After reading one sector, the time it takes to transfer the data to the OS and receive the next request results in missing reading the next sector
• To overcome this, we can use interleaving (see Fig. 10)
• Modern drives overcome interleaving type issues by simply reading the entire track (or part thereof) into the on-disk controller and caching it.

  Figure 10: a) No interleaving b) Single interleaving c) Double interleaving.