Assignment 6, due Feb. 27

Always, on every assignment, please write your name legibly as it appears on your University ID and on the class list! All assignments will be due at the start of class on the day indicated (usually a Friday). The only exceptions to this rule will be by advance arrangement unless there is what insurance companies call "an act of God" - something outside your control. Homework must be turned in on paper and in class! Late work may be turned in to the teaching assistant's mailbox, but see the late work policy. Never push late work under someone's door!

1. Background: Look up CD-ROM, CD-R and CD-RW technologies on the web. Wikipedia is a useful source.

   a) (0.5 points) What is the maximum and average rotational latency of CD-ROM drives operating at 32x speed.

   b) (0.5 points) Explain the most prominent reasons that CD-RW drives do not compete with magnetic disks of the same capacity and similar rotational speed.

2. Background: In older systems, there was only one processor, so all computation involving disk management was done by the operating system.

   In modern systems, since the development of the SCSI interface, it has been common to include a microprocessor or microcontroller as part of the disk drive itself, the disk-local processor. This processor runs a fixed program from ROM provided by the disk vendor. Of course, a significant part of this processor's job is to handle the protocol on the bus connecting the disk to the main system, but it can also take care of hardware specific details while leaving higher level issues to the operating system.

   a) (0.5 points) Give specific examples of functions that the disk-local processor can handle, and examples of functions that are best handled by the operating system.

   b) (0.5 points) Discuss some of the potential pitfalls of this separation of function between the operating system and the disk-local processor.

3. Background: During the late 1980's, IBM PC models became available with far more memory than Microsoft's DOS operating system could use. Some applications simply bypassed the system and directly used the extra memory, if it was available. One popular application was called a RAM-Disk. This appeared, from the operating system's perspective, to be a disk driver, while it used RAM and not any physical disk. It was frequently useful to copy heavily used files to RAM disk on startup, particularly for those who were using floppy disks as their primary disk drives. Recall that most floppy disks were only about one megabyte in capacity.

   A disk cache is a more sophisticated way of using the extra RAM. In this case, the disk cache software looks like a disk drive to the operating system, and it, in turn, sits on top of a real disk drive. Accessing that drive through the cache improves performance for access to recently used sectors.

   a) (0.5 points) What can you do with a RAM disk that you cannot do with a disk cache.

   b) (0.5 points) What does a disk cache do automatically that a user of a RAM disk would have to do by hand before shutting down the system.