COSC 4330                   QUIZ #1                   February 9, 1999

Closed Book.   You can have one sheet of notes.

1.   What is the major advantage of batch systems over bare-bone
     machines?  (20 points)  What replaced batch systems, at least in
     academic environments, and why? (20 points)
 
A:  BATCH SYSTEMS ALLOW MORE EFFICIENT USE OF THE COMPUTER SINCE IT DID
    NOT HAVE TO BE REBOOTED BETWEEN JOBS.   THEY WERE REPLACED IN MANY
    ENVIRONMENTS BY TIME-SHARING SYSTEMS BECAUSE THESE SYSTEMS ALLOW
    INTERACTIVE ACCESS TO THE COMPUTER.

2.   Why do computers have DMA for their disk drives and not for their
     other peripherals? (20 points)
 
A:  DISK DRIVES HAVE MUCH FASTER TRANSFER RATES THAN OTHER PERIPHERALS.
    WITHOUT DMA--OR DIRECT MEMORY ACCESS, THEY WOULD INTERRUPT TOO
    OFTEN THE CPU.

3.  What are the advantages and disadvantages of delaying writes to
    the disk? (20 points)

A:  DELAYING WRITES TO THE DISK REDUCES THE NUMBER OF DISK ACCESS
    BY ALLOWING THE GROUPING OF SEVERAL SMALL WRITES INTO A SINGLE
    I/O OPERATION.  THE ONLY DISADVANTAGE IS THAT WRITES WILL BE LOST
    IF THE SYSTEM CRASHES WHILE SOME WRITES ARE STILL PENDING.

4.   Explain why a kernel should always handle itself short system
     calls.  (20 points)

A:   THE ONLY ALTERNATIVE WOULD BE TO HAVE THE KERNEL FORWARD THE CALL
     TO SOME USER-LEVEL SERVER AND IT WOULD COST TWO EXTRA CONTEXT
     SWITCHES.  THIS IS NOT A PROBLEM FOR COMPLEX SYSTEM CALLS THAT
     ALREADY TAKE A LOT OF TIME.  IT MATTERS MUCH MORE FOR SHORT
     SYSTEM CALLS.

--IMPORTANT----IMPORTANT----IMPORTANT----IMPORTANT----IMPORTANT--
Please do not try to answer these questions by yourself BEFORE
having a clear idea of what's on the test for which you are
studying.

Materials covered in each test DO VARY from semester to semester
because the pace of the course can change, some materials are be
added and other deleted almost every semester.
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