Coursework Part1
Your answers to these problems should contain explanations of the computa-
tions. If you make additional assumptions, state them explicitly.
1. There are three independent jobs. They need 10 s, 15 s and 20 s CPU
time, respectively, and during their executions each of them spends half
a minute waiting for I/O. Compute the minimal overall runtime of these
jobs when they are processed
(a) in a uniprogrammed
[5 marks]
(b) in a multiprogrammed
system.
[10 marks]
Subtotal: [15 marks]
2. A computer has a cache, main memory and a hard disk. If a referenced
word is in the cache, it takes 15 ns to access it. If it is in main memory
but not in the cache, it takes 45 ns to load (the block containing) it into
the cache (this includes the time to originally check the cache), and then
the reference is started again. If the word is not in main memory, it takes
10 ms to load (the block containing) it from the disk into main memory,
and then the reference is started again. The cache hit ratio is 0.5. In the
case of a cache miss, the probability that the word is in the main memory
is 0.7. Compute the average time required to access a referenced word in
this system.
Subtotal: [20 marks]
Total: [35 marks]
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