Hardware Reference
In-Depth Information
10.
Some computers allow I/O directly to user space. For example, a program could start
up a disk transfer to a buffer inside a user process. Does this cause any problems if
compaction is used to implement the virtual memory? Discuss.
11.
Operating systems that allow memory-mapped files always require files to be mapped
at page boundaries. For example, with 4-KB pages, a file can be mapped in starting at
virtual address 4096, but not starting at virtual address 5000. Why?
12.
When a segment register is loaded on the Core i7, the corresponding descriptor is
fetched and loaded into an invisible part of the segment register. Why do you think the
Intel designers decided to do this?
13.
A program on the Core i7 references local segment 10 with offset 8000. The
BASE
field of LDT segment 10 contains 10000. Which page directory entry does the Core i7
use? What is the page number? What is the offset?
14.
Discuss some possible algorithms for removing segments in an unpaged, segmented
memory.
15.
Compare internal fragmentation to external fragmentation. What can be done to allevi-
ate each?
16.
Supermarkets are constantly faced with a problem similar to page replacement in virtu-
al memory systems. They have a fixed amount of shelf space to display an ever-
increasing number of products. If an important new product comes along, say, 100%
efficient dog food, some existing product must be dropped from the inventory to make
room for it. The obvious replacement algorithms are LRU and FIFO. Which of these
would you prefer?
17.
In some ways, caching and paging are very similar. In both cases there are two levels
of memory (the cache and main memory in the former and main memory and disk in
the latter). In this chapter we looked at some of the arguments in favor of large disk
pages and small disk pages. Do the same arguments hold for cache line sizes?
18.
Why do many file systems require that a file be explicitly opened with an
open
system
call before being read?
19.
Compare the bit-map and hole-list methods for keeping track of free space on a disk
with 800 cylinders, each one having 5 tracks of 32 sectors. How many holes would it
take before the hole list would be larger than the bit map? Assume that the allocation
unit is the sector and that a hole requires a 32-bit table entry.
20.
A third hole allocation scheme, in addition to best fit and first fit, is worst fit, where a
process is allocated space from the largest remaning hole. What advantage can be
gained by using the worst fit algorithm?
21.
Describe a purpose for the file open system call that was not mentioned in the text.
22.
To be able to make some predictions of disk performance, it is useful to have a model
of storage allocation. Suppose that the disk is viewed as a linear address space of
N
>> 1 sectors, consisting of a run of data blocks, then a hole, then another run of data
blocks, and so on. If empirical measurements show that the probability distributions
for data and hole lengths are the same, with the chance of either being
i
sectors as 2
−
i
,
what is the expected number of holes on the disk?
