99 lines
4.0 KiB
Markdown
99 lines
4.0 KiB
Markdown
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This program allows you to see how address translations are performed in a
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system with base and bounds registers. As before, there are two steps to
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running the program to test out your understanding of base and bounds. First,
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run without the -c flag to generate a set of translations and see if you can
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correctly perform the address translations yourself. Then, when done, run with
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the -c flag to check your answers.
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In this homework, we will assume a slightly different address space than our
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canonical one with a heap and stack at opposite ends of the space. Rather, we
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will assume that the address space has a code section, then a fixed-sized
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(small) stack, and a heap that grows downward right after, looking something
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like you see in the Figure below. In this configuration, there is only one
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direction of growth, towards higher regions of the address space.
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-------------- 0KB
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| Code |
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-------------- 2KB
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| Stack |
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-------------- 4KB
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| Heap |
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| v |
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-------------- 7KB
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| (free) |
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| ... |
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In the figure, the bounds register would be set to 7~KB, as that represents
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the end of the address space. References to any address within the bounds
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would be considered legal; references above this value are out of bounds and
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thus the hardware would raise an exception.
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To run with the default flags, type relocation.py at the command line. The
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result should be something like this:
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prompt> ./relocation.py
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...
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Base-and-Bounds register information:
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Base : 0x00003082 (decimal 12418)
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Limit : 472
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Virtual Address Trace
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VA 0: 0x01ae (decimal:430) -> PA or violation?
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VA 1: 0x0109 (decimal:265) -> PA or violation?
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VA 2: 0x020b (decimal:523) -> PA or violation?
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VA 3: 0x019e (decimal:414) -> PA or violation?
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VA 4: 0x0322 (decimal:802) -> PA or violation?
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For each virtual address, either write down the physical address it
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translates to OR write down that it is an out-of-bounds address
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(a segmentation violation). For this problem, you should assume a
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simple virtual address space of a given size.
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As you can see, the homework simply generates randomized virtual
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addresses. For each, you should determine whether it is in bounds, and if so,
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determine to which physical address it translates. Running with -c (the
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"compute this for me" flag) gives us the results of these translations, i.e.,
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whether they are valid or not, and if valid, the resulting physical
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addresses. For convenience, all numbers are given both in hex and decimal.
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prompt> ./relocation.py -c
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...
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Virtual Address Trace
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VA 0: 0x01ae (decimal:430) -> VALID: 0x00003230 (dec:12848)
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VA 1: 0x0109 (decimal:265) -> VALID: 0x0000318b (dec:12683)
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VA 2: 0x020b (decimal:523) -> SEGMENTATION VIOLATION
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VA 3: 0x019e (decimal:414) -> VALID: 0x00003220 (dec:12832)
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VA 4: 0x0322 (decimal:802) -> SEGMENTATION VIOLATION
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]
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With a base address of 12418 (decimal), address 430 is within bounds (i.e., it
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is less than the limit register of 472) and thus translates to 430 added to
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12418 or 12848. A few of the addresses shown above are out of bounds (523,
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802), as they are in excess of the bounds. Pretty simple, no? Indeed, that is
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one of the beauties of base and bounds: it's so darn simple!
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There are a few flags you can use to control what's going on better:
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prompt> ./relocation.py -h
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Usage: relocation.py [options]
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Options:
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-h, --help show this help message and exit
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-s SEED, --seed=SEED the random seed
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-a ASIZE, --asize=ASIZE address space size (e.g., 16, 64k, 32m)
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-p PSIZE, --physmem=PSIZE physical memory size (e.g., 16, 64k)
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-n NUM, --addresses=NUM # of virtual addresses to generate
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-b BASE, --b=BASE value of base register
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-l LIMIT, --l=LIMIT value of limit register
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-c, --compute compute answers for me
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]
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In particular, you can control the virtual address-space size (-a), the size
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of physical memory (-p), the number of virtual addresses to generate (-n), and
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the values of the base and bounds registers for this process (-b and -l,
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respectively).
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