A file of 150 blocks is saved on disk using contiguous allocation. How many blocks need to be accessed to find the 100th block in the file?

Question

A file of 150 blocks is saved on disk using contiguous allocation. How many blocks need to be accessed to find the 100th block in the file?

BlackTom AI · Accepted Answer

Begin by restating what the problem asks in context: a file consisting of 150 contiguous blocks on disk, and the task is to locate the 100th block within that file using contiguous allocation.
Option 150: Saying you need to access all 150 blocks would imply you must read from the start up to the 100th to reach it, which isn’t how contiguous allocation is typically leveraged if you know the file’s starting point and block location. In many OS explanations, contiguous allocation allows direct access to any block by computing its address, so requiring all blocks to be accessed is unnecessary.
Option 1: Accessing only a single block to reach the 100th block aligns with the idea that, once the starting block and the block size are known, the OS can compute the physical location of the 100th block and perform a read on just that block. This avoids sequentially reading earlier blocks, assuming random access by block address is supported.
Option 75: This would represent reading halfway through the file, which would be relevant only if you were constrained to sequential traversal from the beginning. Under contiguous allocation, there is no inherent reason you must stop at the halfway point unless you intentionally read forward in order. The phrasing here suggests a misunderstanding of how block addressing works versus reliance on a sequential scan.
Option None of these: This would be chosen only if every other option clearly failed to describe the access pattern correctly. Given the standard model of contiguous allocation permitting direct access to a block given its address, this option is unlikely to be accurate.
To evaluate the options critically, consider the core benefit of contiguous allocation: a file’s blocks occupy a single continuous region, enabling straightforward calculations of a block’s physical location from its index within the file. This contrasts with linked or indexed allocation, where more work may be required to reach a particular block.
In summary, the reasoning for each choice hinges on whether accessing the 100th block requires sequential traversal from the start or whether direct addressing to the 100th block is possible due to contiguity. The option that reflects direct access leverages the primary characteristic of contiguous allocation, while the other options embody misunderstandings about addressing versus traversal.

A file of 150 blocks is saved on disk using contiguous allocation. How many blocks need to be accessed to find the 100th block in the file?单项选择题

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