UNIX for Dummies Questions & Answers

Ok, so I'm trying to finalize my understanding of read/write and kernel interaction.


read():

You have a library function that has as it's first parameter (what the open file to read from is), second parameter( a pointer to a buffer (is this the location of a buffer in the user area or the controller buffer in the kernel?) and a third parameter(# of bytes to read).

So, once the controller stores words from disk into its own buffer, the DMA then transfers the data to the main memory and the CPU gains control. My question is, what now? The purpose was to read a certain amount of words. So, are those words being returned to the caller once they are in the main memory or is an address returned of where in the main memory those words are located?


write():

First param(which file to write to), second param (again, unsure. Is this pointing to a buffer in the user area already filled in with the new words to modify the file with?), third parameter (how much you replace)

Main question entails how this even works as opposed to the read. Does the controller still need to store a certain amount of words from the disk into its own internal buffer?

I understood it like this: Controller's own hardware buffer somehow stores the words to modify the file. The DMA then uses the new block within the controller buffer to modify a file within the main memory.

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