# Understanding User and Kernel Mode

**URL:** <https://discourse.codinghorror.com/t/understanding-user-and-kernel-mode/405>\
**Category:** blog\
**Created:** [January 3, 2008, 12:00am UTC](https://discourse.codinghorror.com/t/understanding-user-and-kernel-mode/405 "2008-01-03T00:00:00Z")\
**Posts on this page:** 1\
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**Author:** ![mccoyn](https://discourse-cdn.codinghorror.com/letter_avatar_proxy/v2/letter/m/87869e/32.png) [@mccoyn](https://discourse.codinghorror.com/u/mccoyn)\
**Post date:** [January 4, 2008, 12:00am UTC](https://discourse.codinghorror.com/t/understanding-user-and-kernel-mode/405/42 "2008-01-04T00:00:00Z")

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I can’t read memory without making an API call?

I believe the CPU/Memory System takes care of this by creating an interrupt to ask the OS where the memory is actually mapped. Some amount of caching is used to make it faster. There is no need to get the OS involved when the read is on the same page as the last one.

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