CODEWITHSUNDEEP
memorymemory-managementoperating-systemkernel
venuswu
venuswu

Reputation: 107

How does the kernel know about segment fault?

When a segment fault occurs, it means I access memory which is not allocated or protected.But How does the kernel or CPU know it? Is it implemented by the hardware? What data structures need the CPU to look up? When a set of memory is allocated, what data structures need to be modified?

Upvotes: 1

Views: 808

Answers (4)

Kuba hasn't forgotten Monica
Kuba hasn't forgotten Monica

Reputation: 98425

It all depends on the particular architecture, but all architectures with paged virtual memory work essentially the same. There are data structures in memory that describe the virtual-to-physical mapping of each allocated page of memory. For every memory access, the CPU/MMU hardware looks up those tables to find the mapping. This would be horribly slow, of course, so there are hardware caches to speed it up.

Upvotes: 0

Stewart
Stewart

Reputation: 4028

To answer the second part of your question, again it depends on hardware and OS. In a typical system (i.e. x86) the CPU consults the segment registers (via the global or local descriptor tables) to turn the segment relative address into a virtual address (this is usually, but not always, a no-op on modern x86 operating systems), and then (the MMU does this bit really, but on x86 its part of the CPU) consults the page tables to turn that virtual address into a physical address. When it encounters a page which is not marked present (the present bit is not set in the page directory or tables) it raises an exception. When the OS handles this exception, it will either give up (giving rise to the segfault signal you see when you make a mistake or a panic) or it will modify the page tables to make the memory valid and continue from the exception. Typically the OS has some bookkeeping which says which pages could be valid, and how to get the page. This is how demand paging occurs.

Upvotes: 1

Code maniac
Code maniac

Reputation: 86

A seg fault generally happens when a process attempts to access memory that the CPU cannot physically address. It is the hardware that notifies the OS about a memory access violation. The OS kernel then sends a signal to the process which caused the exception

Upvotes: 1

Paul R
Paul R

Reputation: 212929

The details will vary, depending on what platform you're talking about, but typically the MMU will generate an exception (interrupt) when you attempt an invalid memory access and the kernel will then handle this as part of an interrupt service routine.

Upvotes: 2

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