Ciro Santilli @cirosantilli 35

When an exception happens, the CPU jumps to an address that the OS had previously registered as the fault handler. This is usually done at boot time by the OS.

This could happen for example due to a programming error:

int *is = malloc(1);
is[2] = 1;

but there are cases where it is not a bug, for example in Linux when:

the program wants to increase its stack.
It just tries to accesses a certain byte in a given possible range, and if the OS is happy it adds that page to the process address space, otherwise, it sends a signal to the process.
the page was swapped to disk.
The OS will need to do some work behind the processes back to get the page back into RAM.
The OS can discover that this is the case based on the contents of the rest of the page table entry, since if the present flag is clear, the other entries of the page table entry are completely left for the OS to to what it wants.
On Linux for example, when present = 0:
- if all the fields of the page table entry are 0, invalid address.
- else, the page has been swapped to disk, and the actual values of those fields encode the position of the page on the disk.

In any case, the OS needs to know which address generated the Page Fault to be able to deal with the problem. This is why the nice IA32 developers set the value of cr2 to that address whenever a Page Fault occurs. The exception handler can then just look into cr2 to get the address.

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GIF  Updated 2025-04-24  +Created 1970-01-01

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Video 1.

It's pronounced GIF by Jehtt (2022)

Source.

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x86 Paging Tutorial / Process memory layout  Updated 2025-04-24  +Created 1970-01-01

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For each process, the virtual address space looks like this:

------------------ 2^32 - 1
Stack (grows down)
v v v v v v v v v
------------------

(unmapped)

------------------ Maximum stack size.


(unmapped)


-------------------
mmap
-------------------


(unmapped)


-------------------
^^^^^^^^^^^^^^^^^^^
brk (grows up)
-------------------
BSS
-------------------
Data
-------------------
Text
-------------------

------------------- 0

The kernel maintains a list of pages that belong to each process, and synchronizes that with the paging.

If the program accesses memory that does not belong to it, the kernel handles a page-fault, and decides what to do:

if it is above the maximum stack size, allocate those pages to the process
otherwise, send a SIGSEGV to the process, which usually kills it

When an ELF file is loaded by the kernel to start a program with the exec system call, the kernel automatically registers text, data, BSS and stack for the program.

The brk and mmap areas can be modified by request of the program through the brk and mmap system calls. But the kernel can also deny the program those areas if there is not enough memory.

brk and mmap can be used to implement malloc, or the so called "heap".

mmap is also used to load dynamically loaded libraries into the program's memory so that it can access and run it.

Stack allocation: stackoverflow.com/questions/17671423/stack-allocation-for-process

Calculating exact addresses Things are complicated by: