linux kernel development

9
votes

I am currently reading 'Linux Kernel Development' by Robert Love and I do not understand what this bit of assembly is doing.

Basically, in each process kernel stack, there is a struct thread_info which resides at the end of the stack. Now, on the x86 architecture, we can apparently grab this (assuming 8KB stack size) by using the following assembly

movl $-8192, %eax
andl %esp, %eax

So basically ANDing the stack pointer by 0xffffe000. I'm confused as to what is going on here? I don't see why masking the least significant 13 bits of %esp takes us to the struct. I know I'll feel stupid once it is explained, but it is bugging me.

Thanks.

1
linuxassemblykernel
I'm currently reading this book and I have even more basic question: how do you know that -8192 is 0xffffe000? How negative numbers are stored in CPU's registers? - patryk.beza
@patryk.beza The negative numbers are stored during 1's and 2's complements Read more here - Chaitanya Tetali

1 Answers

5
votes

The stack grows downwards, so the end of the stack is the lowest address in the stack, and the structure's starting address. And stacks are stored at multiples of 8KB. Therefore, erasing the 13 least significant bits gets the lowest address of the stack and therefore the start of the structure. Does this make sense?