Question

In Xv6, system call results are returned in the eax register. Explain how this happens, from user space making a system call to the kernel space returning the result. Particularly, how the following structure is used in this process

struct trapframe

{

uint edi;

uint esi;

uint ebp;

uint oesp;

uint ebx;

uint edx;

uint ecx;

uint eax;

...

}

Answer 1

The following are the set of information which allows us to resume the execution of the task where we have stopped before because the scheduler selects some other task to run and it pauses the current task. So, it has to save the informationof the task which has been paused. So that it can restore it when it has to run
The main function of each of the registers:

edi: It is the destination index used for string operations
esi: It is the source index used for string operations
ebx: It specifies the base index of the array
ebp: It holds the address of the current stack frame i.e., Stack Base Pointer
eip: It points at the instruction which should be executed called as Instruction Pointer