How to save the registers value using inline Assembler - c++

Question

(Well,this is the first time i ask questions here and English isn't my first language,so please forgive some of my mistakes. And i'm a green hand in programme.)
I met this problem while doing my OS homework, we were asked to simulate the function SwitchToFiber,and my current problem is i don't know how to save the registers value in order to recover the function next time it was called.
I don't know if my problem was clear. Though i don't think my code was useful, i will put them below.
#include <stdio.h>
#define INVALID_THD NULL
#define N 5
#define REG_NUM 32
unsigned store[N][REG_NUM];
typedef struct
{
void (*address) (void * arg);
void* argu;
}thread_s;
thread_s ts[N];
void StartThds();
void YieldThd();
void *CreateThd(void (*ThdFunc)(void*), void * arg);
void thd1(void * arg);
void thd2(void * arg);
void StartThds()
{
}
void YieldThd()
{
thd2((void*)2);
}
void *CreateThd(void (*ThdFunc)(void*), void * arg)
{
ts[(int)arg].address = (*ThdFunc);
ts[(int)arg].argu = arg;
}
void thd2(void * arg)
{
for (int i = 4; i < 12; i++)
{
printf("\tthd2: arg=%d , i = %d\n", (int)arg, i);
//in order to see clearly，i added /t abouve
YieldThd();
}
}
void thd1(void * arg)
{
/*
__asm__(
);
*/
for (int i = 0; i < 12; i++)
{
printf("thd1: arg=%d , i = %d\n", (int)arg, i);
YieldThd();
}
}
int main()
{
//this is my first plan, to store the register value in some static arry
for(int i = 0; i<N; i++)
for(int j = 0; j<REG_NUM; j++)
store[i][j] = 0;
//create the two thread
if (CreateThd(thd1, (void *)1) == INVALID_THD)
{
printf("cannot create\n");
}
if (CreateThd(thd2, (void *)2) == INVALID_THD)
{
printf("cannot create\n");
}
ts[1].address(ts[1].argu); //thd1((void*)1),argu = 1；
// StartThds();
return 0;
}
This is the whole code now i have,because i don't know which part maybe useful, so i put them all above. As you can see, most of them are still empty.

Answer 1

It's possible (as pointed out in comments) that you don't need to write assembly for this, perhaps you can get away with just using setjmp()/longjmp(), and have them do the necessary state-saving.

Answer 2

I have done this before but i always have to look up the details. The following is of course just pseudo-code.
basically what you do is you create a struct with your registers:
typedef struct regs {
int ebx; //make sure these have the right size for the processors.
int ecx;
//... for all registers you wish to backup
} registers;
//when changing from one thread
asm( //assembly varies from compiler to compiler check your manual
"mov ebx, thread1.register.ebx;
mov ecx, thread1.register.ecx;"
// and so on
//very important store the current program counter to the return address of this fu nction so we can continue from ther
// you must know where the return address is stored
"mov return address, thread1.register.ret"
);
//restore the other threads registers
asm(
"mov thread2.register.ebx, ebx;
mov thread2.register.ecx, ecx;
//now restoer the pc and let it run
mov thread2.register.ret, pc; //this will continue from where we stopped before
);
This is more or less the principle of how it works. Since you are learning this you should be able to figure out the rest on your own.