It was so simple to implement system call

system call ：
perform read System call will generate swi Instructions , Put the system call number in r7 In the register
（1） System call entry address

（2） Take out the system call number and put it in scno in  

（3） Find the system call table  

    .type    sys_call_table, #object
ENTRY(sys_call_table)
#include "calls.S" System call table

 （4） How to implement new system calls
① stay printk Implement function body in
void sys_pk()
{
    printk("this is a new sys call\n");
}
②caLLs.S Middle append
③unistd.h Middle append
④ Call in user mode
void pk()
{
        __asm__(
            "ldr r7,=363 \n"
            "swi \n"
            :
            :
            :"memory"
   )
}
void main()
{
        pk();
}