text: #include "types.h"
  #include "x86.h"
  #include "defs.h"
  #include "date.h"
  #include "param.h"
  #include "memlayout.h"
  #include "mmu.h"
  #include "proc.h"
  
  // returns struct proc from proc.c
  extern struct proc * getptable_proc(void);
  
  int
  sys_fork(void)
  {
    return fork();
  }
  
  int
  sys_exit(void)
  {
    exit();
    return 0;  // not reached
  }
  
  int
  sys_wait(void)
  {
    return wait();
  }
  
  int
  sys_kill(void)
  {
    int pid;
  
    if(argint(0, &pid) < 0)
      return -1;
    return kill(pid);
  }
  
  int
  sys_getpid(void)
  {
    return myproc()->pid;
  }
  
  int
  sys_sbrk(void)
  {
    int addr;
    int n;
  
    if(argint(0, &n) < 0)
      return -1;
    addr = myproc()->sz;
    if(growproc(n) < 0)
      return -1;
    return addr;
  }
  
  int
  sys_sleep(void)
  {
    int n;
    uint ticks0;
  
    if(argint(0, &n) < 0)
      return -1;
    acquire(&tickslock);
    ticks0 = ticks;
    while(ticks - ticks0 < n){
      if(myproc()->killed){
        release(&tickslock);
        return -1;
      }
      sleep(&ticks, &tickslock);
    }
    release(&tickslock);
    return 0;
  }
  
  // return how many clock tick interrupts have occurred
  // since start.
  int
  sys_uptime(void)
  {
    uint xticks;
  
    acquire(&tickslock);
    xticks = ticks;
    release(&tickslock);
    return xticks;
  }
  // shuts down the machine
  int
  sys_shutdown(void)
  {
    outb(0x501, 0x0);
    exit();
    return 0;
  }
  
  // To see the current process state
  int
  sys_getptable(void)
  {
    int size;
    char *buf;
    char *s;
    struct proc *p = '\0';
  
    if(argint(0, &size) < 0)
      return -1;
    if(argptr(1, &buf, size) < 0)
      return -1;
    //buf gets updated
    //buf = ptables from ps.s
    s = buf;
    p = getptable_proc();
  
    while(buf + size > s && p->state != UNUSED)
    {
      *(int *)s = p->state;
      s += 4;
      *(int *)s = p->pid;
      s += 4;
      *(int *)s = p->parent->pid;
      s += 4;
      memmove(s, p->name, 16);
      s += 16;
      p++;
    }
    return 0;
  }