// Segments in proc->gdt#define SEG_KCODE 1  // kernel code#define SEG_KDATA 2  // kernel data+stack#define SEG_UCODE 3#define SEG_UDATA 4#define SEG_TSS   5  // this process's task state#define NSEGS     6// Saved registers for kernel context switches.// Don't need to save all the %fs etc. segment registers,// because they are constant across kernel contexts.// Save all the regular registers so we don't need to care// which are caller save.// Don't save %eax, because that's the return register.// The layout of jmpbuf is known to setjmp.S.struct jmpbuf {  int ebx;  int ecx;  int edx;  int esi;  int edi;  int esp;  int ebp;  int eip;};enum proc_state { UNUSED, EMBRYO, SLEEPING, RUNNABLE, RUNNING, ZOMBIE };// Per-process statestruct proc {  char *mem;                // Start of process memory (kernel address)  uint sz;                  // Size of process memory (bytes)  char *kstack;             // Bottom of kernel stack for this process  enum proc_state state;    // Process state  int pid;                  // Process ID  int ppid;                 // Parent pid  void *chan;               // If non-zero, sleeping on chan  int killed;               // If non-zero, have been killed  struct file *ofile[NOFILE];  // Open files  struct inode *cwd;        // Current directory  struct jmpbuf jmpbuf;     // Jump here to run process  struct trapframe *tf;     // Trap frame for current interrupt};// Process memory is laid out contiguously://   text//   original data and bss//   fixed-size stack//   expandable heapextern struct proc proc[];extern struct proc *curproc[NCPU];  // Current (running) process per CPU#define MPSTACK 512// Per-CPU statestruct cpu {  uchar apicid;               // Local APIC ID  struct jmpbuf jmpbuf;       // Jump here to enter scheduler  struct taskstate ts;        // Used by x86 to find stack for interrupt  struct segdesc gdt[NSEGS];  // x86 global descriptor table  char mpstack[MPSTACK];      // Per-CPU startup stack  volatile int booted;        // Has the CPU started?  int nlock;                  // Number of locks currently held};extern struct cpu cpus[NCPU];extern int ncpu;