// Segments in proc->gdt.
// Also known to bootasm.S and trapasm.S
#define SEG_KCODE 1  // kernel code
#define SEG_KDATA 2  // kernel data+stack
#define SEG_KCPU  3  // kernel per-cpu data
#define SEG_UCODE 4
#define SEG_UDATA 5
#define SEG_TSS   6  // this process's task state
#define NSEGS     7

// Saved registers for kernel context switches.
// Don't need to save all the segment registers (%cs, etc),
// because they are constant across kernel contexts.
// Don't need to save %eax, %ecx, %edx, because the
// x86 convention is that the caller has saved them.
// Contexts are stored at the bottom of the stack they
// describe; the stack pointer is the address of the context.
// The layout of the context must match the code in swtch.S.
struct context {
  uint edi;
  uint esi;
  uint ebx;
  uint ebp;
  uint eip;
};

enum procstate { UNUSED, EMBRYO, SLEEPING, RUNNABLE, RUNNING, ZOMBIE };

// Per-process state
struct 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 procstate state;       // Process state
  volatile int pid;            // Process ID
  struct proc *parent;         // Parent process
  struct trapframe *tf;        // Trap frame for current syscall
  struct context *context;     // Switch here to run process
  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
  char name[16];               // Process name (debugging)
};

// Process memory is laid out contiguously, low addresses first:
//   text
//   original data and bss
//   fixed-size stack
//   expandable heap

// Per-CPU state
struct cpu {
  uchar apicid;                // Local APIC ID
  struct context *context;     // Switch here to enter scheduler
  struct taskstate ts;         // Used by x86 to find stack for interrupt
  struct segdesc gdt[NSEGS];   // x86 global descriptor table
  volatile uint booted;        // Has the CPU started?
  int ncli;                    // Depth of pushcli nesting.
  int intena;                  // Were interrupts enabled before pushcli?
  void *tls[2];
};

extern struct cpu cpus[NCPU];
extern int ncpu;

// Per-CPU variables, holding pointers to the
// current cpu and to the current process.
// The __thread prefix tells gcc to refer to them in the segment
// pointed at by gs; the name __thread derives from the use
// of the same mechanism to provide per-thread storage in
// multithreaded user programs.
extern __thread struct cpu *c;       // This cpu.
extern __thread struct proc *cp;     // Current process on this cpu.