现在需要维护系统中的空闲页框数，你需要 - 在`kalloc.c`文件中定义一个全局变量`freeMemCnt`，记录空闲页框数; - 在分配和回收内存时维护这个全局变量； - 然后实现系统调用`int freemem(void)`查询这个变量的值； - 实现以下用户命令`free`，打印空闲页框数。int main(int argc, char **argv) { printf(1,"# of free frames is %d\n",freeMem()); if(fork()==0){ printf(1,"Child, # of free frames is %d\n",freeMem()); }else{ wait(); printf(1,"parent: # of free frames is %d\n",freeMem()); } exit(); }

好的，那么在 `kalloc.c` 中添加全局变量 `freeMemCnt`，并在分配和回收内存时维护这个变量的代码如下：

#include "defs.h"
#include "param.h"
#include "memlayout.h"
#include "mmu.h"
#include "spinlock.h"
#include "proc.h"
#include "x86.h"

static struct run {
  struct run *next;
} *kfreelist;

static uint freeMemCnt = 0; // 新增全局变量

void
kinit1(void *vstart, void *vend)
{
  kfreelist = 0;
  char *p;
  p = (char*)PGROUNDUP((uint)vstart);
  for(; p + PGSIZE <= (char*)vend; p += PGSIZE) {
    kfree(p);
  }
}

void
kfree(char *v)
{
  struct run *r;

  if((uint)v % PGSIZE || v < end || V2P(v) >= PHYSTOP)
    panic("kfree");

  // Acquire the lock on kfreelist
  acquire(&kmem.lock);

  // Add the memory block to kfreelist
  r = (struct run*)v;

  // 维护 freeMemCnt 变量
  freeMemCnt++; 

  r->next = kfreelist;
  kfreelist = r;

  // Release the lock on kfreelist
  release(&kmem.lock);
}

char*
kalloc(void)
{
  struct run *r;

  // Acquire the lock on kfreelist
  acquire(&kmem.lock);

  // Get a memory block from kfreelist
  r = kfreelist;
  if(r)
    kfreelist = r->next;

  // 维护 freeMemCnt 变量
  if (freeMemCnt > 0) {
    freeMemCnt--;
  }

  // Release the lock on kfreelist
  release(&kmem.lock);

  return (char*)r;
}

然后在 `syscall.h` 中定义 `freemem` 系统调用的宏定义：

#define SYS_freemem 23

在 `syscall.c` 中添加 `sys_freemem` 系统调用的函数实现：

extern int sys_freemem(void);

static int (*syscalls[])(void) = {
    ...
    [SYS_freemem]  sys_freemem,
    ...
};

int
sys_freemem(void)
{
    return freeMemCnt;
}

在 `usys.S` 中添加 `freemem` 系统调用的汇编实现：

SYSCALL(freemem)

最后，在用户空间编写 `free.c` 程序，实现以下用户命令 `free`：

#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"

int
main(int argc, char *argv[])
{
    printf("Free page frames: %d\n", freemem());
    exit(0);
}

最后，再使用以下代码测试一下：

#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"

int
main(int argc, char **argv)
{
    printf(1, "# of free frames is %d\n", freemem());
    if (fork() == 0) {
        printf(1, "Child, # of free frames is %d\n", freemem());
    } else {
        wait();
        printf(1, "Parent: # of free frames is %d\n", freemem());
    }
    exit(0);
}