main.c

#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <string.h>

//缓冲池大小
int poolsize;         // default size of text/data/stack

//内存一般分为代码段(text),数据段(data),未初始化数据段(bss),栈(stack),堆(heap)
//这里只关注text, data, stack三段.  其中, stack在内存顶上, 向下增长. 
// +------------------+
// |    stack   |     |      high address
// |    ...     v     |
// |                  |
// |                  |
// |                  |
// |                  |
// |    ...     ^     |
// |    heap    |     |
// +------------------+
// | bss  segment     |
// +------------------+
// | data segment     |
// +------------------+
// | text segment     |      low address
// +------------------+
int *text,            // text segment
     *old_text,        // for dump text segment
     *stack;           // stack
char *data;           // data segment

//寄存器, 真实电脑中有很多个寄存器, 这里只使用四种寄存器: 
// PC 程序计数器，它存放的是一个内存地址，该地址中存放着 下一条 要执行的计算机指令。
// SP 指针寄存器，永远指向当前的栈顶。注意的是由于栈是位于高地址并向低地址增长的，所以入栈时 SP 的值减小。
// BP 基址指针。也是用于指向栈的某些位置，在调用函数时会使用到它。
// AX 通用寄存器，我们的虚拟机中，它用于存放一条指令执行后的结果
int *pc, *bp, *sp, ax; // virtual machine registers

// CPU指令集
enum { LEA ,IMM ,JMP ,CALL,JZ  ,JNZ ,ENT ,ADJ ,LEV ,LI  ,LC  ,SI  ,SC  ,PUSH,
       OR  ,XOR ,AND ,EQ  ,NE  ,LT  ,GT  ,LE  ,GE  ,SHL ,SHR ,ADD ,SUB ,MUL ,DIV ,MOD ,
       OPEN, READ, CLOS, PRTF, MALC, MSET, MCMP, EXIT };

//这是一个虚机的实现. 
int eval() { 
  int op, *tmp;
  while (1) {
    op = *pc++; // get next operation code
    if (op == IMM)       {ax = *pc++;}                                     // load immediate value to ax
    else if (op == LC)   {ax = *(char *)ax;}                               // load character to ax, address in ax
    else if (op == LI)   {ax = *(int *)ax;}                                // load integer to ax, address in ax
    else if (op == SC)   {*(char *)*sp++ = ax;}                       // save character to address, value in ax, address on stack
    else if (op == SI)   {*(int *)*sp++ = ax;}                             // save integer to address, value in ax, address on stack
    else if (op == PUSH) {*--sp = ax;}
    else if (op == JMP)  {pc = (int *)*pc;} 
    else if (op == JZ)   {pc = ax ? pc + 1 : (int *)*pc;}   
    else if (op == JNZ)  {pc = ax ? (int *)*pc : pc + 1;}
    else if (op == CALL) {*--sp = (int)(pc+1); pc = (int *)*pc;}  
    else  if (op == ENT)  {*--sp = (int)bp; bp = sp; sp = sp - *pc++;} 
    else  if (op == ADJ)  {sp = sp + *pc++;}  
    else if (op == LEV)  {sp = bp; bp = (int *)*sp++; pc = (int *)*sp++;}  // restore call frame and PC
    else if (op == LEA)  {ax = (int)(bp + *pc++);}  
    else if (op == OR)  ax = *sp++ | ax;
    else if (op == XOR) ax = *sp++ ^ ax;
    else if (op == AND) ax = *sp++ & ax;
    else if (op == EQ)  ax = *sp++ == ax;
    else if (op == NE)  ax = *sp++ != ax;
    else if (op == LT)  ax = *sp++ < ax;
    else if (op == LE)  ax = *sp++ <= ax;
    else if (op == GT)  ax = *sp++ >  ax;
    else if (op == GE)  ax = *sp++ >= ax;
    else if (op == SHL) ax = *sp++ << ax;
    else if (op == SHR) ax = *sp++ >> ax;
    else if (op == ADD) ax = *sp++ + ax;
    else if (op == SUB) ax = *sp++ - ax;
    else if (op == MUL) ax = *sp++ * ax;
    else if (op == DIV) ax = *sp++ / ax;
    else if (op == MOD) ax = *sp++ % ax;
    else if (op == EXIT) { printf("exit(%d)\n", *sp); return *sp;}
    else if (op == OPEN) { ax = open((char *)sp[1], sp[0]); }
    else if (op == CLOS) { ax = close(*sp);}
    else if (op == READ) { ax = read(sp[2], (char *)sp[1], *sp); }
    else if (op == PRTF) { tmp = sp + pc[1]; ax = printf((char *)tmp[-1], tmp[-2], tmp[-3], tmp[-4], tmp[-5], tmp[-6]); }
    else if (op == MALC) { ax = (int)malloc(*sp);}
    else if (op == MSET) { ax = (int)memset((char *)sp[2], sp[1], *sp);}
    else if (op == MCMP) { ax = memcmp((char *)sp[2], (char *)sp[1], *sp);}
    else {
        printf("unknown instruction:%d\n", op);
        return -1;
    }
  }
  return 0;
}
 
int main(int argc, char **argv)
{
  poolsize = 256 * 1024; // arbitrary size
  // 为虚机分配内存
  if (!(text = old_text = malloc(poolsize))) {
      printf("could not malloc(%d) for text area\n", poolsize);
      return -1;
  }
  if (!(data = malloc(poolsize))) {
      printf("could not malloc(%d) for data area\n", poolsize);
      return -1;
  }
  if (!(stack = malloc(poolsize))) {
      printf("could not malloc(%d) for stack area\n", poolsize);
      return -1;
  }

  memset(text, 0, poolsize);
  memset(data, 0, poolsize);
  memset(stack, 0, poolsize);

  bp = sp = stack + poolsize / sizeof(int);
  ax = 0;

  //以下为测试用的指令. 计算10+20的值. 
  int i = 0;
  text[i++] = IMM;
  text[i++] = 10;
  text[i++] = PUSH;
  text[i++] = IMM;
  text[i++] = 20;
  text[i++] = ADD;
  text[i++] = PUSH;
  text[i++] = EXIT;
  pc = text;

  return eval();
}

//运行本文件: 
//gcc main.c 
//./a.out
//应该输出exit(30)