Dosdbg--一个简单的调试器

代码摘自 《虚拟机的设计与实现----C/C++》，是实模式下的调试器。但是做得很粗糙，我们这里对他的代码进行了简单的分析。

#include

/*global variables-------------------------------------------------*/
char INT1_STR[]="->SINGLED-STEP<-\n";
char INT3_STR[]="->BREAKPOINT<-\n";
unsigned short rCS,rSS,rDS;
short rAX,rBX,rCX,rDX;
unsigned short rIP;
unsigned char traceOn=0;
/*prototypes-------------------------------------------------------*/
void procCmd(int *lptr);
/*definitions------------------------------------------------------*/

void main()
{
    unsigned short oldInt1Seg;
    unsigned short oldInt1Offset;
    unsigned short oldInt3Seg;
    unsigned short oldInt3Offset;

    goto past_interrupts;

    /* Handle INT 1 -----------------------------------------------*/
    int1:
    __asm
    {
        STI
        MOV rCS,CS /* 保存现场 */
        MOV rSS,SS
        MOV rDS,DS
        MOV rAX,AX
        MOV rBX,BX
        MOV rCX,CX
        MOV rDX,DX
        POP CX
        MOV rIP,CX
        POP DX
        POP AX
        AND AX,65279
        PUSH AX
        PUSH DX
        PUSH CX
    }
    printf("%s",INT1_STR);
    printf("next instruction at IP=%u\n",rIP);
    {
        int loop = 1;
        while(loop)
        {
            procCmd(&loop);
        }
    }
    if(traceOn) /* 单步执行下一条指令 */
    {
        __asm
        {
            POP CX
            POP DX
            POP AX
            OR AX,256
            PUSH AX
            PUSH DX
            PUSH CX
        }
    }
    __asm
    {
        MOV AX,rAX
        MOV BX,rBX
        MOV CX,rCX
        MOV DX,rDX
        IRET
    }
    /* Handle INT 3 -----------------------------------------------*/

    int3: /* 同上 */
    __asm
    {
        STI
        MOV rCS,CS
        MOV rSS,SS
        MOV rDS,DS
        MOV rAX,AX
        MOV rBX,BX
        MOV rCX,CX
        MOV rDX,DX
        POP CX
        POP DX
        POP AX
        AND AX,65279
        PUSH AX
        PUSH DX
        PUSH CX
    }

    printf("%s",INT3_STR);
    {
        int loop = 1;

        while(loop)
        {
            procCmd(&loop);
        }
    }
    if(traceOn)
    {
        __asm
        {
            POP CX
            POP DX
            POP AX
            OR AX,256
            PUSH AX
            PUSH DX
            PUSH CX
        }
    }
    __asm
    {
        MOV AX,rAX
        MOV BX,rBX
        MOV CX,rCX
        MOV DX,rDX
        IRET
    }
    /* Execution path begins here --------------------------------*/
    past_interrupts:
    printf("Save old interrupts SEG:OFF\n");
    __asm
    {
        MOV AH,0x35 /* 保存 int1 的cs:off */
        MOV AL,0x1
        INT 0x21
        MOV oldInt1Seg,ES
        MOV oldInt1Offset,BX
        MOV AH,0x35 /* 保存 int3 的cs:off */
        MOV AL,0x3
        INT 0x21
        MOV oldInt3Seg,ES
        MOV oldInt3Offset,BX
    }
    printf("Load new interrupts SEG:OFF\n");
    __asm
    {
        MOV AH,0x25
        MOV AL,0x1
        PUSH DS
        MOV CX,CS
        MOV DS,CX
        MOV DX,OFFSET int1
        INT 0x21
        POP DS
        MOV AH,0x25
        MOV AL,0x3
        PUSH DS
        MOV CX,CS
        MOV DS,CX
        MOV DX,OFFSET int3
        INT 0x21
        POP DS
    }

    /* actually do something here to provoke debugger -------------*/
    __asm
    {
        INT 3
        MOV DX,20
        INC DX
        NOP
        MOV DX,3501
        MOV DX,72
        DEC DX
    }

    printf("Re-loading old interrupts SEG:OFF\n");

    __asm
    {
        PUSH DS
        MOV AH,0x25
        MOV AL,0x1
        MOV DS,oldInt1Seg
        MOV DX,oldInt1Offset
        INT 0x21
        POP DS
        PUSH DS
        MOV AH,0x25
        MOV AL,0x3
        MOV DS,oldInt3Seg
        MOV DX,oldInt3Offset
        INT 0x21
        POP DS
    }
    return;
}/*end main*/

void procCmd(int *lptr)
{
    char ch;

    traceOn=0;

    printf("dbg>");
    scanf("%c",&ch);
    fflush(stdin);

    switch(ch)
    {
        case 'a':
        {
            printf("INT1_STR address=%u\n",INT1_STR);
        }break;
        case 'd': /* 显示寄存器的内容 */
        {
            printf("CS=%u\n",rCS);
            printf("SS=%u\n",rSS);
            printf("DS=%u\n",rDS);
            printf("AX=%d\n",rAX);
            printf("BX=%d\n",rBX);
            printf("CX=%d\n",rCX);
            printf("DX=%d\n",rDX);
        }break;
        case 'i': /* 对ax进行递增操作 */
        {
            rAX++;
            printf("AX=%d\n",rAX);
        }break;
        case 'm': /* 显示一个16字节长的内存块 */
        {
            int i;
            unsigned long address;
            unsigned long limit = rCS+65535;
            unsigned char *sptr;

            printf("memory address>");
            scanf("%lu",&address);
            fflush(stdin);
            sptr = (unsigned char*)address;
            printf("address=%u\n",sptr);
            if(address > limit)
            {
                printf("address is beyond .COM segment\n");
            }
            else
            {
                for(i=0;i<16;i++)
                {
                    if((sptr[i]>0x20)&&(sptr[i]<0x7F))
                    {
                        printf("byte[%lu]=%c\n",address+i,sptr[i]);
                    }
                    else
                    {
                        printf("byte[%lu]=%X\n",address+i,sptr[i]);
                    }
                }
            }
        }break;
        case 'q':
        {
            *lptr=0;
        }break;
        case 't': /* 设置单步调试 */
        {
            traceOn=1;
            printf("trace flag set\n");
            *lptr=0;
        }break;
        default:
        {
            printf("not valid command\n");
        }
    }

    return;

}/*end procCmd*/