Switch语句AVR-GCC

Question

嘿，这只是一个简单的程序，我用c写的，在avr下编译的--gcc。相应的汇编代码也会发布。

仍然不能理解switch语句的汇编代码在做什么，任何帮助都会很好。谢谢。

int main()
{

char myinput;

printf("Which option will you choose:\n");
printf("a) Program 1 \n");
printf("b) Program 2 \n");
scanf("%c", &myinput);

switch (myinput)
    {
                case 'a':
                printf("Run program 1\n");
                break;
        case 'b':
            {
                printf("Run program 2\n");
                printf("Please Wait\n");
                break;
            }
        default:
                printf("Invalid choice\n");
                break;
    }
    return 0;

返回0；

}

汇编代码：

    switch (myinput)
+00000147:   900F        POP       R0             Pop register from stack
+00000148:   900F        POP       R0             Pop register from stack
+00000149:   900F        POP       R0             Pop register from stack
+0000014A:   900F        POP       R0             Pop register from stack
+0000014B:   8189        LDD       R24,Y+1        Load indirect with displacement
+0000014C:   3681        CPI       R24,0x61       Compare with immediate
+0000014D:   F019        BREQ      PC+0x04        Branch if equal
+0000014E:   3682        CPI       R24,0x62       Compare with immediate
+0000014F:   F459        BRNE      PC+0x0C        Branch if not equal
+00000150:   C003        RJMP      PC+0x0004      Relative jump
22:                         printf("Run program 1\n");
+00000151:   E38D        LDI       R24,0x3D       Load immediate
+00000152:   E092        LDI       R25,0x02       Load immediate
+00000153:   C009        RJMP      PC+0x000A      Relative jump
26:                         printf("Run program 2\n");
+00000154:   E48B        LDI       R24,0x4B       Load immediate
+00000155:   E092        LDI       R25,0x02       Load immediate
+00000156:   940E02A9    CALL      0x000002A9     Call subroutine
27:                         printf("Please Wait\n");
+00000158:   E589        LDI       R24,0x59       Load immediate
+00000159:   E092        LDI       R25,0x02       Load immediate
+0000015A:   C002        RJMP      PC+0x0003      Relative jump
31:                         printf("Invalid choice\n");
+0000015B:   E685        LDI       R24,0x65       Load immediate
+0000015C:   E092        LDI       R25,0x02       Load immediate
+0000015D:   940E02A9    CALL      0x000002A9     Call subroutine
38:       }
+0000015F:   E080        LDI       R24,0x00       Load immediate
+00000160:   E090        LDI       R25,0x00       Load immediate
+00000161:   900F        POP       R0             Pop register from stack
+00000162:   91CF        POP       R28            Pop register from stack
+00000163:   91DF        POP       R29            Pop register from stack
+00000164:   9508        RET                      Subroutine return

谢谢你们。

Answer 1

下面是对这些行含义的详细解释：

+0000014B:   8189        LDD       R24,Y+1        Load indirect with displacement

在这里，myinput的值从内存复制到寄存器r24。在内存中，它位于地址Y+1。Y中的值是位于myinput字符之前的字节的地址。

+0000014C:   3681        CPI       R24,0x61       Compare with immediate
+0000014D:   F019        BREQ      PC+0x04        Branch if equal

这里将myinput的值(在r24中)与0x61='a‘(在ascii中)进行比较。如果它们相等(myinput=='a')，则在+00000151: (0x14D+0x4=0x0x151)处继续执行。否则继续执行下一条指令。

+0000014E:   3682        CPI       R24,0x62       Compare with immediate
+0000014F:   F459        BRNE      PC+0x0C        Branch if not equal
+00000150:   C003        RJMP      PC+0x0004      Relative jump

这里再次将myinput值(以r24为单位)与0x62='b‘(以ASCII为单位)进行比较，但现在只有当它们不等于(默认情况)时，才会在+0000015B: (0x14F+0xC=0x0x15B)处继续执行。Else (myinput=='b')下一条指令是跳转到+00000154: (0x150+0x4=0x0x154)。

22:                         printf("Run program 1\n");
+00000151:   E38D        LDI       R24,0x3D       Load immediate
+00000152:   E092        LDI       R25,0x02       Load immediate
+00000153:   C009        RJMP      PC+0x000A      Relative jump

在'a'-casethe中，printf命令的参数(实际上是字符串的内存地址)存储在r25:r24中。然后，执行转移到+0000015D:处switch语句结束之前的最后一个printf调用。

26:                         printf("Run program 2\n");
+00000154:   E48B        LDI       R24,0x4B       Load immediate
+00000155:   E092        LDI       R25,0x02       Load immediate
+00000156:   940E02A9    CALL      0x000002A9     Call subroutine
27:                         printf("Please Wait\n");
+00000158:   E589        LDI       R24,0x59       Load immediate
+00000159:   E092        LDI       R25,0x02       Load immediate
+0000015A:   C002        RJMP      PC+0x0003      Relative jump

在‘b’的情况下，与前面类似，要打印的字符串的地址存储在r25:r24中。然后，执行在存储器地址0x000002A9处调用printf子例程。则第二串的地址被加载到r25:r24，并且执行跳转到在+0000015E:处的switch语句结束之前对printf的最后一次调用。

在这里可以很好地看到，字符串存储在内存中，因为第一个字符串有14 (0xE)个字节(字符)长，位于0x3D02__；下一个字符串位于0x3D02+0xE=0x4D02__!！

31:                         printf("Invalid choice\n");
+0000015B:   E685        LDI       R24,0x65       Load immediate
+0000015C:   E092        LDI       R25,0x02       Load immediate
+0000015D:   940E02A9    CALL      0x000002A9     Call subroutine

在默认情况下，和以前一样，字符串被加载到r25:r24，并调用printf。

38:       }
+0000015F:   E080        LDI       R24,0x00       Load immediate
+00000160:   E090        LDI       R25,0x00       Load immediate
+00000161:   900F        POP       R0             Pop register from stack
+00000162:   91CF        POP       R28            Pop register from stack
+00000163:   91DF        POP       R29            Pop register from stack
+00000164:   9508        RET                      Subroutine return

清理并返回..。

我希望这能对你有所帮助(即使很晚了)，如果不能，希望它能帮助那些在搜索中找到这篇文章的人，同时也有类似的问题！

Answer 2

偏移量0000014C比较"myInput“和”a“(ASCII97，十六进制0x61)，如果相等，则跳转到偏移量00000151 (在BREQ点是PC+4，偏移量0000014D，因为流水线的获取部分在执行部分之前)。

如果比较失败，则将其与'b‘(偏移量0000014E)进行比较，并再次进行相应分支。

如果失败了，我们就跳到最后一个案例。

注意，我们如何调用printf (调用0x2A9)以及它返回的位置有一些微妙之处。