POJ1049 Microprocessor Simulation

题目来源：http://poj.org/problem?id=1049

题目大意：

　　一种小型的微处理器有以下特性：

　　1. 每个字长4bit.

　　2. 地址用2个字进行编码。先高位字后低位字，即高位字大的地址占据内存中靠前的字。

　　3. 内存大小为256个字。

　　4. 有两个微处理器A和B，每个存储一个字。

　　5. 有9个指令编码。每条指令需要至少一个字来存储编码，其中有4条指令含参数，并需要额外的2个字。

　　每4个bit组成的字可以取值0-15(10进制)，下文中我们将用16进制来表示这些数。

　　9条指令说明如下：

Code	Words	Description
0	3	LD: Load accumulator A with the contents of memory at the specified argument.
1	3	ST: Write the contents of accumulator A to the memory location specified by the argumen
2	1	SWP: Swap the contents of accumulators A and B.
3	1	ADD: Add the contents of accumulators A and B. The low word of the sum is stored in A, and the high word in B
4	1	INC: Increment accumulator A. Overflow is allowed; that is, incrementing F yields 0.
5	1	DEC: Decrement accumulator A. Underflow is allowed; that is, decrementing 0 yields F.
6	3	BZ: If accumulator A is zero, the next command to be executed is at the location specified by the argument. If A is not zero, the argument is ignored and nothing happens.
7	3	BR: The next command to be executed is at the location specified by the argument.
8	1	STP: Stop execution of the program.

程序总是最先执行地址00处的指令，然后依次执行后面的指令直到遇到Stop指令。

下面的例子展示了一些片段程序并描述了它的作用。

Program	Description
01A8	Load accumulator A with the contents of memory location 1A (26 in decimal) and stop.
01A512F8	Load accumulator A with the contents of memory location 1A (26 in decimal), decrement it, store the result to memory location 2F, then stop.

输入：输入由若干行组成，每行256个16进制数字（1-9, A-F）.每行表示的是内存的内容，地址编码从00到FF。00地址的内存单元为Stop指令时标志着输入的结束。输入的程序保证了程序的指令不会位于地址F0到FF的内存单元中。

输出：对每个输入的内存块，模拟微处理器的执行，输出程序执行后的每个内存字，格式与输入一致。

---

Sample Input

0102011311321128FF0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

Sample Output

0102011311321128FF1E00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

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标题已经告诉我们方法：模拟。关键是细心，比如数字的INC和DEC要注意从数字到字母的跳变，还有分清楚值和址。勾起了当年做组成原理课程设计的"美好回忆"。=。=

 //////////////////////////////////////////////////////////////////////////
 //        POJ1049 Microprocessor Simulation
 //        Memory: 164K        Time: 0MS
 //        Language: C++        Result: Accepted
 //////////////////////////////////////////////////////////////////////////

 #include <cstdio>

 using namespace std;

 char mem[];
 char A, B;
 int pc;

 int Hex2Dec(char c) {
     if (c >= '' && c <= '') return c - '';
     else return c - 'A' + ;
 }

 int DHex2Dec(char a, char b) {
     return Hex2Dec(a) *  + Hex2Dec(b);
 }

 char Dec2Hex(int i) {
     if (i >= ) return 'A' + i - ;
     else return i + '';
 }

 int main(void) {
     while (scanf("%s", mem) && mem[] != '') {
         pc = ;
         char command;
         char buf;
         int buf1, buf2;
         A = B = '';
         while ((command = mem[pc]) != '') {
             switch (command) {
                 case ''://LD
                     A = mem[DHex2Dec(mem[pc + ], mem[pc + ])];
                     pc += ;
                     break;
                 case ''://ST
                     mem[DHex2Dec(mem[pc + ], mem[pc + ])] = A;
                     pc += ;
                     break;
                 case ''://SWP
                     buf = A;
                     A = B;
                     B = buf;
                     ++pc;
                     break;
                 case ''://ADD
                     buf1 = Hex2Dec(A);
                     buf2 = Hex2Dec(B);
                     B = Dec2Hex((buf1 + buf2) / );
                     A = Dec2Hex((buf1 + buf2) % );
                     ++pc;
                     break;
                 case ''://INC
                     if (A == 'F') A = '';
                     else if (A == '') A = 'A';
                     else ++A;
                     ++pc;
                     break;
                 case ''://DEC
                     if (A == '') A = 'F';
                     else if (A == 'A') A = '';
                     else --A;
                     ++pc;
                     break;
                 case ''://BZ
                     if (A == '') pc = DHex2Dec(mem[pc + ], mem[pc + ]);
                     else pc += ;
                     break;
                 case ''://BR
                     pc = DHex2Dec(mem[pc + ], mem[pc + ]);
                     break;
             }
         }
         printf("%s\n", mem);
     }
     return ;
 }