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对不起，这个问题很长，但我真的迷失了这个任务。我基本上被要求实现 fetchInstruction。我猜，另外两个是我应该使用的辅助函数。我在下面附上了 fetchInstruction.c 代码和 instruction.h 代码。我不期待直接的答案和完整的代码，但是有人可以指出我如何开始这个正确的方向吗？对于那些想知道的人，programMap 指的是内存本身，而 location 是指令的位置。

/* Reads one byte from memory, at the specified address. Stores the
     read value into *value. Returns 1 in case of success, or 0 in case
     of failure (e.g., if the address is beyond the limit of the memory
     size). */
int memReadByte(machine_state_t *state, uint64_t address, uint8_t *value)
{
    if (address > machine_state->programSize)
       return 0;
    *value = machine_state->programMap[address];
    return 1;
}

/* Reads one quad-word (64-bit number) from memory in little-endian
     format, at the specified starting address. Stores the read value
     into *value. Returns 1 in case of success, or 0 in case of failure
     (e.g., if the address is beyond the limit of the memory size). */
int memReadQuadLE(machine_state_t *state, uint64_t address, uint64_t *value)
{

    return 0;
}

/* Fetches one instruction from memory, at the address specified by
     the program counter. Does not modify the machine's state. The
     resulting instruction is stored in *instr. Returns 1 if the
     instruction is a valid non-halt instruction, or 0 (zero)
     otherwise. */
int fetchInstruction(machine_state_t *state, y86_instruction_t *instr)
{

    return 0;
}

typedef enum y86_icode {
  I_HALT      = 0x0,
  I_NOP       = 0x1,
  I_RRMVXX    = 0x2,
  I_IRMOVQ    = 0x3,
  I_RMMOVQ    = 0x4,
  I_MRMOVQ    = 0x5,
  I_OPQ       = 0x6,
  I_JXX       = 0x7,
  I_CALL      = 0x8,
  I_RET       = 0x9,
  I_PUSHQ     = 0xA,
  I_POPQ      = 0xB,
  I_INVALID   = 0xE,
  I_TOO_SHORT = 0xF
} y86_icode_t;

typedef enum y86_reg {
  R_RAX = 0x0,
  R_RCX = 0x1,
  R_RDX = 0x2,
  R_RBX = 0x3,
  R_RSP = 0x4,
  R_RBP = 0x5,
  R_RSI = 0x6,
  R_RDI = 0x7,
  R_R8  = 0x8,
  R_R9  = 0x9,
  R_R10 = 0xA,
  R_R11 = 0xB,
  R_R12 = 0xC,
  R_R13 = 0xD,
  R_R14 = 0xE,
  R_NONE = 0xF
} y86_register_t;

typedef enum y86_condition {
  C_NC = 0x0,
  C_LE = 0x1,
  C_L  = 0x2,
  C_E  = 0x3,
  C_NE = 0x4,
  C_GE = 0x5,
  C_G  = 0x6
} y86_condition_t;

typedef enum y86_operation {
  A_ADDQ = 0x0,
  A_SUBQ = 0x1,
  A_ANDQ = 0x2,
  A_XORQ = 0x3,
  A_MULQ = 0x4,
  A_DIVQ = 0x5,
  A_MODQ = 0x6
} y86_operation_t;

typedef struct instruction {

  y86_icode_t    icode;
  uint8_t        ifun;
  y86_register_t rA;
  y86_register_t rB;
  uint64_t       valC;
  
  uint64_t       location;
  uint64_t       valP;
} y86_instruction_t;

#define CC_ZERO_MASK     0x1
#define CC_SIGN_MASK     0x2
#define CC_CARRY_MASK    0x4
#define CC_OVERFLOW_MASK 0x8

typedef struct machine_state {
  
  uint8_t *programMap;
  uint64_t programSize;
  
  uint64_t programCounter;

  uint64_t registerFile[16];

  uint8_t conditionCodes;
  
} machine_state_t;
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