Question

因此，根据我所看到的几个来源，在32位mips中不可能使用32位立即数，因为机器指令是32位并且存储立即数值在机器说明书内。据我所知，最大的立即数可以是16位，为剩下的指令留出空间。但在MARS 4.5 mips中，此指令工作正常（使用默认设置）：

#include<iostream>
using namespace std;

class linkedList{

public:
  int content;
  linkedList* pointerToBelow;
};

void freeTheMemory(linkedList *top) {

  if(top != nullptr){
    linkedList* temp = top->pointerToBelow;
    delete top;
    freeTheMemory(temp); // temp is now the top
  }
}

void push(linkedList* top, int x){

  // create new node, assign x
  linkedList *node = new linkedList;
  node->content = x;

  // new node points to top element in the stack
  node->pointerToBelow = top->pointerToBelow;

  // top now points to the new node
  top->pointerToBelow = node;
}

void pop(linkedList* top ){

  linkedList* toDelete = top->pointerToBelow;

  // case 1: stack is empty
  if(toDelete == nullptr)
    return; // do nothing

  // case 2: deleting top element
  top->pointerToBelow = toDelete->pointerToBelow;
  delete toDelete;
}

void printList(linkedList *top) {
  top = top->pointerToBelow;

  while(top != nullptr) {
    cout << top->content << " ";
    top = top->pointerToBelow;
  }
}

int main(){

int x;
linkedList *top = new linkedList;

top->pointerToBelow = nullptr; // indicates stack is empty

cout<<"Enter the elements of your list: ";

while(x != -9) {
   cin >> x;

    if(x! =- 9) {
      if(x>0)
        push(top, x);
      else
        pop(top);
    }
  }

  printList(top);
  freeTheMemory(top);

  return 0;
}

这组装并且运行正常，这让我感到困惑，因为我认为它不应该出于上述原因（32位机器代码怎么能保持32位立即？）。我最好的猜测是MARS中的ori实际上运行伪指令，当直接字段高于0xffff时，lui＆or; s和ori定期加载，但我不确定。有人可以对此有所了解吗？

Answer 1

如果您注意到在火星4.5中组装该指令时，您会得到三条指令

lui $1,0xFFFFFFFF
ori $1,$1,0x0000FFFF
or $9,$0,$1

这仍然有点误导，但基本上它会为你修复它。

为什么MARS可以立即使用32位？

1 个答案: