直接映射Cache模拟器

直接映射是Cache中结构最简单的一种。

一、Cache的大小和结构

直接映射Cache的结构如下：（因为每组只有一行，所以下文中行、组代表相同含义）

代码如下：

本实验采用总容量16KB，每一组（行）64字节，则共256组（行）

typedef unsigned char      UINT8;
typedef unsigned short     UINT16;
typedef unsigned int       UINT32;
typedef int                INT32;
typedef unsigned long long UINT64;                                          // 数字表示数据的位数
#define DCACHE_SIZE 16384                                                   // 总大小16KB
#define DCACHE_DATA_PER_LINE 64                                             // 每行64字节
#define DCACHE_DATA_PER_LINE_ADDR_BITS GET_POWER_OF_2(DCACHE_DATA_PER_LINE) // 2^6=64,即64字节需要6位地址
#define DCACHE_SET (DCACHE_SIZE / DCACHE_DATA_PER_LINE)                     // 256行
#define DCACHE_SET_ADDR_BITS GET_POWER_OF_2(DCACHE_SET)                     // 2^8=256,即256行需要8位地址

定义一个DCache数组，每个元素表示一行（组）的结构：

struct DCACHE_LineStruct
{UINT8 Valid;                     // 只要从主存加载过数据到该行，Valid变为1UINT8 Dirty;                    // 标识该行是否被修改过（写回策略）UINT64 Tag;                        // 行标记，用于区分映射到该行的不同地址的数据UINT8 Data[DCACHE_DATA_PER_LINE]; // 数据块，每个1字节，共64字节
} DCache[DCACHE_SET];

DCache的初始化：

void InitDataCache()
{UINT32 i;for (i = 0; i < DCACHE_SET; i++){DCache[i].Valid = 0; // 最初从未从主存加载过数据，Valid=0DCache[i].Dirty = 0; // 最初数据未被修改过，Dirty=0}
}

二、Cache对读、写操作的处理

参数：Address主存地址；
operation操作类型，'R’读’W’写；
DataSize数据的大小（字节数）；
StoreValue: 当执行写操作的时候，需要写入的数据；
LoadResult: 当执行读操作的时候，从Cache读出的数据

首先要对地址进行切分，得到标记、行号、块偏移

 CacheLineAddress = (Address >> DCACHE_DATA_PER_LINE_ADDR_BITS) % DCACHE_SET;   // Cache的行号，在直接映射中，就是组号（每组1行）BlockOffset = Address % DCACHE_DATA_PER_LINE;                                     // 块偏移，数据起始字节所在的位置AddressTag = (Address >> DCACHE_DATA_PER_LINE_ADDR_BITS) >> DCACHE_SET_ADDR_BITS; // 地址去掉DCACHE_SET、DCACHE_DATA_PER_LINE，剩下的作为Tag

根据行号索引到对应组，再根据有效位和标记判断是否命中

（一）命中时

即满足两个条件：索引到的行有效，且标记位与AddressTag相同

1. 读操作

根据需要的字节数直接读取即可
注意要对齐到相应字节边界，如4字节：BlockOffset & 0xFC，即把最后2位设置为0，一定为4的倍数

         ReadValue = 0;switch (DataSize){case 1: // 1个字节ReadValue = DCache[CacheLineAddress].Data[BlockOffset + 0];break;case 2:                                // 2个字节BlockOffset = BlockOffset & 0xFE; // 需对齐到2字节边界ReadValue = DCache[CacheLineAddress].Data[BlockOffset + 1];ReadValue = ReadValue << 8;ReadValue |= DCache[CacheLineAddress].Data[BlockOffset + 0];break;case 4:                                // 4个字节BlockOffset = BlockOffset & 0xFC; // 需对齐到4字节边界ReadValue = DCache[CacheLineAddress].Data[BlockOffset + 3];ReadValue = ReadValue << 8;ReadValue |= DCache[CacheLineAddress].Data[BlockOffset + 2];ReadValue = ReadValue << 8;ReadValue |= DCache[CacheLineAddress].Data[BlockOffset + 1];ReadValue = ReadValue << 8;ReadValue |= DCache[CacheLineAddress].Data[BlockOffset + 0];break;case 8:                                // 8个字节BlockOffset = BlockOffset & 0xF8; // 需对齐到8字节边界ReadValue = DCache[CacheLineAddress].Data[BlockOffset + 7];ReadValue = ReadValue << 8;ReadValue |= DCache[CacheLineAddress].Data[BlockOffset + 6];ReadValue = ReadValue << 8;ReadValue |= DCache[CacheLineAddress].Data[BlockOffset + 5];ReadValue = ReadValue << 8;ReadValue |= DCache[CacheLineAddress].Data[BlockOffset + 4];ReadValue = ReadValue << 8;ReadValue |= DCache[CacheLineAddress].Data[BlockOffset + 3];ReadValue = ReadValue << 8;ReadValue |= DCache[CacheLineAddress].Data[BlockOffset + 2];ReadValue = ReadValue << 8;ReadValue |= DCache[CacheLineAddress].Data[BlockOffset + 1];ReadValue = ReadValue << 8;ReadValue |= DCache[CacheLineAddress].Data[BlockOffset + 0];break;}*LoadResult = ReadValue;

2.写操作

由于修改了Cache的值，涉及到要不要修改相应主存地址的值，有两种策略：写直达和写回。
一般采用效率更高的写回
即先不修改主存内的数据，一直用Cache中的那个，直到其将要被驱逐时，再修改主存的值。
实现起来需要引入Dirty标记，逻辑为：新加载到Cache一行时，其Dirty设置为0，表示未被修改。当用写操作修改了Cache中这行的数据后，其Dirty设置为1，表示其已经和主存的值不同了。如果要驱逐这行，驱逐前要把这行修改过的块存到它原来所在的地址。

         switch (DataSize){case 1: // 1个字节DCache[CacheLineAddress].Data[BlockOffset + 0] = StoreValue & 0xFF;break;case 2:                               // 2个字节BlockOffset = BlockOffset & 0xFE; // 需对齐到2字节边界DCache[CacheLineAddress].Data[BlockOffset + 0] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 1] = StoreValue & 0xFF;break;case 4:                               // 4个字节BlockOffset = BlockOffset & 0xFC; // 需对齐到4字节边界DCache[CacheLineAddress].Data[BlockOffset + 0] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 1] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 2] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 3] = StoreValue & 0xFF;break;case 8:                                 // 8个字节BlockOffset = BlockOffset & 0xF8; // 需对齐到8字节边界DCache[CacheLineAddress].Data[BlockOffset + 0] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 1] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 2] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 3] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 4] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 5] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 6] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 7] = StoreValue & 0xFF;break;}DCache[CacheLineAddress].Dirty = 1;//由于写入后数据被修改，Dirty设为1

（二）未命中

需要从主存中加载数据到Cache中。
在此之前，根据写回策略，如果这一行已经被修改（Dirty=1）,需要先倒推出其在主存的地址，并存回去。

     if (DCache[CacheLineAddress].Dirty){UINT64 OldAddress;                  // 计算原地址，OldAddress = > (Tag,Set,0000)OldAddress = ((DCache[CacheLineAddress].Tag << DCACHE_SET_ADDR_BITS) << DCACHE_DATA_PER_LINE_ADDR_BITS) | ((UINT64)CacheLineAddress << DCACHE_DATA_PER_LINE_ADDR_BITS); // 从Tag中恢复旧的地址StoreDataCacheLineToMemory(OldAddress, CacheLineAddress);DCache[CacheLineAddress].Dirty = 0; // 将新加载一行，Dirty设为1}

再加载数据，设置有效位和标记

     LoadDataCacheLineFromMemory(Address, CacheLineAddress);DCache[CacheLineAddress].Valid = 1;DCache[CacheLineAddress].Tag = AddressTag;

1.读操作

不用读了

2.写操作

写回策略一般搭配写分配，即加载到Cache后只修改Cache中的数据，主存不变
和命中时的写一样

         switch (DataSize){case 1: // 1个字节DCache[CacheLineAddress].Data[BlockOffset + 0] = StoreValue & 0xFF;break;case 2:                               // 2个字节BlockOffset = BlockOffset & 0xFE; // 需对齐到2字节边界DCache[CacheLineAddress].Data[BlockOffset + 0] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 1] = StoreValue & 0xFF;break;case 4:                               // 4个字节BlockOffset = BlockOffset & 0xFC; // 需对齐到4字节边界DCache[CacheLineAddress].Data[BlockOffset + 0] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 1] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 2] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 3] = StoreValue & 0xFF;break;case 8:                                 // 8个字节BlockOffset = BlockOffset & 0xF8; // 需对齐到8字节边界DCache[CacheLineAddress].Data[BlockOffset + 0] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 1] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 2] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 3] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 4] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 5] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 6] = StoreValue & 0xFF;StoreValue = StoreValue >> 8;DCache[CacheLineAddress].Data[BlockOffset + 7] = StoreValue & 0xFF;break;}

(三）Load和Store函数

1.Load函数

从内存加载一块数据到Cache中

void LoadDataCacheLineFromMemory(UINT64 Address, UINT32 CacheLineAddress)
{// 一次性从Memory中将DCACHE_DATA_PER_LINE数据读入某个Data Cache行// 提供了一个函数，一次可以读入8个字节UINT32 i;UINT64 ReadData;UINT64 AlignAddress;UINT64 *pp;AlignAddress = Address & ~(DCACHE_DATA_PER_LINE - 1); // 地址必须对齐到DCACHE_DATA_PER_LINE (64)字节边界pp = (UINT64 *)DCache[CacheLineAddress].Data;for (i = 0; i < DCACHE_DATA_PER_LINE / 8; i++){ReadData = ReadMemory(AlignAddress + 8LL * i);if (DEBUG)printf("[%s] Address=%016llX ReadData=%016llX\n", __func__, AlignAddress + 8LL * i, ReadData);pp[i] = ReadData;}
}

2.Store函数

从Cache中把一行存回主存

void StoreDataCacheLineToMemory(UINT64 Address, UINT32 CacheLineAddress)
{// 一次性将DCACHE_DATA_PER_LINE数据从某个Data Cache行写入Memory中// 提供了一个函数，一次可以写入8个字节UINT32 i;UINT64 WriteData;UINT64 AlignAddress;UINT64 *pp;AlignAddress = Address & ~(DCACHE_DATA_PER_LINE - 1); // 地址必须对齐到DCACHE_DATA_PER_LINE (64)字节边界pp = (UINT64 *)DCache[CacheLineAddress].Data;WriteData = 0;for (i = 0; i < DCACHE_DATA_PER_LINE / 8; i++){WriteData = pp[i];WriteMemory(AlignAddress + 8LL * i, WriteData);if (DEBUG)printf("[%s] Address=%016llX ReadData=%016llX\n", __func__, AlignAddress + 8LL * i, WriteData);}
}