So I've created a direct-mapped write-back cache simulator.
In testing my stuff, I wrote to address 0x14c the value "99", it was originally a value "4C" Then I read in value 0x348, which makes the write back function occur because both of those addresses below to the same slot number, just different tag.
Basically, I then need to write all the data in the slot from the "cache" object to the main_mem object. I use System.array copy.
I can SEE that the value 99 IS successfully written back to the main memory array.
But when I want to read address 14C again (which should pull it's whole block including that 99), it prints what was there initially, not reflecting that change I wrote.
Does System.arraycopy not pull the data from the array starting at specific index? Does it just count up from the value of the first index?
Here is my read() and write() methods and the output
public static void readAddress() {
System.out.println("What address? ");
address = keyboard.nextInt(16);
startAddress = address & 0x758;
tag = (address >> 6) & 0x1F;
slot = (address >> 3) & 0x7;
//Valid bit is 0, empty slot
if (cache[slot].getValidBit() == 0) {
cache[slot].setValidBit(1);
cache[slot].setTag(tag);
cache[slot].setStartAddress(startAddress);
System.arraycopy(main_Mem, main_Mem[startAddress], cache[slot].dataBlock, 0, cacheSize);
System.out.println("Cache Miss");
System.out.print("The value at that address is: ");
System.out.printf("%X", 0xFF & address);
System.out.println();
System.out.println();
}
//Valid bit 1 but tags don't match
else if (cache[slot].getValidBit() == 1 && cache[slot].getTag() != tag) {
System.out.println("Cache Miss ");
if (cache[slot].getDirty() == 1) {
System.out.println("This is a dirty slot!");
//copy contents of slot back into main memory before loading new block
System.out.println("Slot is dirty and will be written back, val for 14c is " +cache[slot].dataBlock[4]);
System.arraycopy(main_Mem, cache[slot].getStartAddress(), cache[slot].dataBlock, 0, cacheSize);
System.out.println("Everything should have been copied to main by now. The value for 332 is " + main_Mem[332]);
}
startAddress = address & 0x7F8;
cache[slot].setTag(tag);
cache[slot].setStartAddress(startAddress);
//set dirty back to 0, incase it was 1
cache[slot].setDirty(0);
for (int i = 0; i < cacheSize; i++) {
for (int j = cache[slot].getStartAddress(); j<cacheSize; j ++) {
cache[slot].dataBlock[i] = main_Mem[j];
}
}
//System.arraycopy(main_Mem, main_Mem[startAddress], cache[slot].dataBlock, 0, cacheSize);
System.out.print("The value at that address is: ");
System.out.printf("%X", 0xFF & address);
System.out.println();
System.out.println();
}
//Valid bit 1 and tags match, hit
else if (cache[slot].getValidBit() == 1 && tag == cache[slot].getTag()) {
System.out.println("Cache Hit");
System.out.print("The value at that address is: ");
System.out.printf("%X", 0xFF & address);
System.out.println();
System.out.println();
}
menu();
}
public static void writeAddress() {
System.out.println("What address do you want to write to? ");
address = keyboard.nextInt(16);
System.out.println("And what value do you want to write to it? ");
int input = keyboard.nextInt(16);
startAddress = address & 0x758;
tag = (address >> 6) & 0x1F;
slot = (address >> 3) & 0x7;
//Valid bit 1, tag matches, hit, just modify value
if (cache[slot].getValidBit() != 0 && cache[slot].getTag() == tag) {
System.out.println("Cache Hit");
System.out.printf("%X", 0xFF & address);
for (int i = 0; i <8; i++) {
if (cache[slot].dataBlock[i] == (0xFF & address)) {
cache[slot].dataBlock[i] = input;
cache[slot].setDirty(1);
}
}
}
//Valid bit 1, tags don't match-Check dirty bit and write back first if valid
else if (cache[slot].getValidBit() == 1 && cache[slot].getTag() != tag) {
if (cache[slot].getDirty() ==1) {
//copy contents of slot back into main memory before loading new block
for (int i = 0; i < cacheSize; i++) {
for (int j = startAddress; j<cacheSize; j++) {
cache[slot].dataBlock[i] = main_Mem[j];
}
}
//System.arraycopy(cache[slot].dataBlock, 0, main_Mem, cache[slot].getStartAddress(), cacheSize);
}
System.out.println("Cache Miss");
cache[slot].setValidBit(1);
cache[slot].setTag(tag);
cache[slot].setDirty(1);
cache[slot].setStartAddress(startAddress);
//copy new block into cache now
System.arraycopy(main_Mem, main_Mem[startAddress], cache[slot].dataBlock, 0, cacheSize);
for (int i = 0; i <8; i++) {
if (cache[slot].dataBlock[i] == (0xFF & address)) {
System.out.println("Writing over the value of that address now...");
cache[slot].dataBlock[i] = input;
}
}
}
//Empty slot, no need to write back
else if (cache[slot].getValidBit() == 0) {
System.out.println("Cache Miss");
System.out.println("Setting the dirty bit to 1");
System.out.println("Dirty bit was " + cache[slot].getDirty());
cache[slot].setValidBit(1);
cache[slot].setTag(tag);
cache[slot].setDirty(1);
System.out.println("And is now " +cache[slot].getDirty());
cache[slot].setStartAddress(startAddress);
//copy from main mem to cache
System.arraycopy(main_Mem, main_Mem[startAddress], cache[slot].dataBlock, 0, cacheSize);
//writes to selected value in the cache
for (int i = 0; i <8; i++) {
if (cache[slot].dataBlock[i] == (0xFF & address)) {
System.out.println("Writing over the value of that address now...");
cache[slot].dataBlock[i] = input;
}
}
}
menu();
}
Output:
This is a cache simulator, type in what you want to do and press enter
[r] to read [w] to write [d] to display
w
What address do you want to write to?
14c
And what value do you want to write to it?
99
Cache Miss
Setting the dirty bit to 1
Dirty bit was 0
And is now 1
Writing over the value of that address now...
This is a cache simulator, type in what you want to do and press enter
[r] to read [w] to write [d] to display
r
What address?
348
Cache Miss
This is a dirty slot!
Slot is dirty and will be written back, val for 14c is 153
Everything should have been copied to main by now. The value for 332 is 76
The value at that address is: 48
This is a cache simulator, type in what you want to do and press enter
[r] to read [w] to write [d] to display
"This is a dirty slot!"...one vowel away from being hilarious!