CODEWITHSUNDEEP
cachingmemory
drleifz
drleifz

Reputation: 179

Understanding Cache memory

I am trying to understand how cache memory reads and writes. Also I am trying to determine the hit and miss rate. I have tried reading and reading the textbook "Computer Systems - A Programmer Perspective" over and over and can't seem to grasp this idea. Maybe someone can help me understand this:

I am working with a two-dimensional array which has 480 rows and 640 columns. The cache is direct-mapped and 64 KB with 4 byte lines. Below is the C-code:

struct pixel {
char r;
char g;
char b;
char a;
};
struct pixel buffer[480][640];
register int i, j;
register char *cptr;
register int *iptr;

sizeof(char) == 1 (meaning an index in the array consists of 4 byte each (if I am understanding that correctly)). The buffer begins at memory address 0 and the cache is initially empty (cold cache). The only memory accesses are to the entries of the array. All other variables are stored in registers.

for (j=0; j < 640; j++) {
for (i=0; i < 480; i++){
buffer[i][j].r = 0;
buffer[i][j].g = 0;
buffer[i][j].b = 0;
buffer[i][j].a = 0;
}
}

For the code above then it is initializing all the elements in the array to 0, so it must be writing. I can see that this is bad locality because the array is writing column by column instead of row by row. Doesn't that affect the miss rate? I am trying to determine the miss rate for this code based on the cache size. I think the miss rate is 100% and if the locality was row by row then it would be 25%. But I am not totally understanding how cache-memory works so... Can anyone tell me something that could help me understand this better?

Upvotes: 0

Views: 260

Answers (1)

user3815757
user3815757

Reputation: 161

I would recommend you to watch the whole Tutorial if you are a beginner. But for your question, lecture 27 to 31 would explain everything.

https://www.youtube.com/watch?v=tGarzP488Wc&index=29&list=PL2F82ECDF8BB71B0C IISc Bangalore.

Upvotes: 1

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