# Set Associative Mapping | Practice Problems

## Set Associative Mapping-

Before you go through this article, make sure that you have gone through the previous article on Set Associative Mapping.

In set associative mapping,

• A particular block of main memory can be mapped to one particular cache set only.
• Block ‘j’ of main memory will map to set number (j mod number of sets in cache) of the cache.
• A replacement algorithm is needed if the cache is full.

In this article, we will discuss practice problems based on set associative mapping.

## Problem-01:

Consider a 2-way set associative mapped cache of size 16 KB with block size 256 bytes. The size of main memory is 128 KB. Find-

1. Number of bits in tag
2. Tag directory size

## Solution-

Given-

• Set size = 2
• Cache memory size = 16 KB
• Block size = Frame size = Line size = 256 bytes
• Main memory size = 128 KB

We consider that the memory is byte addressable.

### Number of Bits in Physical Address-

We have,

Size of main memory

= 128 KB

= 217 bytes

Thus, Number of bits in physical address = 17 bits

### Number of Bits in Block Offset-

We have,

Block size

= 256 bytes

= 28 bytes

Thus, Number of bits in block offset = 8 bits

### Number of Lines in Cache-

Total number of lines in cache

= Cache size / Line size

= 16 KB / 256 bytes

= 214 bytes / 28 bytes

= 64 lines

Thus, Number of lines in cache = 64 lines

### Number of Sets in Cache-

Total number of sets in cache

= Total number of lines in cache / Set size

= 64 / 2

= 32 sets

= 25 sets

Thus, Number of bits in set number = 5 bits

### Number of Bits in Tag-

Number of bits in tag

= Number of bits in physical address – (Number of bits in set number + Number of bits in block offset)

= 17 bits – (5 bits + 8 bits)

= 17 bits – 13 bits

= 4 bits

Thus, Number of bits in tag = 4 bits

### Tag Directory Size-

Tag directory size

= Number of tags x Tag size

= Number of lines in cache x Number of bits in tag

= 64 x 4 bits

= 256 bits

= 32 bytes

Thus, size of tag directory = 32 bytes

Also Read- Practice Problems On Direct Mapping

## Problem-02:

Consider a 8-way set associative mapped cache of size 512 KB with block size 1 KB. There are 7 bits in the tag. Find-

1. Size of main memory
2. Tag directory size

## Solution-

Given-

• Set size = 8
• Cache memory size = 512 KB
• Block size = Frame size = Line size = 1 KB
• Number of bits in tag = 7 bits

We consider that the memory is byte addressable.

### Number of Bits in Block Offset-

We have,

Block size

= 1 KB

= 210 bytes

Thus, Number of bits in block offset = 10 bits

### Number of Lines in Cache-

Total number of lines in cache

= Cache size / Line size

= 512 KB / 1 KB

= 512 lines

Thus, Number of lines in cache = 512 lines

### Number of Sets in Cache-

Total number of sets in cache

= Total number of lines in cache / Set size

= 512 / 8

= 64 sets

= 26 sets

Thus, Number of bits in set number = 6 bits

### Number of Bits in Physical Address-

Number of bits in physical address

= Number of bits in tag + Number of bits in set number + Number of bits in block offset

= 7 bits + 6 bits + 10 bits

= 23 bits

Thus, Number of bits in physical address = 23 bits

### Size of Main Memory-

We have,

Number of bits in physical address = 23 bits

Thus, Size of main memory

= 223 bytes

= 8 MB

### Tag Directory Size-

Tag directory size

= Number of tags x Tag size

= Number of lines in cache x Number of bits in tag

= 512 x 7 bits

= 3584 bits

= 448 bytes

Thus, size of tag directory = 448 bytes

## Problem-03:

Consider a 4-way set associative mapped cache with block size 4 KB. The size of main memory is 16 GB and there are 10 bits in the tag. Find-

1. Size of cache memory
2. Tag directory size

## Solution-

Given-

• Set size = 4
• Block size = Frame size = Line size = 4 KB
• Main memory size = 16 GB
• Number of bits in tag = 10 bits

We consider that the memory is byte addressable.

### Number of Bits in Physical Address-

We have,

Size of main memory

= 16 GB

= 234 bytes

Thus, Number of bits in physical address = 34 bits

### Number of Bits in Block Offset-

We have,

Block size

= 4 KB

= 212 bytes

Thus, Number of bits in block offset = 12 bits

### Number of Bits in Set Number-

Number of bits in set number

= Number of bits in physical address – (Number of bits in tag + Number of bits in block offset)

= 34 bits – (10 bits + 12 bits)

= 34 bits – 22 bits

= 12 bits

Thus, Number of bits in set number = 12 bits

### Number of Sets in Cache-

We have-

Number of bits in set number = 12 bits

Thus, Total number of sets in cache = 212 sets

### Number of Lines in Cache-

We have-

Total number of sets in cache = 212 sets

Each set contains 4 lines

Thus,

Total number of lines in cache

= Total number of sets in cache x Number of lines in each set

= 212 x 4 lines

= 214 lines

### Size of Cache Memory-

Size of cache memory

= Total number of lines in cache x Line size

= 214 x 4 KB

= 216 KB

= 64 MB

Thus, Size of cache memory = 64 MB

### Tag Directory Size-

Tag directory size

= Number of tags x Tag size

= Number of lines in cache x Number of bits in tag

= 214 x 10 bits

= 163840 bits

= 20480 bytes

= 20 KB

Thus, size of tag directory = 20 KB

## Problem-04:

Consider a 8-way set associative mapped cache. The size of cache memory is 512 KB and there are 10 bits in the tag. Find the size of main memory.

## Solution-

Given-

• Set size = 8
• Cache memory size = 512 KB
• Number of bits in tag = 10 bits

We consider that the memory is byte addressable.

Let-

• Number of bits in set number field = x bits
• Number of bits in block offset field = y bits

### Sum of Number Of Bits Of Set Number Field And Block Offset Field-

We have,

Cache memory size = Number of sets in cache x Number of lines in one set x Line size

Now, substituting the values, we get-

512 KB = 2x x 8 x 2y bytes

219 bytes = 23+x+y bytes

19 = 3 +x + y

x + y = 19 – 3

x + y = 16

### Number of Bits in Physical Address-

Number of bits in physical address

= Number of bits in tag + Number of bits in set number + Number of bits in block offset

= 10 bits + x bits + y bits

= 10 bits + (x + y) bits

= 10 bits + 16 bits

= 26 bits

Thus, Number of bits in physical address = 26 bits

### Size of Main Memory-

We have,

Number of bits in physical address = 26 bits

Thus, Size of main memory

= 226 bytes

= 64 MB

Thus, size of main memory = 64 MB

## Problem-05:

Consider a 4-way set associative mapped cache. The size of main memory is 64 MB and there are 10 bits in the tag. Find the size of cache memory.

## Solution-

Given-

• Set size = 4
• Main memory size = 64 MB
• Number of bits in tag = 10 bits

We consider that the memory is byte addressable.

### Number of Bits in Physical Address-

We have,

Size of main memory

= 64 MB

= 226 bytes

Thus, Number of bits in physical address = 26 bits

### Sum Of Number Of Bits Of Set Number Field And Block Offset Field-

Let-

• Number of bits in set number field = x bits
• Number of bits in block offset field = y bits

Then, Number of bits in physical address

= Number of bits in tag + Number of bits in set number + Number of bits in block offset

So, we have-

26 bits = 10 bits + x bits + y bits

26 = 10 + (x + y)

x + y = 26 – 10

x + y = 16

Thus, Sum of number of bits of set number field and block offset field = 16 bits

### Size of Cache Memory-

Cache memory size

= Number of sets in cache x Number of lines in one set x Line size

= 2x x 4 x 2y bytes

= 22+x+y bytes

= 22+16 bytes

= 218 bytes

= 256 KB

Thus, size of cache memory = 256 KB

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Summary
Article Name
Set Associative Mapping | Practice Problems
Description
Practice Problems based on set associative mapping. Set associative mapping is a cache mapping technique that allows to map a particular block of main memory to one particular cache set only.
Author
Publisher Name
Gate Vidyalay
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