Cache Set Index Calculator
Enter a memory address and cache geometry to calculate its cache set index, cache-line number, block offset, tag, and exact address-bit breakdown for a power-of-two set-associative cache.
What Is a Cache Set Index?
A cache set index identifies which cache set a memory block maps to in a set-associative CPU cache. The address is conceptually divided into a tag, set-index field, and block or line offset.
The block offset selects a byte inside the cache line. The set index chooses the cache set. The remaining higher-order address portion acts as the tag used to distinguish different memory blocks that map to the same set.
How to Calculate a Cache Set Index
Cache Set Index Formula
First convert the byte address into a cache-line number by dividing by the cache-line size.
Then reduce the cache-line number modulo the number of cache sets.
0x12345 in a 32 KiB, 64-Byte, 8-Way Cache
What Is the Cache Block Offset?
The block offset identifies the exact byte within the selected cache line. For a 64-byte line, valid offsets range from 0 through 63.
Because 64 equals 2⁶, a 64-byte line uses six low-order
address bits for the block offset.
How Many Bits Are Used for the Set Index?
If the number of sets is an exact power of two, the number of index bits is the base-2 logarithm of the set count.
A cache with 64 sets therefore requires 6 set-index bits because
64 = 2⁶.
How the Cache Tag Is Calculated
After the block-offset and set-index portions are removed, the remaining higher-order address value is the cache tag.
In a power-of-two cache, this corresponds to shifting the address right by the total number of offset and index bits.
Tag, Set Index, and Offset Address Fields
A conventional cache address decomposition can be visualized as three fields.
Cache Set Index in a Direct-Mapped Cache
A direct-mapped cache has associativity 1. Each set contains exactly one cache line, so the number of sets equals the total number of lines.
The set-index calculation itself remains the same:
lineNumber mod numberOfSets.
How Associativity Changes the Number of Sets
Increasing associativity means more cache lines belong to each set, so the number of sets decreases for the same total cache size and line size.
| Cache Size | Line Size | Ways | Total Lines | Sets |
|---|---|---|---|---|
| 32 KiB | 64 B | 1 | 512 | 512 |
| 32 KiB | 64 B | 2 | 512 | 256 |
| 32 KiB | 64 B | 4 | 512 | 128 |
| 32 KiB | 64 B | 8 | 512 | 64 |
| 32 KiB | 64 B | 16 | 512 | 32 |
What Is a Cache-Line Number?
The cache-line number is the memory address divided by the line size using integer division. All byte addresses within the same aligned cache line have the same line number.
For a 64-byte line, addresses 0 through 63 belong to memory line 0, addresses 64 through 127 belong to line 1, and so on.
Why Cache Geometry Usually Uses Powers of Two
Power-of-two line sizes and set counts allow cache fields to correspond to fixed groups of address bits. This makes cache lookup efficient in digital hardware.
For example, a 64-byte line uses six low-order bits for the byte offset. If the cache has 128 sets, seven additional bits identify the set.
Cache Set Index Examples
| Address | Cache | Line | Ways | Sets | Set Index |
|---|---|---|---|---|---|
| 0x12345 | 32 KiB | 64 B | 8 | 64 | 13 |
| 0x1000 | 32 KiB | 64 B | 8 | 64 | 0 |
| 0x1040 | 32 KiB | 64 B | 8 | 64 | 1 |
| 0x2000 | 32 KiB | 64 B | 8 | 64 | 0 |
| 0x12345 | 16 KiB | 64 B | 4 | 64 | 13 |
Why Different Addresses Can Map to the Same Cache Set
The set index uses only part of the address. Different memory blocks whose line numbers differ by a multiple of the number of sets therefore map to the same set.
Their tag values differ, allowing the cache to distinguish them. In a set-associative cache, multiple such blocks can coexist in different ways of the same set until the set reaches its capacity.
Cache Set Index and Conflict Misses
When many actively used memory blocks map to the same cache set, they compete for the limited number of ways available in that set.
If more distinct blocks must remain active than the associativity permits, cache lines can evict one another and cause conflict misses.
A set-index calculator can therefore help when studying array strides, data-layout behavior, cache conflicts, or low-level performance patterns.
Where Cache Set Calculations Are Useful
Practice decomposition of addresses into tag, index, and block-offset fields.
Investigate whether address patterns repeatedly target the same cache sets.
Reason about memory layout and cache-line placement in low-level software.
Study how arrays and records map into a set-associative cache.
Analyze cache geometry when working with known processor configurations.
Verify manual cache-address calculations and bit-field exercises.
This Calculator Models Conventional Cache Indexing
The tool models a conventional physically indexed or directly address-based power-of-two set calculation using the cache geometry you provide.
Real processors can use more complex mechanisms, including virtual indexing, address hashing, slice selection, skewed organizations, undocumented mappings, or other microarchitecture-specific behavior.
Common Cache Set Index Mistakes
Using the byte address directly modulo the number of sets
The block-offset portion must first be removed. Use the cache-line number, not the raw byte address, when calculating the set.
Ignoring associativity
Associativity changes how many sets the cache contains. A 32 KiB 8-way cache has fewer sets than a 32 KiB direct-mapped cache with the same line size.
Confusing total lines with number of sets
Total cache lines equal cache size divided by line size. Number of sets is total lines divided by associativity.
Using bits before validating powers of two
A clean fixed-bit index interpretation requires power-of-two line size and set count.
Assuming all real CPUs use simple indexing
Some processors use additional hashing or slice-selection functions, especially in higher-level shared caches.