Cache Address Decode
Split a 32-bit byte address into tag, index and word offset fields for a small direct-mapped cache.
Task
Use a 16-line cache with 4 words per line. Produce a 24-bit tag, 4-bit line index and 2-bit word offset from address bits.
Examples
- Complete the required behavior and preserve it under the stated control conditions.
Constraints
- Use synthesizable SystemVerilog unless this is a verification-only stage.
What this problem is testing
A strong answer translates the requirement into synthesizable state and combinational logic, states reset and update semantics precisely, and handles boundary conditions rather than only the nominal case.
Interview habit: before coding, state reset behavior, the state that must be stored, and the conditions that update each output.
Verification plan
- Check reset or initialization before normal operation.
- Exercise the nominal sequence cycle by cycle.
- Test boundaries such as zero, maximum count, wraparound, full/empty, or simultaneous controls when they apply.
- Prove the design holds state when an enable or qualifying condition is absent.
Edge cases worth testing
- First and last byte within a line.
- Two addresses that map to the same set.
Common mistakes
- Incomplete combinational assignments that infer a latch.
- Blocking/non-blocking assignments that do not match combinational versus sequential intent.
- Parameter-width and boundary errors.
- Passing the happy path while missing reset, overflow, simultaneous operations, or illegal requests.
Interview follow-up questions
- How would you parameterize this design without introducing width or boundary bugs?
- What changes if reset timing or polarity changes?
Concepts to review
cache, addressing, tag/index/offset
Open Cache Address Decode in the HDLForge interactive editor.
Related problems
- 2:1 Multiplexer — Easy
- Parameterized Counter — Easy
- Priority Encoder — Medium
- Synchronous FIFO — Hard
- Shift Register — Easy
Explore the RTL Design Practice & Interview Preparation hub.