Counted Synchronous FIFO
Build an 8-entry synchronous FIFO with an occupancy count.
Task
Increment count on successful write-only cycles, decrement on read-only cycles, hold on simultaneous read/write.
Examples
- Implement the stated behavior for all legal input combinations.
Constraints
- Use synthesizable SystemVerilog.
- Do not use delays.
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
- Write when full and read when empty.
- Simultaneous read and write.
- Pointer wraparound and transitions into and out of full/empty.
- Read-after-write behavior.
- Address boundary values.
- Simultaneous accesses or collisions when the interface permits them.
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?
- How would this change for an asynchronous FIFO?
- What read-during-write behavior do you expect from the target memory?
Concepts to review
fifo, count, memory
Open Counted Synchronous FIFO in the HDLForge interactive editor.
Related problems
- 2:1 Multiplexer — Easy
- Parameterized Counter — Easy
- Priority Encoder — Medium
- Synchronous FIFO — Hard
- Shift Register — Easy
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