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CLARA Gap-10 — audit_log_ring_buffer

Summary

Implements a 64-entry circular ring buffer for on-chip inference event logging (CLARA Gap-10).

Entry Format (48 bits)

bits [47:32] = timestamp[15:0]   — free-running 16-bit timestamp
bits [31:28] = event_type[3:0]   — event class (0..15)
bits [27:0]  = data[27:0]        — inference result / operand payload

Architecture

• Write port: single-port sequential write; head pointer auto-advances (mod 64); wrapped flag set on first full rotation
• Read port: rd_en advances rd_ptr through all 64 entries; rd_data provides the 48-bit entry; rd_valid indicates entry validity
• Status: head_ptr[5:0], wrapped, buf_full, buf_empty, audit_ok
• uio integration: host dumps all 64 entries by asserting rd_en 64 times and reading rd_data

Files

File	Lines	Purpose
src/audit_log_ring_buffer.v	151	RTL — ring buffer, Verilog-2005, R-SI-1 clean
test/tb_audit_log_ring_buffer.v	303	Testbench — 5 scenarios, 35 assertions

Verification

iverilog -g2001 -Wall src/audit_log_ring_buffer.v test/tb_audit_log_ring_buffer.v
vvp a.out
=== SUMMARY: 35 PASS, 0 FAIL ===
ALL ASSERTIONS PASS

5 scenarios:

• S1 — Empty read: buf_empty=1, rd_valid=0, head_ptr=0 after reset
• S2 — Single write/read: rd_data matches {0xABCD, 0xF, 0x1234567}
• S3 — Wrap-around at 64: wrapped=1, head_ptr wraps to 0, overwrite verified
• S4 — Simultaneous wr_en + rd_en: write and read pointer coherent
• S5 — Reset mid-operation: buffer clears, re-write works correctly

Constraints

• Pure Verilog-2005 (no logic, typedef, enum, SV constructs)
• R-SI-1 clean: zero arithmetic * operators in synthesizable code
• Cell budget: ~400 cells (0.8% of GAMMA 48K tile budget)
• No external IP (compiles from src/ only)

PhD Anchors

• Chapter 8 (AGI Driver) — inference event logging for driver accountability
• Chapter 18 (Audit Chain) — on-chip forensic record keeping
• Chapter 36 (Holographic) — ring buffer as circular memory primitive
• DOI: 10.5281/zenodo.19227877