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This wiki explains, in technical depth, what every value spd_matchtable.py
reads or computes actually means physically — not just the abbreviation,
but why the parameter exists in DDR4's protocol, what happens if you get
it wrong, and why this tool handles it the way it does. The tool's own
--selftest and source comments establish correctness; this wiki is
where the why lives.
Written to be read start to finish by someone who's never tuned RAM timings before, but precise enough to be useful to someone who has.
Timings — what each one physically gates
- Primary Timings CL tRCD tRP tRAS tRC — the core row/column access chain
- Refresh Timing tRFC — why bigger modules need longer refresh pauses
- Bank Group and Activation Timings — tFAW, tRRD_S/L, tCCD_L — current and bus-sharing limits
- Write Recovery and Turnaround — tWR, tWTR_S/L, tRTP — the write side nobody talks about
- Command Rate and tCWL — the two numbers SPD doesn't give you, and how this tool infers them
The data model
- SPD Structure and Timebase — how a byte-oriented EEPROM encodes fractional nanoseconds
- Module Organization — density, width, ranks, and where the capacity formula comes from
- Module Type and ECC Compatibility — why RDIMM+UDIMM or ECC+non-ECC won't POST
- XMP Profiles Explained — what's actually stored in that embedded profile, and why mixed kits usually can't use it
Methodology
- Worst Case Methodology — why "take the max per parameter" is almost right, and the one place it isn't
- Voltage — what VDD actually trades off, physically
- FAQ and Design Decisions — why not bash, why read-only, why offline files, why 80 columns, "why not just trust XMP"
Every byte offset and formula referenced across these pages was
independently verified against this project's own reference kit — a live
4-module mixed DDR4 set (2× Kingston KF3200C16D4/16GX dual-rank, plus
KF432C16BB3/16 and KF432C16BB/8 single-rank) — cross-checked against
decode-dimms output and, for the XMP region decode-dimms doesn't parse
at all, against integralfx/DDR4XMPEditor.
See the main repo's RELEASE_NOTES.md
for the specific verification history, including one case where an
external review's own claimed byte offset was wrong and had to be
re-derived from the struct layout before being trusted.
Stated plainly: this tool, this wiki, and the surrounding repo were built in collaboration with Claude (Anthropic). The correctness/safety fixes in v1.0.1 were prompted by an independent code review from Kimi (Moonshot AI) — which is also the "external review" referenced above, including the case where its own suggested byte offset was itself wrong. Every AI-suggested change, from either model, was checked against real reference hardware and a second independent source before being trusted — that verification discipline is the actual subject of this wiki, not a disclaimer bolted onto otherwise-unverified output.