A retention substrate is its coordinate.

CPU register

SILICON · FLIP-FLOP

Sub-millisecond retention, clock-tied refresh, single-word capacity, sub-femtojoule write, indexed access. The cell most other memory is benchmarked against.

L1 cache line

SILICON · SRAM

Tag-matched content-addressable access, implicit write as a side-effect of memory traffic. Sits ~6 orders above Landauer. The first cell where the access regime differs from the substrate's address line.

DRAM row

SILICON · 1T1C

Sixty-four-millisecond half-life, sustained-refresh hold cost, destructive sense-amp read. The canonical ρ ≫ 1 substrate — information persists indefinitely on a substrate that decays in milliseconds.

NAND flash page

SILICON · FLOATING GATE

Ten-year retention spec, near-zero hold cost, asymmetric write-erase-read, LDPC-protected. Pays its energy entirely at write time, then leaves the substrate alone.

Hopfield network

ALGORITHMIC · SPIN GLASS

Capacity ≤ 0.138 N before catastrophic first-order phase transition. Hebbian write, content-addressable recall by attractor dynamics. Interference-limited — the failure mode is not gradual.

Transformer KV cache

ALGORITHM ON GPU

Per-sequence volatile working memory at ~20 orders above Landauer per output bit. Implicit write as a forward-pass side-effect. Quadratic compute scaling with length is its capacity-failure mode.

RAG vector store

HYBRID · ANN INDEX

Externalized semantic store. Embeds and writes imperatively. Recall pays a fetch-and-recompute cost; verifiability is bolted on via citation passthrough, not native.

Working memory (Baddeley)

BIOLOGICAL · NEURAL

Twenty-second half-life without rehearsal, four-plus-or-minus-one chunks, attentional refresh, context-reinstatement access. Five orders of magnitude more efficient than the LLM working-memory equivalent.

Hippocampal episodic engram

BIOLOGICAL · CA3 + DG

Single-shot write, sparse pattern-separated capacity, context-cued recall. Reconsolidates on every retrieval. Consolidates to neocortex over sleep — the CLS handoff.

Memory B-cell population

BIOLOGICAL · CLONAL

Decade-scale information persistence on a substrate that turns over in months. Affinity selection during germinal-center reaction — the immune system's V₅ verifiability. The DRAM-refresh pattern at the cellular timescale.

Synthetic DNA archive

MOLECULAR · OLIGONUCLEOTIDE

Millennium retention at near-zero hold cost. Massive density (~2 PB/g demonstrated). Pays at synthesis and sequencing — the deepest write-recall asymmetry on the table.

Aboriginal songline

LANDSCAPE + ORAL + RITUAL

Fifty-thousand-year retention via continuous ceremonial refresh. Landscape-indexed (the loci of place itself). Multimodal cross-check — song, dance, petroglyph, ceremony, place — gives V₅ verifiability without writing.