Backward-Latent Erratum

Definition

A backward-latent erratum is an erratum that was reported in a design strictly before being reported in an earlier design. Formally, it is the temporal dual of a forward-latent erratum:

• Forward-latent erratum — reported in one design and later reported in a later design.
• Backward-latent erratum — reported in a design and later reported in an earlier design.

This notion of "latency" is purely chronological across a vendor's published errata documents; it captures bugs that, by their first public sighting, appear on a newer product and only later surface as documented defects on a predecessor.

Context: RemembERR Classification

The term is introduced and analyzed in the paper RemembERR: Leveraging Microprocessor Errata for Design Testing and Validation (USENIX MICRO 2022). The authors use forward- and backward-latent errata as one axis of their empirical analysis of cross-generation bug propagation in commercial CPUs, complementing their classification of triggers, contexts, effects, and suggested workarounds.

Observations in the RemembERR Dataset

• The authors present forward-latent and backward-latent errata plots (their Figure 5) for the Intel Core CPU families.
• The AMD errata documents lack sufficient chronological information to support the same forward/backward latency analysis.
• The plot shows a salient portion of backward-latent errata around the year 2015, which the authors suggest may reflect a period at Intel where fewer resources were devoted to errata documentation or disclosure, allowing older designs to accumulate later-documented defects.
• Identical errata that recur across designs are merged in the analysis to avoid double-counting.

Related Concepts

• Bug heredity — the broader notion of how a defect is inherited, shared, or reappears across successive microprocessor designs; backward-latency is one operationalization of heredity over the timeline of public errata disclosure.

Practical Significance

Backward-latent errata are interesting for design testing and validation because they indicate that a bug class seen first on a newer core also exists — or was only later documented — in an older core. For CPU vendors, identifying backward-latent cases helps prioritize mitigations (e.g., BIOS/firmware workarounds, software conditions, peripheral conditions) across the installed base, not just on the newest silicon. In the RemembERR study, around 28.9% (AMD) and 35.9% (Intel) of all errata fall into categories where some kind of mitigation is required beyond "no fix" or "absent," underscoring why tracking backward propagation matters for both performance and security.

Limitations

• The classification relies on the availability of dated, per-design errata documents. AMD's publicly available errata documents do not provide the chronology needed to compute backward-latent sets, so the analysis in RemembERR is reported only for Intel Core CPUs.
• "Latency" here is measured against the date of public errata disclosure, not against the actual introduction date of the defect; a backward-latent erratum may in reality have existed long before its first public sighting on the newer design.