9. Write Strategy (A/B Buffer)

Design intent

NFC writes are not atomic at the hardware level. A power loss or tap interruption mid-write can leave the card in a partially written state. The A/B buffer strategy ensures the card always has one known-good buffer, so an interrupted write never corrupts the live card state.

How it works

The card payload is stored in two mirrored buffers (Zone A and Zone B). At any moment, one buffer is the active buffer (indicated by activePtr in the trailer) and the other is the shadow buffer.

A write always targets the shadow buffer first. Only after the write is verified does activePtr flip to make the new buffer authoritative. If the write fails or cannot be verified, the active buffer is unchanged and the card remains in its previous valid state.

Safety guarantees

• The active buffer is never touched until the new buffer is fully verified.
• An interrupted write leaves the shadow buffer in an inconsistent state, which is detectable and recoverable.
• The strategy prevents half-written state from being treated as valid on the next read.

Exact write procedure and verification steps: Tech Specs §7 Write & Update Strategy.