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G-code Motion & Position-Command Coverage

Status: Audit published (issue #154, credits upstream xyz-tools/gcode-preview#179, 2026-07-23). Updated 2026-07-25 for E10 phase 1 (DD-010): M82/M83, G90/G91, and the G92 E-datum are now modeled.

This page records which position-affecting G/M-codes the parser/interpreter (@chestnutlabs/gcode-parser) honors. The interpreter began as a byte-exact port of the inherited xyz-tools engine (E1 golden-gated); E10 (motion-model correctness) layers a firmware-conditioned modal model on top of it. Capability honesty (DD-001) means naming the remaining gaps plainly — and disclosing them at runtime via the extrusionMode / positioningMode capabilities and the g92-xyz-unhandled warning.

Coverage

Code(s) Effect on position Status Evidence
G0 / G1 linear move honored core motion
G2 / G3 arc move (I/J center or R) incl. full circles honored (XY plane only) E2/E3
G20 / G21 units inch / mm honored units channel
G28 homing (position reset to origin) honored interpreter
T0–T7 tool select honored tool channel
G90 / G91 absolute / relative positioning honored (E10 phase 1, #155) — positioningMode capability motion-model.test.ts
M82 / M83 extruder absolute / relative honored (E10 phase 1, #156) — delta-based classification, extrusionMode capability motion-model.test.ts
G92 set-position / coordinate offset partial — E-datum honored (E10 phase 1); X/Y/Z work-offset NOT honored (warns g92-xyz-unhandled, phase 3 #158) motion-model.test.ts
G17 / G18 / G19 arc plane select (XY / XZ / YZ) NOT honored — arcs assume XY (phase 2 #157) code (i/j only)
G53 / G54–G59 machine / work coordinate systems NOT honored (phase 3 #158) interpreter switch
G4 dwell n/a (not position-affecting) —

Firmware conditioning (DD-010)

Absolute is the power-on default (Marlin/RepRapFirmware convention). Whether G90/G91 also switch the extruder mode is firmware-specific, so it is gated on parseOptions.extruderFollowsPositioning: Marlin/Klipper set it true (E follows G90/G91 unless M82/M83 override); RepRapFirmware leaves it false (XYZ and E independent). Explicit M82/M83 always win. When neither the mode nor a firmware hint is known, the capability is disclosed as inferred, never known — the stack does not fabricate a mode it cannot prove.

Resolved reproductions (originally captured 2026-07-23)

[G91] relative: `G1 X10` then `G1 X10` after `G91` → now ends X=20 ✓ (was 10)
[M82] abs-E:    an E-unchanged move is now classified TRAVEL ✓; extrusion distance no longer inflated
[G92 E] datum:  `G92 E0` now resets the extruder datum so the next per-move delta is correct ✓

This is why the 7 absolute-E dialect fixtures' extrusionDistance dropped at E10 phase 1: the old engine summed raw (cumulative) E words; the delta-based model sums true per-move deltas. All other geometry (positions, kinds, layers, source bytes) stayed byte-identical.

Remaining gaps

  • G92 X/Y/Z work-offset — surfaced as the g92-xyz-unhandled warning rather than a silent datum shift; lands with coordinate systems in phase 3 (#158).
  • G17 / G18 / G19 — non-XY arcs (mainly CNC); arcs still assume the XY plane. Phase 2 (#157).
  • G53 / G54–G59 — CNC work-coordinate systems; rare in FDM. Phase 3 (#158).

Motion-model changes alter interpreter state and therefore IR positions, so each remains contract-sensitive and DD-gated (DD-010: explicit golden-regen + capability honesty). Tracking issues: #155 (G90/G91 + G92 E — shipped), #156 (M82/M83 — shipped), #157 (arc planes), #158 (coordinate systems + G92 XYZ).