CNA-BUG-085: GameTime.IsRunningSlowly follows FNA's lag counter, not XNA 4.0's rule

CNA snapshot 009d40f5  ·  Known Issues › Current bugs  ·  source links pinned to 009d40f5

CNA sets IsRunningSlowly after five accumulated extra steps and clears it when the lag counter returns to zero; XNA 4.0 reports it when two multi-step ticks occur within 20 ticks and assigns it before each Update, so the flag differs tick by tick.

Identifier

CNA-BUG-085

Category

Bug

Subsystem

Core & runtime

Status

Open

Verified against

CNA 009d40f5 (009d40f5dd085c4e674d3479675fac84b12b3e0a)

Severity

Low (a triage suggestion, not a project priority)

Evidence basis

Source-verified: read at TARGET, not executed

Tests touching this area

None

Affected contract

Microsoft::Xna::Framework::GameTime::IsRunningSlowly as set by Game::Tick

Expected behaviour

XNA 4.0's Game.Tick (IL) keeps updatesSinceRunningSlowly1/2: a tick that needs more than one step shifts counter 1 into counter 2 and zeroes counter 1; drawRunningSlowly = updatesSinceRunningSlowly2 < 20; and the value is written to GameTime.IsRunningSlowly before every Update of the tick.

Actual behaviour at TARGET

CNA's fixed-step branch in Game.cpp uses FNA's updateFrameLag_: it adds stepCount - 1 per tick, sets the flag when the lag reaches 5, clears it only when the lag is back to 0, decrements the lag after single-step ticks, and assigns the flag after the tick's updates. A single two-step hitch never sets the flag in CNA; two hitches 15 ticks apart set it in XNA and not in CNA; and CNA's flag can stay set long after XNA's has cleared.

Source locations

• modules/runtime/src/Game.cpp — Game::Tick fixed-step branch (updateFrameLag_ and setIsRunningSlowlyProperty)
• modules/runtime/tests/Microsoft/Xna/Framework/GameTimeTests.cpp — GameTime property tests only

Evidence

Checked by reading the named sources at 009d40f5; nothing was built or executed for this entry. XNA rule read from the genuine XNA 4.0 Game IL; no oracle run compares the two tick by tick.

Focused reproduction

No focused reproduction is known. Nothing has been invented here; the evidence above is what exists.

Current tests

GameTimeTests.cpp checks the property's storage only; nothing drives Tick through slow frames.

Regression test

A fake-clock Game test with a scripted sequence of frame durations that asserts XNA's IsRunningSlowly value at each Update (derived from the IL rule).

Blast radius

Games that adapt quality or skip work when IsRunningSlowly is set. Games that ignore the flag are unaffected.

Workaround

No workaround is known.