A replay drifts from the original because the game steps by the measured frame time

Symptoms

The port replays the same inputs as a recording of the original, and the two drift apart after a few seconds. The drift changes from run to run of the original.

Cause

The game does not run a fixed step. Each frame it measures the time since the last frame and moves the world by that much. Hardwar sets a global to the milliseconds since the last frame, capped at 166 ms, and keeps the same delta in seconds × 16,384 beside it. The first two frames after a reset have a delta of 0, and another function can set the delta from outside (observed, FUN_004b3564, FUN_004b3658). POD clamps its frame time and splits a long frame into equal steps (the game's clock). The sequence of frame times is an input, as real as the controls.

What to do

Record the frame delta as an input of every frame, beside the controls, and feed the port the same deltas on replay. Find the timer reads first (timeGetTime, QueryPerformanceCounter, GetTickCount, rdtsc: ./gameframe run insns GAME.EXE '^rdtsc' and the imports), then the global that holds the delta, its cap, and any frames that force it to 0. Record those rules in the clock document.

• Under the emulator, set the delta global directly before each step.
• From a running original, read the delta with memrec at each frame, or log the timer's return values with a call trace.
• For tests, a fixed delta that the original also accepts (one that is under the cap) gives runs that repeat.