# Replay Replay is what makes a recording useful. A branch can play back what its parent recorded and Machine checks, step by step, that it gets the same result. ```bash machine replay app/try-1 ``` Replay is strict. An entry that does not reproduce what was recorded means the run has **diverged**. Before giving up on it, Machine replays that entry again from the nearest checkpoint before it, twice by default. Use `--retry` to change that, or `--retry 0` to try each entry once. `--to` stops at a given entry instead of the end. The host can interrupt an entry too. A firmware interrupt on the computer running Machine makes one entry's instruction and branch counts unusable. That says nothing about your run, so it is not a divergence. Replay prints `Interrupted: entry N, crossing 1: host SMI`, goes back to the nearest checkpoint before the entry and replays it again, counted against the same `--retry`. A recording does the same on its own, see [Driving it](running-a-machine.md#driving-it). If a run does diverge, [`machine diff`](commands.md#machine-diff) shows how. ```bash machine diff app/try-1 machine diff app/try-1 --entry 5000 --show regs,stack ``` With no second argument, `diff` compares the entry against the run it was replayed from, and says `✔ No differences` when they match. Otherwise it prints a hash by default, and `--show` takes any of the sections in its help: registers, stack, page tables, timers, devices, memory and more. Name another run to compare two recordings. A difference there only means the recordings differ. It says nothing about determinism. There is also [`machine play`](commands.md#machine-play), which plays a log back without checking for divergence. It is for when you want to see what happened, not whether it repeated. By default it plays the parent's log, so it needs a branch. To play another run's log, name it: [`machine play app/try-1 app`](commands.md#machine-play). ```bash machine play app/try-1 -n 16240:16243 machine play app/try-1 --include irq random ```