The stages of loading a grf file
Processing of .grf files is done in several passes, most of which happen repeatedly, once per activations, which is whenever a game is started/loaded or the "Apply" button in the GRF Status Window is pressed. Each pass processes all files in the order they appear in the GRF Status Window.
Essentially, initialization is a two-pass system (first resolve jump targets, then initialize everything) that happens only once, activation is a two-pass system (first define cargos and reserve GRM IDs, then activate) that happens multiple times, and testing is a one-pass system run per click on a red/green flag in the GRF Status Window.
A .grf file is activated only if it was active when the game was started. If a game is loaded, only its active .grfs will be reactivated. A .grf file is considered active if its action 8 has been processed, i.e. its action 8 hasn't been skipped using an action 7.
The following table shows exactly which actions are excuted in which passes (X=action executed, I=action initialized, empty=action ignored).
|8||When executed for an inactive file, skips remainder of file.|
|12||If TTDPatch encounters any Action12 during any stage (including "Load"), Unicode handling is enabled.|
|14||Processed by OpenTTD only when scanning for files. Not processed during the normal loading/activation.|
- During reservation stage especially cargo- and railtypes are reserved, so their existence can be tested by all GRFs in activation stage independent of loading order.
- 0.6 2.5 Processed during activation in TTDPatch 2.0.1 alpha 51 and later.
- Processed during initialization only if bit 7 of severity is set.