5.1 Start position / 5.2 Header / 5.3 First instruction / 5.4 Main routine (V6) / 5.5 Initial execution point (other versions)
A routine is required to begin at an address in memory which can be represented by a packed address (for instance, in Version 5 it must occur at a byte address which is divisible by 4).
A routine begins with one byte indicating the number of local variables it has (between 0 and 15 inclusive).
In Versions 1 to 4, that number of 2-byte words follows, giving initial values for these local variables. In Versions 5 and later, the initial values are all zero.
Execution of instructions begins from the byte after this header information. There is no formal 'end-marker' for a routine (it is simply assumed that execution eventually results in a return taking place).
In Version 6, there is a "main" routine (whose packed address is stored in the word at $06 in the header) called when the game starts up. It is illegal to return from this routine.
In all other Versions, the word at $06 contains the byte address of the first instruction to execute. The Z-machine starts in an environment with no local variables from which, again, a return is illegal.
Note that it is permissible for a routine to be in dynamic memory. Marnix Klooster suggests this might be used for compiling code at run time!
In Versions 3 and 4, Inform always stores 0 as the initial values for local variables.
Contents / Preface / Overview
Section 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13 / 14 / 15 / 16
Appendix A / B / C / D / E / F