hugo-book2_05.asciidoc

Grammar

The grammar table starts immediately following the header (at $40, or 64 bytes into the .HEX file). It is used for matching against the player’s input line to determine the verbroutine to be called, and if applicable, the object(s) and xobject (i.e, the indirect object).

Note	If the input line begins with an object instead of a verb — i.e., if it is directed toward a character, as in “Bob, get the object”, then grammar is matched against the phrase immediately following the initial object.

The grammar table is comprised of a series of verb or xverb (i.e., non-action verb) blocks, each beginning with either verb ($2C) or xverb ($2D). A $FF value instead of either verb or xverb indicates the end of the grammar table. A grammar table that looks like

000040: FF

has no entries.

Following the verb type indicator is a single byte giving the number of words (i.e., synonyms) for this particular verb. Following that are the dictionary addresses of the individual words.

Think of the simple grammar definition:

verb "get", "take"
    * object DoGet

If this were the first verb defined, the start of the grammar table would look like:

000040: 2C 02 x2 x1 y2 y1

where $x1x2 is the dictionary address of “get”, and $y1y2 is the dictionary address of “take”.

With v2.5 was introduced a separate — although rarely used — variation to the verb header. A verb or xverb definition can contain something like

verb get_object

where get_object is an object or some other value. In this case, the verb word is get_object.noun instead of an explicitly defined word. The grammar table in this case would look like

000040: 2C 01 FF FF 4A x2 x1

where $FFFF is the signal that instead of a dictionary word address, the engine must read the following discrete value, where $4A is the object# token, and $x1x2 is the object number of get_object. This extension is provided so that grammar may be dynamically coded and changed at runtime. (See also Book I on dynamic grammars definitions.)

Following the verb header giving the number of verb words and the dictionary address of each is one or more grammar lines, each beginning with a * signifying the matched verb word.

Note	For an elaboration of valid grammar syntax specification, please see the Hugo Manual.

Grammar lines are encoded immediately following the verb header, so that in the first example given above,

verb "get", "take"
    * object DoGet

becomes:

000040: 2C 02 x2 x1 y2 y1
000046: 08 66 48 r2 r1
00004B: FF

where $r1r2 is the indexed routine address of DoGet.

The $FF byte marks the end of the current verb definition. Immediately following this is either another verb or xverb token, or a second $FF to indicate the end of the verb table.