Appendix E. Statistics

The shortest files are those dating from the time of the Zork trilogy, at about 85K; middle-period Version 3 games are typically 105K, and only the latest use the full memory map. In Versions 4 and 5, only Trinity, A Mind Forever Voyaging and Beyond Zork use the full 256K. Border Zone and Sherlock, for instance, are about 180K. (The author’s short story Balances is about 50K, an edition of Adventure takes 80K, and Curses takes 256K (it’s padded out to the maximum size with background information; the actual game comprises only about 245K). Under Inform, the library occupies about 35K regardless of the size of game.)

Zork I uses only about 5500 opcodes, but the number rises steeply with later games; Hollywood Hijinx has 10355 and, e.g.Moonmist has 15900 (both these being Version 3). Against this, A Mind Forever Voyaging has only 18700, and only Trinity and Beyond Zork reach 32000 or so. (Inform games are more efficiently compiled and make better use of common code—the library—so perform much better here: the old Version 3, release 10 of Curses (128K long, and a larger game than any Infocom Version 3 game) has only 6720 opcodes.)

This varies greatly with the style of game. Zork I has 110 rooms and 60 takeable objects, but several quite complex games have as few as 30 rooms (the mysteries, or ’Hitch-hikers’). The average for Version 3 games is 69 rooms, 39 takeable objects.

A Mind Forever Voyaging contains many rooms (178) but few objects (30). Trinity, a more typical style of game, contains 134 rooms and 49 objects: the Version 5 Curses has a few more of each. Of the Version 6 games, only Zork Zero scores highly here, with 215 rooms and 106 objects. The average for Version 4/5 games is 105 rooms and 54 objects.

The total number of objects tends to be close to the limit of 255 in Version 3 games. Curses contains 508.

Early games such as Zork I know about 600 words, but again this rises steeply to about 1000 even in Version 3. Later games know 1569 (Beyond Zork) to the record, 2120 (Trinity). (This is achieved by heroic inclusion of unlikely synonyms: e.g. the Japanese lady with the umbrella can be called WOMAN, LADY, CRONE, MADAM, MADAME, MATRON, DAME or FACE with any of the adjectives OLD, AGED, ANCIENT, JAP, JAPANESE, ORIENTAL or YELLOW.) Version 6 games have smaller dictionaries. So has Curses, at 1364.

(a) Of the 1426854 opcodes in the shipped Infocom story files in Paul David Doherty’s collection, here are the top and bottom ten most popular. (Leaving out those which never occur and so score 0: nop, art_shift, piracy and the two post-Infocom opcodes, print_unicode and check_unicode.)

So about 2/3rd s of all opcodes are those in the top ten; 1 in 8 opcodes is a je, and only 1 in 710000 is a print_form.

(b) An experiment (conducted with the help of Kevin Bracey) sheds some light on the opcodes most frequently interpreted in typical play. Two very different games (Zork I, Version 5 “solid gold” edition; Museum of Inform, a complex Inform example game) were played for about 50000 cycles of the Z-machine (about 20 moves in Zork I, rather less in the Museum). The following table records all opcodes with a frequency of at least 1% (i.e., 0.01):

Adventure games spend most of the time parsing, and the differences between these tables reflect different parser designs (byte arrays versus word arrays and arrays stored in properties) as well as different compiler code generators (Inform does not use inc_chk or dec_chk, so it uses inc, dec, jl and jg correspondingly more). In the case of Zork I, about a third of all opcodes are branches: in the case of Museum, almost half.