This page documents information about . Many of the tricks demonstrated here are near impossible in real time and documented for the purposes of creating Tool-assisted Speedruns.
Zork I: The Great Underground Empire runs on the
Z-machine,
which is famously portable and runs on many different operating systems and consoles.
Some of the topics on this page are particular to the Apple II implementation,
while others apply to any version.
Zork I appeared in
many revisions over several years.
Revision 88 (1983) is the most common and widely distributed revision;
it's the "canonical" revision to play
unless you have a particular reason to choose a different revision.
The game by default runs in "brief" mode,
which means the long description of a room is shown only the first time you enter it,
and the short description is shown thereafter.
There is also "verbose" mode, which always shows the long description,
and "superbrief" mode, which always shows the short description.
Superbrief mode (which can be entered using the command super)
is about 20% faster than brief mode in the Apple II version.
The Apple II version can run in 40-column mode or 80-column mode.
80-column mode is visually nicer, because it has uppercase and lowercase letters,
but it is estimated to be about 6% slower.
Of particular interest for a TAS is the "raft of holding" bug:
27. THE RAFT OF HOLDING: When you put objects in the raft and then deflate it, they effectively cease to exist, allowing you to carry as much weight and as many objects as the raft will hold (it won't hold the gold coffin, unfortunately, and of course sharp objects are out). This is because the game handles the pile of plastic and the raft as two separate objects, and there's no provision for handling any objects inside the raft when the switch is made. -- Stu Galley in The New Zork Times
Stacking commands
It's possible to enter multiple consecutive commands
on one line by separating the commands with dots.
(The queue of commands gets stored in a variable called
P-CONT.)
For example, at the start of the game, you can enter
n.n.u.get egg.d to get to the tree, climb the tree, get the egg, and climb down again.
Doing this is generally slightly faster than entering the commands on separate lines.
There are limitations:
many game events, such as
entering the Loud Room
or
initiating combat,
will clear P-CONT.
If you queue up commands that produce more than a screen's worth of output,
you still have to enter inputs to clear the "[MORE]" prompts that appear.
RNG
The RNG in the Apple II version uses the time delay while waiting
for input to be typed,
as well as the ASCII codes of the typed characters.
You can manipulate RNG by altering the case of entered commands,
e.g. get egg versus get egG versus Get EGG.
The Apple II Z-Machine interpreter (ZIP) uses the
KEYIN
random number generator built into the Apple II firmware,
which rapidly increments a 16-bit counter at addresses 0x4e and 0x4f
(which the ZIP calls RNUM1 and RNUM2)
while waiting for a key to be pressed.
The ZIP's GETKEY subroutine
calls
the Apple II RDKEY subroutine,
which in turn calls KEYIN.
KEYIN returns an ASCII code in the A register and leaves
RNUM1 and RNUM2 in some hard-to-predict state.
At the end of GETKEY, the ZIP further tweaks the random number state by
adding the ASCII code into RNUM1 and xoring it into RNUM2:
ADC RNUM1 ;FUTZ WITH RANDOM
STA RNUM1
EOR RNUM2
STA RNUM2
Object have a SIZE property
that represents their weight.
The most weight you can carry is
100 units (LOAD-MAX).
The limit is lowered when you are injured
(LOAD-ALLOWED).
The weight limit is a hard limit that may not be crossed.
There is also a soft limit on the number of items you are carrying.
Beyond 8 items, there is a
random chance that you will fumble
when trying to pick up an additional item.
The movement of the thief is
not random.
He warps from room to room
in a set and predictable schedule.
He is invisible while doing so.
When in the same room as the player, there is a random chance
that he will become visible and encounter the player.
The Visible Zorker
is a playable online version that shows an execution call stack
and has easy cross-references to the source code.
It uses revision 88.
If you click "Show addresses" under the hamburger menu,
you'll find decimal addresses that correspond to the disassembly.
See also: The Visible Zorker (2025).
zork1-disassembly.txt
is disassembly of the compiled Z-machine code.
The hexadecimal addresses in the left column correspond to the decimal addresses
in The Visible Zorker.