@get_prop_len 0  



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I'm not sure if the latest spec (spec 1.0, June 1997) reflects this required behavior, but for the record, this is what Jzip does, and why it does it when you call the opcode @get_prop_len with an argument of zero.

the z-machine list emails

Date: Tue, 04 Aug 1998 14:04:52 +0100
From: Kevin Bracey
To: z-machine@gmd.de
Subject: [z-machine] GET_PROP_LEN 0
Me again.

What should GET_PROP_LEN 0 return? My reading of the Standard says that
it's illegal, but I've just caught Shogun (322.89706) doing it.

This arose after I started making Zip 2000 a lot stricter about memory
accesses, which is of course just asking for trouble :)

If, as your first command, you say "STRAIGHTEN THE WHEEL", the following
piece of code gets executed:

 dc94:  GET_PROP_ADDR   L00,#20 -> L0b
 dc98:  GET_PROP_LEN    L0b -> -(SP)
 dc9b:  DIV             (SP)+,#02 -> L0c
 dc9f:  DEC_CHK         L08,#00 [TRUE] RTRUE
 dca3:  LOADW           L07,L08 >> L09
 dca7:  STORE           L06,L09
 dcaa:  JE              L09,"no.word" [TRUE] dc9f
 dcb1:  SCAN_TABLE      L06,L0b,L0c >> -(SP) [TRUE] dc9f
 dcb9:  JE              L06,"closed","shut" [FALSE] dcc6
 dcc1:  TEST_ATTR       L00,#11 [FALSE] dc9f
 dcc6:  JE              L06,"open" [FALSE] RET #FALSE
 dccc:  TEST_ATTR       L00,#11 [TRUE] dc9f
 dcd1:  RFALSE          

where L00 is the object number of the wheel (269, IIRC). The wheel
doesn't have property $20, so GET_PROP_ADDR returns 0. Zip 2000 1.30
then promptly aborts with "Attempt to access memory location &FFFFFFFF,
outside story file".

Logic, and this code, suggests that GET_PROP_LEN 0 should return 0.


Kevin Bracey, Senior Software Engineer
Acorn Computers Ltd                           Tel: +44 (0) 1223 725228
Acorn House, 645 Newmarket Road               Fax: +44 (0) 1223 725328
Cambridge, CB5 8PB, United Kingdom            WWW: http://www.acorn.co.uk/
Date: Tue, 4 Aug 1998 09:01:02 -0700 (PDT)
From: Andrew Plotkin
To: Z-Machine List
Subject: Re: [z-machine] GET_PROP_LEN 0

On Tue, 4 Aug 1998, Kevin Bracey wrote:

> What should GET_PROP_LEN 0 return? My reading of the Standard says that
> it's illegal, but I've just caught Shogun (322.89706) doing it.

Welcome to the wonderful world... As best I can figure, it's illegal, and
Shogun has a bug in it. 

The original ZIP source will behave unpredictably. It will try to read the
byte before the beginning of Z-machine memory, which is a malloced
chunk, so what you get depends on the malloc implementation.

My solution to this mess is to print a warning, *visible to the player*,
and then return zero. By default, such warnings are only printed once per
game session, so they don't interfere *too* much with the playing
experience. There are options to hide such warnings entirely, to show them
every time, and to treat them as fatal errors.

(However, I actually forgot to test for GET_PROP_LEN 0. Oops. I test for
GET_PROP_ADDR 0 and so on, but I missed that case. A note for the TODO
I think it would be a mistake to say that GET_PROP_LEN 0 is legal, and
always returns zero. That would effectively extend the spec and de-qualify
a lot of existing interpreters. I *don't* say that to coddle the
interpreter authors (I think they should *all* test these conditions, and
at the very least behave predictably!) But if it's legal, then more Inform
games will be written which *do* stuff like this. And then players with
older interpreters will encounter more crashes.

If we say it's illegal, and interpreters print warnings so that authors
*know* when they're doing something illegal, then future Inform games will
be more likely to adhere to the spec. (The spec we have now, I mean.)

BTW, I know I've posted this rant before (about GET_PROP_ADDR and so on)
and I'm sure you remember it. I apologize. I think this kind of thing is
important; it's the difference between a reliable software platform and an
unusable one.


"And Aholibamah bare Jeush, and Jaalam, and Korah: these were the

Date: Tue, 4 Aug 1998 17:12:07 +0100 (BST)
From: Graham Nelson
Subject: Re: [z-machine] GET_PROP_LEN 0
To: Z-Machine Mailing List

On Tue 04 Aug, Kevin Bracey wrote:
> Me again.
> What should GET_PROP_LEN 0 return? My reading of the Standard says that
> it's illegal, but I've just caught Shogun (322.89706) doing it.

Thereby hangs a tale.  Early releases of Inform relied on
get_prop_len returning 0, because the .# operator, as in,

   x = spell_book.#spell_list;

was supposed to return 0 if the object didn't possess the given
property.  On some versions of Zip and ITF, this does indeed
work.  These days Inform would compile



    1  +0001f     get_prop_addr obj identifier -> TEMP1 
    1  +00023     jz           TEMP1 to L1 if TRUE
    1  +00027     get_prop_len TEMP1 -> TEMP1 
    1  +0002a    .L1

(well, actually it compiles .# to something much more
complicated in Inform 6, but this is the interesting part).
Since get_prop_addr definitely does return 0 if the property
doesn't exist, the above code is protected against this
contigency.  In short, no Inform-compiled game can ever produce
the "bad" case except by the direct result of the author's
folly in using assembly language.

Personally I would prefer the standard to require that

   get_prop_len 0
returns 0, since it's not a time-critical operation and why
not be safe rather than sorry.  At the time we made the decision
on this, I'm pretty sure we didn't know about Shogun's behaviour
requiring it.

Graham Nelson | graham@gnelson.demon.co.uk | Oxford, United Kingdom

Copyright © 2000 John Holder