Path: utzoo!attcan!uunet!know!zaphod.mps.ohio-state.edu!rpi!uupsi!sunic!liuida!d88ricwe
From: d88ricwe@odalix.ida.liu.se (Rickard Westman)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Assembly question from turnip
Keywords: can allocmem crash?
Message-ID: <1990Oct27.114320.20536@ida.liu.se>
Date: 27 Oct 90 11:43:20 GMT
References: <2728cc8d-a00comp.sys.amiga.tech@tronsbox.xei.com>
Sender: news@ida.liu.se (News Subsystem)
Organization: CIS Dept, Univ of Linkoping, Sweden
Lines: 36

dfrancis@tronsbox.xei.com (Dennis Francis Heffernan) writes:

>	Well, I've hit a good one.  I've finally gotten around to trying to l
>learn assembly on this beast, and my first program crashes.  Not surprising.
>WHERE it crashes is- the first call to AllocMem.
>	Consider the following:

>getvarmem     move #4,a6                    ;get memory for variables

Try            move.l 4,a6

>              move.b #60,D0

Try            move.l #60,D0

>              move.l #0,D1
>              jsr _LVOAllocMem(A6)

All system calls expect longwords as arguments(*).  If you do "move.b #60,D0"
only the lower 8 bits of D0 are set - the rest of them could be set to 
anything.  If you don't use a size designator at all (as in the first line)
most assemblers will default to word size, leaving you with the same type of 
problem.  So - *always* use ".l" when dealing with system call arguments.

This is not why your program crashed, however.  To call AllocMem() you need
the exec.library base pointer in a6.  This pointer is stored at the fixed 
address 4.  But remember that it is a *pointer* to the library base, not the
library base itself.  You need one more level of indirection, which is what you
get if you use "move.l 4,a6" instead of "move.l #4,a6".

/RiW

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(*)  This may not be entirely true.  I think there is one or two functions
     that use words, but they are clearly an exception.