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From: meekins@anaconda.cis.ohio-state.edu (Tim Meekins)
Newsgroups: comp.sys.apple2
Subject: Re: ML subroutines (passing parameters in ML)
Message-ID: <112122@tut.cis.ohio-state.edu>
Date: 24 Apr 91 21:05:50 GMT
References: <3397@kluge.fiu.edu> <13845@ucrmath.ucr.edu>
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Organization: The Ohio State University, Department of Computer and Information Science
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In article <13845@ucrmath.ucr.edu> rhyde@musial.ucr.edu (randy hyde) writes:
>There are many different ways to pass parameters in assembly language.
>
[snip]

>4) Pass parameters on the stack.  This is how most HLLs pass parameters.
>It is slow, big, and accessing the parameters is inconvenient,
>especially on processors like the 65816 which don't have a frame pointer
>register.  Nonetheless, there are some advantages to this technique: 
>It's easy to understand, easy to verify, and naturally supports
>recursion and reentrancy.  It supports any number of parameters (unlike
>registers) and is easily expanded.
>

Hard to do on a GS? it's quite simple actually if you map the the direct page
to the stack. So parameter 1 becomes dP location 0, 2 become 2, 3 becomes 4,
etc (assuming words and no local variables and so forth). This remapping
also allows local variables to used more efficiently. In fact, subroutines 
written this almost always tend to use direct page variables more often than
absolute addressing variables. This large use of the DP more than makes up
for the small overhead of setting up the stack. Although the set up code
may look nasty to the meek, it is quite simple and macros exist for automating
it. For example, my macro library has a macro called 'subroutine' which works
similar to Orca/M's 'subroutine' macro. All you do is define your local
varibales using equates then invoke the subroutine macro, listing each
parameter and their lengths. It almost looks like a Pascal procedure
definition.



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