Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!jarthur!nntp-server.caltech.edu!toddpw
From: toddpw@nntp-server.caltech.edu (Todd P. Whitesel)
Newsgroups: comp.sys.apple2
Subject: Re: Multitasking on a II
Message-ID: <1990Dec14.060502.1328@nntp-server.caltech.edu>
Date: 14 Dec 90 06:05:02 GMT
References: <&RF^T6-@rpi.edu> <10063@darkstar.ucsc.edu> <7TF^W^=@rpi.edu>
Organization: California Institute of Technology, Pasadena
Lines: 24

gwyn@smoke.brl.mil (Doug Gwyn) writes:

>In article <7TF^W^=@rpi.edu> floyd@pawl.rpi.edu (Patrick J Wetmore) writes:
>Without an MMU, the main difference in operation is that relocation
>must be provided as task images are loaded into memory for execution
>(except if position-independent code is involved, but normally that
>imposes too stiff a speed penalty and requires cooperating compilers).

position-independent code is easier than it looks. you just have to be
prepared to do without an index register.

for example, assuming PHK PLB is already in effect, execute
	PER baseaddress
	PLX
and voila, you can access 64K starting at baseaddress by using Abs,X --
	LDA	|variable-baseaddress,x		;'|' forces Abs addressing
since you can use nearly any register/memory instruction with Abs,X this
is a big win, because you can do the same operations on globals as you can
on locals (direct page). I have pondered the idea of writing a tiny C compiler
that outputs ORCA/M assembly (I'm not about to tackle OMF, ok?) but I've got
other, more immediately useful projects to worry about.

Todd Whitesel
toddpw @ tybalt.caltech.edu