Amiga Computing - Commodore Is Awesome

Paul Overac delves
beneath the surface
of the Amiga's
program scatter
coding routines
Ai
taking a look at the routines that ore used
0 ver to lood the next and few run issues Amiga I am programs. going to The be
illOrce•
to - files used to store an executable program on disk,
• in other words the sacaled Ain igaDOS 'Load Files'.
A load file consists of a number of units known as
sta
'hunks' which, amongst other things, contain program
code, rtin static data such as graphics and text, and
details g of any uninitialised space needed by the pro-
ppm. oWhen
a user runs such a program, either from
ithe Workbench n or by typing its name at a Shel win-
t
dow, on AmigaDOS loading routine is brought into
reion.
f
The first thing that happens is that a header block
ois
read and the space for a hunk table, used during
r the loading operations, is alocated Immediately after
tesis,
appropriate amounts of memory for the various
hcode,
data and bss luninihalised spacer hunks present
in the load file ore alocated, with the hunk con
e
tnts being read into the ac
s
qBecause
separate memory alocations for the various
e
uired hunks are made, the hunk information basicaly just
dgels loaded i into any suitable memory area Mot is
m e m o r y
scm:viable
(with the loading routine performing any
b l o c k s .
ccenchee
address relocation calculations and creating
My segment code links that may be required(
u
Pogrom segments produced in this way are linked
skigeohor
using on arrangement known as a 'seglist'
scmd the i original DOS routine for scatter loading o
oprogram
into memory_ loadSegl), for example will
ncreate
just such a list, and return a pointer to it, if the
s load is successful, At some stage, of course, any pro.
gram loaded by this routine will need to be removed
a
horn memory and the routine used to perform this
roperation
is caled UnLoadSegg.
e
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turn Woe seetist_p
uteri: d l
It
Prototype: POOL UnLoadOes(OPTS)
Function 2 : The Amiga D OS se gme nt unloa ding routine
ler
Function 1 : The
Amiga D OS sca tte r
loa ding sa glia l
cre a tion routine
pot Oslo: fi lenase_p . p o i n t e r to s null terminated
string
pointer too tog List or NIAL
if error errors
seglist.p p o i n t e r to s 'seise
StgettS B o o l e a n succesettoitore
qtrli:ator
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Scatteri
brain
PLACING A LOAD FILE
INTO MEMORY
The use of the LoaciSegji routine itself is easy. Load a pointer to the filename into
•
register di and then make a conventional DOS library call it. t
i
prive.t
c f a s h i oMASTS n :
itt teeese,ell Lead hi./ nose
lostiStg
o
The result which comes bock in dO should, _DOSB of course, be checked for elf Vt and often
a convenient way of doing this will be e to sstore e the pointer in memory and test the
zero flag after the move has been completed like this:
erne. L eD,steitst_p
beq.s .error
preserve EN. pointer for unfolding
Having got the seglist equivalent of a load file safely into memory, the DOS library
provides routines, including one coiled CreateProc(), which allows the code to be
brought to life as a separate process. There are, however, other uses for the loading
routines - you can, for example, pass files containing just image data structures
through the conventional assemble/link stages, producing graphics data that can be
scatter loaded. When it comes to using functions like intuition's Drcrwimage() and so
on you do, however, need to be able to locate the start of the real data within each
segment.
Individual segments which make up a seglist are tied together by means of pointers
(lICPE pointers) with a NULL value signifying the end of the seglist chain. If we
then take the starting BPTR seglist pointer provided by a routine such as LoaciSegt),
left shift it by two bit positions and add 4, we end with on address that is the start of
the real segment information. Here's a typical piece of example code for scatter
loading a load file into memory and locating the entry point of the first segment:
move.1
CALLSTS
sove.1
berr.s
'ALL
•tfOrkt
sone.l
If i lenese,d1
LoodSeg,_DO10e se
dO,s es ti st_s
.error
02,dO
04,d0
d0,11
As far as file loading
AmigaDOS routines take
look at how seglists can
next month.
111•11111•111111111111•1
preserve BCPL pointer for unloading
el points to tntry point
and segfist creation is concerned that's basically it - the
care of all the difficult stuff. The next thing to do is take a
be used and, needless to say, this part of the story comes
SEGLIST POINTERS
AmigaDOS has its roots in cm operahr
system called Tripos, much of which wo'
written in a language called BCPI
Amigo coders in general do not have to
worry about BCPL except for the fact
that it uses a form of long word addressing
based on the normal 680x0-style
memory addresses divided by four. lf,
therefore, an Amiga DOS function is specified
as returning a BPTR, i.e. a BCPLtype
pointer, you have to multiply that
AMIGA COMPUTING
FEBRUARY 1996
sove.I
Leki
Weiss
111•111111111'
uret
...lciress by four before using or.
If an AmigaDOS function returns a
EIPTR, then any equivalent der:notation
function will also expect a BPTR as well.
This being so, it's usually best to store
these pointers in BCPL form and then
convert them just before use like this:
Sep1_00 UPI. fors
02,d0 r o o t slobine
A.