Amiga Computing - Commodore Is Awesome

ler code, even when reasonably
0 Vs well a documented, well known fact is rarely that assem- easy to
understand unless you have a good idea of
what the code is supposed to be doing in
tie first place. Needless to say, this makes
-
',luage
coders look far more difficult than
a-
ne
Db of making an Amiga display 'smooth
y ;croll' and since it seemed to me this would
or
:e an area many of you would be interested
f e
in, I've chosen to devote some time to
pust
ta
this subject.
In fact, over the next two instalments I'll
hl
no modifying and extending the 680x0
e l
code provided last month in order to pro-
ty
duce a demo that vertically smooth scrolls
an ra
Intuition screen. Before explaining how
this ir
type of smooth scrolling is actually
done, ce
however, let me kill off one false trail.
k.
Those of you who have the Amiga's
graphics s A library documentation may have
seen ut
that there is a library routine called
ScrolIVPort() sy
that can be used to produce
display
ep
scrolling effects. The plain truth is
that
d i while, in theory, this routine could conceivably
bc
be used to produce smooth
scrolls,
y a the results obtained by using this
function
al
are lust not good enough.
The autodocs themselves mention that
sc
the ScrolIVPort() function is slow and can
sa
produce visible 'hashing' of the display, So,
if es
ScrolIVPort() can't be used, is there an
alternative me
course ot action available to us?
The bi
answer here is very definitely yes, but
in ln
order to appreciate it it's necessary to be
clear y p in your mind how Amiga displays are
generated. Io
ai
All displays are created by allocating
blocks n of memory called 'bitplanes,' in
which
-t
each 'bit' represents a pixel position
on i the display. Normal displays will contain
a s number of separate bitplanes and by
taking
t
the appropriate pixel bit from
each bitplane, the Amiga's display hard-
h
ware is able to generate a colour register
number. e
F
Values stored in each colour register
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nothing but
an old otp.
05moothq!
,*;„6,
determine the actual colours seen on the
screen, and it's because a screen's colour
register number range depends on the
number of bitplanes being used that the
colours available with different screen
types varies. A one bitplane screen can
only have two colours (corresponding to
any single bit in the bitplane, being either a
0 or a 1). two bitplane screens can have
four colours (each bit from each bitplane is
combined to produce one of four values
00, 01, 10, or 11) and so on.
MEMORY
Although display bitplanes have to be
stored in chip memory (because they need
to be accessible to the custom chips) they
do not have fixed positions as such. In fact,
when a screen is opened the bitplanes will
be allocated in any convenient area of chip
memory available.
Needless to say this means the graphics
system needs to have some way of identifying
the position of these bitplanes and
the structure used is called a BitMap_ This
structure can be found in the graphics/06
include file and you'll notice from the
description below that it contains space for
Smooth I S one of the 177051'
LISP& ttirk5 1317 d501fing 58111111 fimiga
(oder ran learn about th15 month
Paul thiErdd 5tdrt5 Pllfildinifig how it 1.5
Some simple
smooth scroll
rputinos in
pction
Amiga Computing
MAY 1995
TUTORIAL
Part 10
done u5kg a55embN language
up to eight bitplane pointers:
STUCTUIE DitAR.D
VOID bo_ D yte sPe rtoo
VOID
DTI bo_ Fie gs
etvE be _ le pth
VOID blue d
PERT De _ PLa ne o, Doe
the OitpLeoes
LABEL bo_ SU EOF
;R iote rs Co
Because the BilMap's bitplane pointers
define the memory locations used to produce
the display that appears on your monitor,
you might expect that, by arranging for
a display's bitplane pointers to be
increased by an amount which corresponds
to the pixel-width of the screen, it would
be possible to shift the display memory
downwards by one line.
Similarly, by decreasing those pointers
by the same amount the display might be
expected to shift upwards one line.
Although very close to the truth, this
doesn't work because a part of the story is
still missing. While the BitMap structure
certainly defines the initial display memory
being used, the Amiga's graphics coprocessor
(the 'Copper') which handles the
display generation doesn't actually collect
its bitplane information from this source
Instead it uses copies of the bitplane pointers
that have been embedded into a sehes
of instructions called a 'copper list'.
Once Intuition has opened a screen and
generated these copper instructions, the
bitplane pointers held in the BitMap struc•
hire's are essentially redundant as far as
the display generation process is concerned.
The important bitplane pointer
91