Amiga Computing - Commodore Is Awesome

92
TUTORIAL
values are those in the hardware copper
list, so if beplane pointer adjustment needs
to be done quickly, this is the place to do it.
In short we need to search this list, find
the instructions which set up the biplane
pointers at the start of each display frame,
and alter these.
To avoid visible display disturbance. it's
obvious that such adjustments need to be
made at times when the Copper is not trying
to read the bitpiane addresses itself. The
secret here lies in realising that the Copper
re-initialisation occurs as part of the housekeeping
that goes on during vertical blanking
intervals. A number of options are available
but I'll discuss these when I deal with
the scroll code itself.
As you increase (or decrease) a screen's
bitplane pointers, the effect will be to bring
new bitplane memory into the visible display
area. If, therefore, you set up a normal
sized screen, the result of any bitplane
pointer adjustment will be to move the
memory area being used for the real display
640 pixels wide
Within the screen data that Intuition sets up there is
a pointer to a View structure and the first thing we
need to do is get the address of this using Intuition's
ViewAddress() function. The result comes back in
de and can be used immediately, because within the
View structure there is a field called v_LOFCprList.
This points to a cprlist structure which provides a
pointer to the copper list the hardware actually executes.
The end result is that we identify the sten of
the required hardware list using this sort of code:
CALLSTS Vie cAddre ss, _ EntuitionSist
loce.1 dO, a 0 copy to an address righte r
acce.1 c_ LAFC prtistia 0 ), a 0
cri_start(a0),a0 pointe r to sta rt
List Amm in e t
Once we've got to this stage a loop needs to be
used to locate the appropriate bitplane instructions.
Copper instructions consist of two lb-bit words
where the first word specifies the instruction type
and the second specifies a data item. The instruction
we need to locate is the first of a series of
Figure 1 : Only the lowe r
port of each! Chole r,* will
iso vfuibl. whe n the
'croon first ope ns.
Top part of display will not be visible
when screen first opens
This lower part of display will be
visible when screen opens
Amiga Computing
MAY 1995
EALISTS Vie cAddre ss, _ I ntoitiontise
love d
sove.t
one.1
march copia
bed.'
eddo.t
Pre.'
alecceendedlo_ I
note.
Listing t TimGoppor tAit se a rching code in its glory!
outside the memory holding valid screen
graphics information. This usually means
that rubbish gets displayed on the screen.
so the secret is to create an oversized
display-memory area so that you only ever
scroll within the bounds of whatever valid
graphics data you've set up.
CREATIONS
As far as screen and window creation is
concerned, very few changes are needed
to last month's code The screen's title and
.
4
t
i
o
.
p
o•move
instructions that copy bitplane pointer values
iinto
the Amiga's bitplane hardware registers.
n As the hardware registers can only hold 16-brt val-
tues
it actually requires a pair of registers, and there-
e
fore two separate copper instructions, to store a full
r
32-bit memory pointer. It just so happens that the
first pair of high and low bitplane registers set up in
Intuition-created copper lists are those of the lowest
numbered bitplane. conventionally known as
bitplane 1.
The corresponding registers, called BPL1PTH
and SPIAPTL. have absolute addresses of $0080
and $00e2 above the custom chips base address,
so what we need is a search loop which moves
through a hardware copper list two words at a time.
Riff)lane start
identified by
onginal bitplane
Searching the hardware copper I15t
comparing the first word of each instruction until it
finds one which starts with Sea
.search copi.c
becks
addo.t
lion
114,te0) look at insteoction
Jearchend found e l instruction
14,10 move to nest instruc—
Oraa a e a rch N I A keep searching
limAr
0,10 o 0 A T to an Address re giste r
c_LOFiprtistialThs0
cr1_startfa0t,10 s t a r t of tilt nom in 10
ASe0,fa0
[oat a t instruction
.seerchend f o u s d e0 instruction
ei,10 m o v e to next instruction
.sesrch and keep searching
02a0 e v e r to 2nd instruction war
•0,rappertistic a n d save Pointe r
title bar will be removed by setting
SA_Oulet tag to true and, because a
play window is needed that is free fr
any of Intuition's system gadgets. I
going to turn the window into one of
'invisible' borderless backdrop variety.
To implement these latter changes I
the WA_Borderless and WA_Backdroe
tags to true, add false WA_DepthGadgee
and WA_CloseGadget tags, and modify
the window's title and drag bar tags. At
these lags are defined in the system head
ers but, as per usual, I'll be defining ir%
own versions so that those of you withou'
the official includes can still assemble the
code. The end result is that you'll fine
these changed tag definitions incorporate
into the window's tag list:
dc.IWA_Dragaer,TALSE
dc.tiktittediALL
dc.tdA_Bordertess,TRUE
dc.t4A_Baccdrop,ILIE
dc.tilt_DepthGadget, FALSE
dc.tilA_CloseGadget, FALSE
Screen and window sizes also have to
be changed and here I have, somewhat I
arbitrarily, chosen to use a high-res 640
pixel wide screen with a height of 600
lines. The resulting arrangement of the
screen's bitplanes in memory, relative to
what you as a viewer would see on you
monitor, is shown in figure 1. 4 7 0
,
When the loop terminates, the first bitplane oriented
copper instruction has been found, so just add two
to it so that a0 is pointing to the second word (the
data word) of the instruction, and then you can store
the result for future use
.
.searctend Addo.1 $2,e0 e o n to 2nd !Attraction
cord
move.1 e0,coonertist_n e n d sive
pointer
If we put all these ideas together we end up with
the code shown in listing 1. Don't worry if some of
the ideas seem like magic to start with — just concentrate
on understanding the basis of what we're
doing. The important thing is that this code fragment
enables us to find out the whereabouts of the
instructions which set up those bitplane addresses
in the copper list Intuition places. The only bad news
from your point of view is that you now have to wait
until next month's instalment to see how we make
use of this informationl