Using Players to any extent from BASIC is
complex and I have therefore come up with a routine that takes away all
the fuss and allows a shape to be placed anywhere on the screen with
ease. I have also incorporated a routine that allows different 'frames'
to be used to achieve animation and I hope that readers will find this
program of use in their own games.
The program uses machine code residing in
page 6 of memory but allows horizontal and vertical movement to be
controlled easily and rapidly from BASIC. There is also the facility for
animation sequences up to 48 frames long. The machine code routine used
is tied to the Vertical Blank Interrupt system to allow smooth movement.
The accompanying program demonstrates
what is possible with the aid of a Shamus type character. Lines 1000 to
1080 hold the data for the machine code and lines 100 to 155 hold the
data for the frames required. It is a good idea to save the program
before you attempt to run it as any errors in the data statements could
cause the system to crash. If you have a 16K system, the 144 in line 10
should be changed to 48.
To initialise the routine, the following
BASIC command is used at the beginning of the program
A=USR(1536,PMSTART)
PMSTART is the page number of the
beginning of the player/missile table and would normally be above the
display memory.
Each frame is defined on a grid of 16
rows of 8 columns in a similar manner to the way characters are defined
and up to 48 different patterns may be stored. The memory map for single
line resolution players is normally as figure 1.
The program makes use of the unused 768
bytes to store the data for each frame. Thus frame 0 occupies PMSTART*256
to PMSTART*256+15, frame 1 occupies PMSTART*256+16 to PMSTART*256+31,
frame 2 occupies PMSTART*256+32 to PMSTART*256+47 and so on.
Players are placed on the screen by the
following command
A=USR(1568,P,X,Y,F)
the variables used are shown in figure 2.
Each player can access any frame,
regardless of whether it is being used by another. The horizontal and
vertical co-ordinates refer to the top left hand corner of the player,
so to centre it around point X,Y, the values X4,Y-8 would have to be
substituted into the USR call. Animation may be achieved simply by
modifying the X,Y and F values and repeatedly calling the routine.
To incorporate the routine into you own
programs, just omit lines from 100 to 999 and continue programming as
usual.
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