Amiga Format 39/af039a.adf/month1.doc

Text File | 1992-08-24 | 5KB | 143 lines

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*                   oo                              oo                           *
*                 \(  )/      Bullfrog Demo      \(  )/                           *
*                 ^ ^^ ^         Month 1         ^ ^^ ^                           *
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So you want to be a games programmer.  Well then read on, and you should get a
few helpful hints.  Included on the disk is a simple shell for writing programs
in assembler.  It sets you up with all you need to swap screens, draw sprites,
and so on...


With this shell you don't have to wade through loads of reference manuals to
find out how to set up screens, or display sprites, its been done for you.  Over
the course of several months we will be putting together a very simple game, but
there are several things you will need:-

    * Enthusiasm.  Yep I know, sounds obvious but you will always be just a
      standard programmer unless you really want to do it.
    * It would be nice if you read a 68000 assembler book.
    * Do you know what binary / hexidecimal are, no then thry and find out, get
      one of those boring reference manauls out the libarary.


Ok, thats the boring introduction done, lets have a look at the disk.  If you
first assemble Demo.s and then run Demo.  All that should happen is that we
display a sprite on the screen.  To exit from the program press the Q Key.
Great.  There you go.  What?  You want more?

Then lets try and move the sprite.  Have a look in draw.s in the routine

_draw_all

you will see we have

        moveq.w    #0,d0
        moveq.w    #0,d1
        moveq    #0,d2

now if we change these to

        move.w    man_x,d0
        move.w    man_y,d1
        move.w    man_frame,d2

Right, now the impatient ones of you will have assembled that.  No change, of
course not, but if you go into move.s and remove the indicated ;'s our sprite
should start to move to the right and return to the start when he reaches the
end of the screen.  The way these instructions work is as follows.  First we
increment the value of the man x co-ordinate by 1 every turn.  The value is then
compared with 304 and if it is greater than this the position is reset to 0
otherwise we jump over the reset command.  The value is then stored back into
man_x ready for the next draw.

You can change the value in the add command to increase the speed of movement.

A small challenge for you, see if you can write the code to move the man down
the screen.  The code will be basically the same as the move x stuff, but
remember the screen size is 320 by 200 pixels, and that the label lessthanx
has already been used, so use something different.  A note on labels, try
and keep them meaningful, 6 labels with the name .here is not very helpful.

We should have, by now, got the man moving down and across the screen, but thats
the problem he is not moving.  Lets put a bit of animation into the man.
To do this we took the same routine as was used in the movement and changed a
few things.  Be careful, we only have 4 frames of animation so dont set the
compare to high.

Compile this and have a look.  He is changing frames just a little bit fast,
too fast in fact so we need to slow him down.  To do this change the line
    cmp    #4,d0
to    cmp    #4*4,d0

thats not quite all, we need to scale the number back down.  To do this
after the line
    move.w    man_frame,d2
in draw.s, place the following piece of code:-
    asr.w    #2,d2

Assemble this, and the man should be walking a bit slower.


COMMAND OVERVIEW

move.w    1,2
The move command is one of the most heavily used commands for simplicity we
are using move.w the w means word sized data.  It is the equivelent of the
basic LET statement.  It loads 1 into 2.

add.w    1,2
The add command does just that adds the value of 1 to 2.

sub.w    1,2
The sub command is similar to the add but does subtracts instead.

cmp.w    1,2
The compare command compares the value of 1 with the value of 2.  When used
in conjunction with a branch you get the equivelent of the basic IF statement

Branches
The branch checks on the flags set by the compare statement.  There are 15
different versions of this command:-
    bra        Branch always.  Same as GOTO
    bcc        Branch if carry clear
    bcs        Branch if carry set
    beq        Branch if equal
    bge        Branch if greater than or equal to
    bhi        Branch if higher
    ble        Branch if less than or equal to
    bls        Branch if low or same
    blt        Branch if less than
    bmi        Branch if minus
    bne        Branch if not equal to
    bpl        Branch if plus
    bvc        Branch if overflow clear
    bvs        Branch if overflow set

Dont worry if you dont understand what all of these do, we want be using all of
them.  I hope.  Anyway, when the condition is true the branch is taken, if not
them we move onto the next instruction.

Labels
You will notice that some of the labels have a '.' in front of them, these are
called local labels.  Now that means that you can use words like .finished more
than once, though not inside the same routine.  Routines are defined by labels
which have not got a period '.' in front of them and are finished with an RTS
command.

jsr
Jump to subroutine.  This command is basically the same as the GOSUB command.

rts
This is basically the same as the RETURN command.

asr.w    1,2
The asr command means Arithmetic Shift Right and works by moving a binary
number to the right the define number of times.
eg.        on a asr.w    #1,10
we get an answer of 5. This is because 10 in binary looks like this:-
    1010 = 10
and when we move it to the right by one it becomes:-
    0101 = 5