Floppyshop 2/Floppyshop - 2.zip/Floppyshop - 2.iso/diskmags/0022-3.564/dmg-3557/news.txt/stos4.asc

Text File | 1989-04-05 | 7KB | 168 lines

                    STOS BASIC TUTORIAL - PART FOUR
                    -------------------------------

                       Written by Lozian for STEN
                       --------------------------


Last  time  we concentrated on getting a picture to appear on  the  screen
from  the  memory  bank,and  hopefully  you  have  mastered   this.   This
articlewill  move  on (excuse the pun),  to show you how  to  scroll   the
screen you have designated to appear, hopefully in the direction that  you
require.

So let us begin by booting up your COPY (Your COPY only), of STOS.


Direct Commands - RESERVE AS DATASCREEN 5:LOAD "\STOS\PIC.PI1",5


What  we  have done is reserve memory for the screen so that  we  are  not
continually  accessing  the drive and have loaded the screen into  BANK  5
ready  for  recall when required.  Screen PIC.PI1 is in the  STOS  folder,
hence the syntax,but you can load in any screen of your choice into a BANK
of  your  choice  provided  you  follow  the  rules  as  above.  The  only
stipulation  as  regards BANK of your choice,is to be careful if  you  are
using accessories and pick the QUIT & GRAB option,since the accessory  may
try,and succeed,to grab the BANK space you have reserved.

You  are normally warned about this from the accessory.  The  manual  does
explain which BANK's are used by SPRITES etc.

Now  study the following listing and I will endeavour to explain  what  is
happening with the help of REM statements.

 10 MODE 0:FLASH OFF:HIDE:CURS OFF:KEY OFF:BACK=LOGIC
 20 GET PALETTE (5):REM Get correct screen colours.
 30 APPEAR 5,30:REM Recall screen from BANK 5,fade in no.30 (or choose).
 40 DEF SCROLL 1,0,0 TO 320,200,1,-1:REM 1st parameter is scroll no.and
    you can define up to sixteen scroll zones,2nd and 3rd parameters are
    top left of scroll area TO 4th and 5th being bottom right of scroll
    area (full screen in this example).6th parameter defines direction
    of scroll in pixels, negative being left and positive being right.
    7th parameter defines direction up (negative) or down (positive) by
    no.of pixels at each scroll call.
 50 SCROLL 1:REM Call scroll previously defined.
 60 SCREEN COPY 5 TO LOGIC:REM Copy back original screen.
 70 GOTO 50:REM Enter loop to show effect.

Remember to RUN it.

This  routine  gives  a jittering screen providing  your  display  is  not
already like this.

To break out of this program use Control+C but specially note that if  the
screen is being updated it may be a moment or two before your keypress  is
read. So be patient.

Now try this.

  10 MODE 0:FLASH OFF:HIDE:CURS OFF:KEY OFF:BACK=LOGIC
  20 GET PALETTE (5)
  30 APPEAR 5,22
  40 DEF SCROLL 1,0,0 TO 320,200,0,1:REM Try last parameter 5.
  50 FOR Y=0 TO 100:REM Try inserting STEP 5.
  60 SCREEN COPY BACK,0,0,320,200 TO LOGIC,0,1:REM Try last parameter 5.
  70 SCROLL 1
  80 NEXT Y
  90 CLS
 100 APPEAR 5,15
 110 GOTO 35

This is a down scroll.  Try changing the parameters in the lines that  are
specified,but change them all equally first and view the effect,  then try
different values in different lines to clarify what is actually going  on.
Please remember that you are specifying amount of pixels to move and large
numbers are inclined to be jerky.

A further routine.

  10 MODE 0:FLASH OFF:HIDE:CURS OFF:KEY OFF:BACK=LOGIC
  20 GET PALETTE (5)
  30 APPEAR 5,12
  40 DEF SCROLL 1,0,0 TO 320,200,1,1
  50 FOR Y=0 TO 100
  60 SCREEN COPY BACK 0,0,320,200 TO LOGIC 1,1
  70 SCROLL 1
  80 NEXT Y
  90 CLS
 100 APPEAR 5,50
 110 GOTO 40

This is a down right scroll.(mmm). Try an up left scroll.

After you have defined a scroll area and direction etc.  it can be  called
at any time.  If a CLEAR command intervenes in the program,the scroll area
will  have to be defined again,but of course a CLEAR command also  creates
other problems,and is to be avoided.

I  will  briefly mention another set of SCROLL commands.  Do  not  confuse
these commands with the previously explained scrolling operations.


 SCROLL ON - A new line is automatically inserted when the CURSOR reaches
             past the bottom of the screen.(Default Mode)

 SCROLL OFF - When the CURSOR reaches past the bottom of the screen it
              will reappear at the top.

 SCROLL UP - Moves the section of the current window above the CURSOR
             one line up.

 SCROLL DOWN - Will scroll the area below the CURSOR one line down,and
               the bottom line will be overwritten.


Whether  these  commands are of any use in your program's I leave  you  to
find  out.  One  point  to note is that if a screen is  displayed  and  it
occupies the bottom line,a call to SCROLL ON in a loop will keep the  full
screen displayed for the duration of the loop.

If you are experiencing problems with synchronisation of your screens, try
the  command  WAIT VBL in your program.  This waits for a  vertical  blank
before updating the screen and may cure problems with screens or sprites.

The  ORBIT  game on the games disk makes heavy use of sprites  and  it  is
worth studying the listing. LOAD up "ORBIT.BAS" and list the program for a
look,  controlling  the listing with the SPACEBAR.  If you have a  printer
attached,  use LLIST to get a hardcopy.  STOS makes it easy to search  for
particular  commands to examine.  Use from DIRECT MODE the command  SEARCH
"sprite"  and  STOS  will  find the  first  occurrence  of  the  parameter
specified  and present you with the line containing it on the  screen.  If
you wish to continue to the next occurrence of the same item,  use  SEARCH
by itself with no parameters. If no further occurrence exists you will get
the  report SEARCH FAILED.  Note especially that the SEARCH function  will
look  for the parameters quoted,and will differentiate  between  uppercase
and lowercase.  To clarify,  keywords entered from the EDITOR in uppercase
are converted to lowercase by STOS on entry to the program line.  Got  it!

So SEARCH "DSPRITE" will find DSPRITE but will not find  dsprite.  DSPRITE
of course can only be a variable.  This confused me for a  start.  Certain
basic keywords cannot be used as variables OR part of a variable,  notably
TO,STEP,THEN,ELSE,XOR,OR,AND,GOTO,GOSUB,MOD and AS as opposed to the other
keywords which can be used.  Are you still there?  So STEPS is not allowed
as a variable,  but DPOKE is. POKER is not allowed so it would appear that
you  can  only use the PERMITTED keywords as variables after  a  preceding
letter. Phew!!

Next time - Creating your own customised drop down or be dropped MENU bar.
How to install and get running.  Incorporating icons in the MENU bar,  and
for some added spice,design your own fully functional mouse pointer.  This
will introduce you to designing sprites with the very user friendly SPRITE
designer.  Have fun and remember I only had 512k to play with and a  half-
meg drive so don't be put off by memory or drive limitations. Have fun and
read about the variables again....

                                                (c)  Lozian MCMXC11.