Home > Atari Memories > #Assembly > Let’s Make a Game!
Let’s Make a Game!
Step 5: Automate Vertical Delay
By Darrell Spice, Jr. (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
As an Amazon Associate I earn from qualifying purchases. (I get commissions for purchases made through certain links on this page.)
Original Blog Entry
For this update, we're going to double the Y range of the player objects. To use the new Y value for the 2LK we just need to divide it in half using the LSR command. The remainder of the divide, which ends up in the Carry flag, will conveniently tell us if we need to turn on Vertical Delay.
This routine preps the 2LK data for player0 and turns on VDELP0 if required (if you're wondering, VDELP0 is turned off in VerticalSync):
; prep Humanoid's Y position for 2LK
ldx #1 ; preload X for setting VDELPx
lda ObjectY ; get the human's Y position
lsr ; divide by 2 for the 2LK position
sta Temp ; save for position calculations
bcs NoDelay0 ; if carry is set we don't need Vertical Delay
stx VDELP0 ; carry was clear, so set Vertical Delay
NoDelay0:
; HumanDraw = ARENA_HEIGHT + HUMAN_HEIGHT - Y position
lda #(ARENA_HEIGHT + HUMAN_HEIGHT)
sec
sbc Temp
sta HumanDraw
; HumanPtr = HumanGfx + HUMAN_HEIGHT - 1 - Y position
lda #<(HumanGfx + HUMAN_HEIGHT - 1)
sec
sbc Temp
sta HumanPtr
lda #>(HumanGfx + HUMAN_HEIGHT - 1)
sbc #0
sta HumanPtr+1
One minor problem with the prior 2LK was that player1 could not show up on the topmost scanline of the Arena:
Closeup:
To fix this, we'll modify the kernel to prime GRP1 before it enters the loop that draws the Arena:
ldy #ARENA_HEIGHT+1 ; 2 7 - the arena will be 180 scanlines (from 0-89)*2
; prime GRP1 so player1 can appear on topmost scanline of the Arena
lda #BOX_HEIGHT-1 ; 2 9 - height of the box graphics,
dcp BoxDraw ; 5 14 - Decrement BoxDraw and compare with height
bcs DoDrawGrp1pre ; 2 16 - (3 17) if Carry is Set, then box is on current scanline
lda #0 ; 2 18 - otherwise use 0 to turn off player1
.byte $2C ; 4 22 - $2C = BIT with absolute addressing, trick that
; causes the lda (BoxPtr),y to be skipped
DoDrawGrp1pre: ; 17 - from bcs DoDrawGRP1pre
lda (BoxPtr),y ; 5 22 - load the shape for the box
sta GRP1 ; 3 25 - @0-22, update player1 to draw box
dey ; 2 27
ArenaLoop: ; 13 - from bpl ArenaLoop
The 2LK calculations for player1 used to be the same as for player0, but now must be modified to compensate for the priming of GRP1:
; prep box's Y position for 2LK
lda ObjectY+1 ; get the box's Y position
clc
adc #1 ; add 1 to compensate for priming of GRP1
lsr ; divide by 2 for the 2LK position
sta Temp ; save for position calculations
bcs NoDelay1 ; if carry is set we don't need Vertical Delay
stx VDELP1 ; carry was clear, so set Vertical Delay
NoDelay1:
; BoxDraw = ARENA_HEIGHT + BOX_HEIGHT - Y position + 1
; the + 1 compensates for priming of GRP1
lda #(ARENA_HEIGHT + BOX_HEIGHT +1)
sec
sbc Temp
sta BoxDraw
; BoxPtr = BoxGfx + BOX_HEIGHT - 1 - Y position
lda #<(BoxGfx + BOX_HEIGHT - 1)
sec
sbc Temp
sta BoxPtr
lda #>(BoxGfx + BOX_HEIGHT - 1)
sbc #0
sta BoxPtr+1
Added GRP1 priming which allows player1 to cover full Arena:
Closeup:
Lastly, I added a new Box graphic for player1:
The ROM and the source are at the bottom of my blog entry.
Other Assembly Language Tutorials
Be sure to check out the other assembly language tutorials and the general programming pages on this web site.
#ad Amazon: Atari 2600 Programming
#ad Amazon: 6502 Assembly Language Programming
#ad Atari 2600 Programming for Newbies
|
|
|
Table of Contents for Let’s Make a Game!
Step 1: Generate a Stable Display
Step 3: Score and Timer Display
Step 5: Automate Vertical Delay
Step 8: Select and Reset Support
Step 12: Add the Missile Objects
How to get started writing 6502 assembly language. Includes a JavaScript 6502 assembler and simulator.
Atari Roots by Mark Andrews (Online Book)
This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).
Machine Language For Beginners by Richard Mansfield (Online Book)
This book only assumes a working knowledge of BASIC. It was designed to speak directly to the amateur programmer, the part-time computerist. It should help you make the transition from BASIC to machine language with relative ease.
The 6502 Instruction Set broken down into 6 groups.
Nice, simple instruction set in little boxes (not made out of ticky-tacky).
The Second Book Of Machine Language by Richard Mansfield (Online Book)
This book shows how to put together a large machine language program. All of the fundamentals were covered in Machine Language for Beginners. What remains is to put the rules to use by constructing a working program, to take the theory into the field and show how machine language is done.
An easy-to-read page from The Second Book Of Machine Language.
6502 Instruction Set with Examples
A useful page from Assembly Language Programming for the Atari Computers.
Continually strives to remain the largest and most complete source for 6502-related information in the world.
By John Pickens. Updated by Bruce Clark.
Guide to 6502 Assembly Language Programming by Andrew Jacobs
Below are direct links to the most important pages.
Goes over each of the internal registers and their use.
Gives a summary of whole instruction set.
Describes each of the 6502 memory addressing modes.
Describes the complete instruction set in detail.
HTMLified version.
Nick Bensema's Guide to Cycle Counting on the Atari 2600
Cycle counting is an important aspect of Atari 2600 programming. It makes possible the positioning of sprites, the drawing of six-digit scores, non-mirrored playfield graphics and many other cool TIA tricks that keep every game from looking like Combat.
How to Draw A Playfield by Nick Bensema
Atari 2600 programming is different from any other kind of programming in many ways. Just one of these ways is the flow of the program.
Cart Sizes and Bankswitching Methods by Kevin Horton
The "bankswitching bible." Also check out the Atari 2600 Fun Facts and Information Guide and this post about bankswitching by SeaGtGruff at AtariAge.
Atari 2600 programming specs (HTML version).
Atari 2600 Programming Page (AtariAge)
Links to useful information, tools, source code, and documentation.
Atari 2600 programming site based on Garon's "The Dig," which is now dead.
Includes interactive color charts, an NTSC/PAL color conversion tool, and Atari 2600 color compatibility tools that can help you quickly find colors that go great together.
The Atari 2600 Music and Sound Page
Adapted information and charts related to Atari 2600 music and sound.
A guide and a check list for finished carts.
A multi-platform Atari 2600 VCS emulator. It has a built-in debugger to help you with your works in progress or you can use it to study classic games.
A very good emulator that can also be embedded on your own web site so people can play the games you make online. It's much better than JStella.
If assembly language seems a little too hard, don't worry. You can always try to make Atari 2600 games the faster, easier way with batari Basic.
Did you know that Trump's rushed experimental rona jab has less than one percent overall benefit? It also has many possible horrible side effects. Some brainwashed rona jab cultists claim that there are no victims of the jab, but person after person will post what the jab did to them or a family member on web sites such as Facebook and Twitter and they'll be lucky if they don't get banned soon after. Posting the truth is “misinformation” don't you know. Awakened sheep might turn into lions, so powerful people will do just about anything to keep the sheep from waking up.
Check out these videos:
Take a look at my page called The H Word and Beyond. You might also want to look at my page called Zinc and Quercetin. My sister and I have been taking those two supplements since summer of 2020 in the hopes that they would scare away the flu and other viruses (or at least make them less severe).
Some people appear to have a mental illness because they have a vitamin B deficiency. For example, the wife of a guy I used to chat with online had severe mood swings which seemed to be caused by food allergies or intolerances. She would became irrational, obnoxious, throw tantrums, and generally act like she had a mental illness. The horrid behavior stopped after she started taking a vitamin B complex. I've been taking #ad Jarrow B-Right for many years. It makes me much easier to live with.
Unfermented soy is bad! “When she stopped eating soy, the mental problems went away.” Fermented soy doesn't bother me, but the various versions of unfermented soy (soy flour, soybean oil, and so on) that are used in all kinds of products these days causes a negative mental health reaction in me that a vitamin B complex can't tame. The sinister encroachment of soy has made the careful reading of ingredients a necessity.
If you are overweight, have type II diabetes, or are worried about the condition of your heart, check out the videos by Ken D Berry, William Davis, and Ivor Cummins. It seems that most people should avoid wheat, not just those who have a wheat allergy or celiac disease. Check out these books: #ad Undoctored, #ad Wheat Belly, and #ad Eat Rich, Live Long.
Negative ions are good for us. You might want to avoid positive ion generators and ozone generators. Whenever I need a new air cleaner (with negative ion generator), I buy it from surroundair.com. A plain old air cleaner is better than nothing, but one that produces negative ions makes the air in a room fresher and easier for me to breathe. It also helps to brighten my mood.
Never litter. Toss it in the trash or take it home. Do not throw it on the ground. Also remember that good people clean up after themselves at home, out in public, at a campsite and so on. Leave it better than you found it.
Climate Change Cash Grab = Bad
Seems like more people than ever finally care about water, land, and air pollution, but the climate change cash grab scam is designed to put more of your money into the bank accounts of greedy politicians. Those power-hungry schemers try to trick us with bad data and lies about overpopulation while pretending to be caring do-gooders. Trying to eliminate pollution is a good thing, but the carbon footprint of the average law-abiding human right now is actually making the planet greener instead of killing it.
Watch these two YouTube videos for more information:
Disclaimer
View this page and any external web sites at your own risk. I am not responsible for any possible spiritual, emotional, physical, financial or any other damage to you, your friends, family, ancestors, or descendants in the past, present, or future, living or dead, in this dimension or any other.
Use any example programs at your own risk. I am not responsible if they blow up your computer or melt your Atari 2600. Use assembly language at your own risk. I am not responsible if assembly language makes you cry or gives you brain damage.