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
A quiet game is playable, but it's more fun with sound. The TIA produces 2 channel sound. The channels are known as channel 0 and channel 1. There are 3 registers for each channel to control the sound produced:
It's common to have a hand full of routines that handle sound effects. For Collect, these routines can be found in the new source code file sfx.asm. To add the file to collect.asm we use the include command:
Besides the file, we also need to allocate 2 RAM variables to keep track of the sound effects:
; indexes for sound effect driver SFX_LEFT: ds 1 ; stored in $B1 SFX_RIGHT: ds 1 ; stored in $B2
The routines in sfx.asm are:
sfx.asm also includes a couple data tables that SFX_UPDATE uses to update the 6 TIA registers. Yes, only 2 tables even though there are 3 registers per channel to update. AUDCx and AUDVx both only require a nybbles worth of data, so their information is combined into a single byte to save ROM.
The tables are defined like this for Collect:
SFX_F: .byte 0, 31 ; collide .byte 0, 0, 0, 0, 1, 1, 1, 2, 2, 2, 3, 3, 3 ; collect .byte 0, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8 ; ping .byte 0, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31 ; game over SFX_CV: .byte 0,$8f ; collide sfxCOLLIDE = *-SFX_CV-1 .byte 0,$6f,$6f,$6f,$6f,$6f,$6f,$6f,$6f,$6f,$6f,$6f,$6f ; collect sfxCOLLECT = *-SFX_CV-1 .byte 0,$41,$42,$43,$44,$45,$46,$47,$48,$49,$4a,$4b,$4c,$4d,$4e,$4f ; ping sfxPING = *-SFX_CV-1 .byte 0,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf,$cf ; game over sfxGAMEOVER = *-SFX_CV-1
Each .byte line contains the data for a single sound effect. The two tables are used together, so data in the first .byte line of SFX_F goes along with the data in the first .byte line in SFX_CV. The number of values must be the same in each table and each .byte line. The first value in each .byte line should be 0, it denotes end-of-sfx (though if you have a long-duration sound effect you could span it over multiple .byte lines).
Table SFX_CV looks a little complicated because of the extra lines such as sfxPING = *-SFX_CV-1. All those are doing is calculating the value to be used when you trigger a sound effect. You can name your sound effects whatever you want, just make sure it's followed by = *-SFX_CV-1 (also make sure you have a space before and after the equal sign).
Lets look at a single sound effect (that's been slightly changed from above for clarity) to explain how the data is used:
SFX_F: .byte 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 ; collect SFX_CV: .byte 0,$64,$65,$66,$67,$68,$69,$6a,$6b,$6c,$6d,$6e,$6f ; collect sfxCOLLECT = *-SFX_CV-1
To trigger the sound effect, you'll use this in your code:
ldy #sfxCOLLECT ; select sound effect jsr SFX_TRIGGER ; and trigger it
By triggering the sound effect, the value of sfxCOLLECT will be stored in one of the RAM variables. The value of sfxCOLLECT points to the 12 in the SFX_F table and the $6f in the SFX_CF table. When SFX_UPDATE is called (via jsr SFX_UPDATE), the 12 goes into AUDFx while the $6f will be split into two parts with the $6 going into AUDCx and the $f going into AUDFx. Lastly the pointer will be updated so it now points to 11 and $6e. After the next update they'll point to 10 and $6d, and so on until they point to 0 which means the end of the sound effect.
The order of the sound effects, as listed in the tables, is used to denote priority. The first sound effect has the lowest priority, so if both channels are busy and you try to trigger sound effect sfxCOLLIDE, nothing will happen. If both are busy and you try to trigger sfxGAMEOVER, the last sound effect, then one of the current sound effects will be aborted so sfxGAMEOVER can be heard.
SFX_UPDATE is called during Overscan:
OverScan: sta WSYNC ; Wait for SYNC (halts CPU until end of scanline) lda #2 ; LoaD Accumulator with 2 so D1=1 sta VBLANK ; STore Accumulator to VBLANK, D1=1 turns image output off ; set the timer so the total number of scanlines ends up being 262 lda #35 sta TIM64T jsr SFX_UPDATE ; update sound effects
While SFX_TRIGGER is called from a number of locations throughout the program. One of them is a tick sound that plays during the last 8 ticks of the timer:
DecrementTimer: lsr TimerPF+5 ; PF2 right side, reversed bits so shift right rol TimerPF+4 ; PF1 right side, normal bits so shift left ror TimerPF+3 ; PF0 right side, reversed bits so shift right lda TimerPF+3 ; only upper nybble used, so we need to put bit 3 into C lsr lsr lsr lsr ror TimerPF+2 ; PF2 left side, reversed bits so shift right rol TimerPF+1 ; PF1 left side, normal bits so shift left ror TimerPF ; PF0 left side, reversed bits so shift right lda TimerPF+1 ; PF1 from left side and #%00011111 ; check the lower 5 bits bne NoTickSfx ; branch if there's a value in the lower 5 bits ldy #sfxPING ; else do a sound effect jsr SFX_TRIGGER NoTickSfx:
In playing the game, I noticed it was harder to locate the box drawn by the ball, because it is the same color as the playfield, so I modified the program so it scores 2 points instead of just 1.
TestCollisions: ... bit CXP0FB ; N=player0/playfield, V=player0/ball bvc notP0BL ; if V is off, then player0 did not collide with ball ldy #0 ; which score to update ldx #4 ; which box was collected jsr Collect2ptBox ; update score and reposition box ... bit CXP1FB ; N=player1/playfield, V=player1/ball bvc notP1BL ; if V is off, then player1 did not collide with ball ldy #1 ; which score to update ldx #4 ; which box was collected jsr Collect2ptBox ; update score and reposition box
Collect2ptBox is a new routine that falls into CollectBox:
Collect2ptBox: lda #2 ; 2 point box .byte $2C ; BIT with absolute addressing, trick that ; causes the lda #1 to be skipped over CollectBox: lda #1 ; 1 point per box sed ; SEt Decimal flag clc ; CLear Carry bit adc Score,y ; add to player's current score
The ROM and the source are at the bottom of my blog entry.
Other Assembly Language Tutorials
Step 13: Add Sound Effects
This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).
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).
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.
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.
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.
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.
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.
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).
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.
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.
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:
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.