Thanks to Matt Jernigan for compiling and contributing this page.
(Adapted by Duane Alan Hahn, a.k.a. Random Terrain.)
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Page Table of Contents
For those looking for the missing part 2, here is Matt Jernigan's interpretation of what it was set to cover (more or less).
This section is taken from the Stella Programmer's Guide by Steve Wright 12/03/79.
|20||HMP0||1||1||1||1||.||.||.||.||Horizontal Motion Player 0|
|21||HMP1||1||1||1||1||.||.||.||.||Horizontal Motion Player 1|
|22||HMM0||1||1||1||1||.||.||.||.||Horizontal Motion Missile 0|
|23||HMM1||1||1||1||1||.||.||.||.||Horizontal Motion Missile 1|
|24||HMBL||1||1||1||1||.||.||.||.||Horizontal Motion Ball|
Horizontal motion allows the programmer to move any of the 5 graphics objects relative to their current horizontal position. Each object has a 4 bit horizontal motion register (HMP0, HMP1, HMM0, HMM1, HMBL) that can be loaded with a value in the range of +7 to -8 (negative values are expressed in two's complement from). This motion is not executed until the HMOVE register is written to, at which time all motion registers move their respective objects. Objects can be moved repeatedly by simply executing HMOVE. Any object that is not to move must have a 0 in its motion register. With the horizontal positioning command confined to positioning objects at 15 color clock intervals, the motion registers fills in the gaps by moving objects +7 to -8 color clocks. Objects can not be placed at any color clock position across the screen. All 5 motion registers can be set to zero simultaneously by writing to the horizontal motion clear register (HMCLR).
There are timing constraints for the HMOVE command. The HMOVE command must immediately follow a WSYNC (Wait for SYNC) to insure the HMOVE operation occurs during horizontal blanking. This is to allow sufficient time for the motion registers to do their thing before the electron beam starts drawing the next scan line. Also, for mysterious internal hardware considerations, the motion registers should not be modified for at least 24 machine cycles after an HMOVE command.
These addresses write data (horizontal motion values) into the horizontal motion registers. These registers will cause horizontal motion only when commanded to do so by the horiz. move command HMOVE. The motion values are coded as shown below:
|0||1||1||1||+7||Move left indicated number of clocks|
|1||1||1||1||-1||Move right indicated number of clocks|
Warning: These motion registers should not be modified during the 24 computer cycles immediately following an HMOVE command. Unpredictable motion values may result.
Not all objects position exactly the same. Double-wide and quad-wide player sprites shift +1 color clock. Missiles and the ball shift -1 color clock.
For example, to compensate for this, the quad-wide bridge sprite in River Raid is curiously set to an x-position of 63 (instead of 64) before its fine-positioning subroutine is called. This quirk will shift the bridge +1 color clock and draw it at the intended x-position of 64.
This quirk is discussed in more detail here:
INX ; 2 x = 0! (player jet) ... LDY playerX ; 3 y = player's x-pos ... TYA ; 2 a = y JSR SetPosX ; 6 x-position player jet INX ; 2 x = 1 (enemy object) ... INX ; 2 x = 2 (player missile) LDA missileX ; 3 a = missile's x-pos JSR SetPosX ; 6 x-position missile JSR DoHMove ; 6 do HMOVE ... SetPosX SUBROUTINE ; calculates the values and positions objects: ; INPUT: ; A = x-position ; X = object to position (0=P0, 1=P1, 2=M0, 3=M1, 4=BL) JSR CalcPosX ; 6 $FAEF SetPosX2: STA HMP0,X ; 4 fine position the object specified in X INY ; 2 get past the 68 pixels of horizontal blank INY ; 2 INY ; 2 STA WSYNC ; 3 wait for sync; wait for TIA to finish line .waitPos: DEY ; 2 BPL .waitPos ; 2/3 5-cycle loop = 15 TIA color clocks (pixels) STA RESP0,X ; 4 RTS ; 6 ... CalcPosX SUBROUTINE ; calculates values for fine x-positioning: ; ; Basically, divides pos by 15 and stores the int result of that. ; Then, the mod of that is adjusted to equal 6 + HMP1. ; HMPx bits 4..7: Offset value: ; 0000 ($00): No offset ; 0001 ($10): Left 1 clock ; 0010 ($20): Left 2 clocks ; 0011 ($30): Left 3 clocks ; 0100 ($40): Left 4 clocks ; 0101 ($50): Left 5 clocks ; 0110 ($60): Left 6 clocks ; 0111 ($70): Left 7 clocks ; 1000 ($80): Right 8 clocks ; 1001 ($90): Right 7 clocks ; 1010 ($A0): Right 6 clocks ; 1011 ($B0): Right 5 clocks ; 1100 ($C0): Right 4 clocks ; 1101 ($D0): Right 3 clocks ; 1110 ($E0): Right 2 clocks ; 1111 ($F0): Right 1 clock ; ; INPUT: ; A = x-position ; RETURN: ; Y = coarse value for delay loop (shifted to lower 4 bits), 1 loop = 5 clocks = 15 pixels ; A = fine value for HMPx, HMMx, or HMBL TAY ; 2 $FDD8 Y = A INY ; 2 Y++ (setup for div 15; causes 15 to overflow) TYA ; 2 A = Y AND #$0F ; 2 A chopped to lower 4 bits 00001111 (fine pos value + 1) STA temp2 ; 3 $ED temp2 = A (fine pos value + 1) TYA ; 2 A = Y LSR ; 2 A chopped to upper 4 bits and shifted to lower 4 bits (A div 16) LSR ; 2 LSR ; 2 LSR ; 2 TAY ; 2 Y = A, backup shifted bits to Y CLC ; 2 CF = 0 ADC temp2 ; 3 A = A + temp2 (shifted bits added to change this from div 16 to div 15) CMP #15 ; 2 is A < 15? (look for div 15 overflow) BCC .skipIny ; 2 yes, skip to EOR (no div 15 overflow) SBC #15 ; 2 no, A = A - 15 (CF = 1 but not needed here) INY ; 2 Y++ (yes on the div 15 overflow so one-up Y) .skipIny: EOR #$07 ; 2 A = A XOR 00000111, 7's complement, sets offset for 6 + HMPx ; NOTE: Any JSR here will use 20 cycles instead of 8 ; but saves 1 byte of code overall (3 bytes instead of 4). Mult16: ASL ; 2 A's lower 4 bits shifted to upper 4 ASL ; 2 ASL ; 2 ASL ; 2 Wait12: RTS ; 6 ... DoHMove: STA WSYNC ; 3 wait for sync; wait for TIA to finish line STA HMOVE ; 3 must follow WSYNC if horizontal motion desired RTS ; 6
Hopefully, after this, Session 24 will make more sense, which discusses other algorithms for horizontal positioning.
Other Assembly Language Tutorials
Session 22: Sprites, Horizontal Positioning (Part 2)
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.
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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.