Attract mode code breakdown

[JCHarrist]

I know I'm not the only attract mode geek here so I decided to brush up on my assembly skills by breaking down the attract mode code.  I've always been kinda fascinated by the seemingly endless number of possible outcomes.

It turns out that it is incredibly simple, with only 28 lines of code to handle Jumpmans movements. The movements are completely hardcoded with no randomness and no barrel/fireball/ladder detection. However, the barrels and fireballs behave exactly like they do in a game, so that is where the variability comes from.

It only becomes interesting if Jumpman is able to grab the hammer and actually smash something because when the screen freezes during the smashes, the script keeps running and the variable number and timing of the smashes is what makes the many different outcomes possible.

The input sequence table starts at 21D1 and only has 15 different moves. The first byte is the input. The second byte is the duration of the FOLLOWING input.

I've expanded on Jeff Willms commenting.

Code: [Select]

21D1  80 FE ; jump
21D3  01 C0 ; run right for 254 frames
21D5  04 50 ; climb ladder for 192 frames( it only takes 54 frames to climb the short ladder so he pauses at the top for 138 frames)
21D7  02 10 ; run left for 80 frames
21D9  82 60 ; jump left for 16 frames
21DB  02 10 ; run left for 96 frames
21DD  82 CA  ; jump left for 16 frames
21DF  01 10 ; run right for 202 frames
21E1  81 FF ; jump right for 16 frames (grabs hammer)
21E3  02 38 ; run left for 255 frames
21E5  01 80 ; run right for 56 frames
21E7  02 FF ; run left for 128 frames
21E9  04 80 ; up for 255 frames
21EB  04 60 ; up 128 frames
21ED  80 ; jump for 96 frames

The code that calls the inputs follows directly after it. It's interesting to note that Jumpman occasionally makes it further than the 15 inputs in the table at which point the code starts using itself as input data. It will take some more experimentation to define those inputs although #16 and #17 seem to make Jumpman run right for at least 220 frames or so.

Code: [Select]

21EE  11D121    LD      DE,#21D1 ; load DE with #21D1 (address of start of input table data)
21F1  21CC63    LD      HL,#63CC ; load HL with #63CC (63CC holds the sequence # of the current input)
21F4  7E        LD      A,(HL) ; load A with input sequence #
21F5  07        RLCA    ; multiply A X 2
21F6  83        ADD     A,E ; add A to E (E = hex D1/ decimal 209)
21F7  5F        LD      E,A ; put A into E (this returns DE to the address of the current input )
21F8  1A        LD      A,(DE) ; load A from the address of the current input from table
21F9  321060    LD      (#6010),A ; output A to Jumpman
21FC  2C        INC     L ; increment HL by 1 to 63DC (63DC holds the number of frames remaining to execute the input)
21FD  7E        LD      A,(HL) ; load the # of frames remaining to execute into A
21FE  35        DEC     (HL) ; decrement the # of frames remaining by 1
21FF  A7        AND     A ; clear carry flag
2200  C0        RET     NZ ; return if A is not zero

2201  1C        INC     E ; if A is zero increment E by 1 (advances to next input duration)
2202  1A        LD      A,(DE) ; load A with input duration
2203  77        LD      (HL),A ; store input duration into HL (address 63DC)
2204  2D        DEC     L ; decrement HL to address 63CC
2205  34        INC     (HL) ; increment the input sequence #
2206  C9        RET      ; return


Earlier we were discussing this is in the shoutbox  and Chris P was doing some experimentation using invinciblity cheat and it seems that in order for Jumpman to make it to the third girder he has to smash exactly 3 objects AND be standing at the base of a ladder when the hammer expires.

Discuss, fellow attract mode geeks!

[marinomitch13]

Code: [Select]

21E9  04 80 ; up for 255 frames
21EB  04 60 ; up 128 frames
21ED  80 ; jump for 96 frames


So, based on this (the fact that there are no potential movements to the right after climbing to the third girder), it is impossible for Jumpman to get to the 4th girder... correct?

[JCHarrist]

Code: [Select]

21E9  04 80 ; up for 255 frames
21EB  04 60 ; up 128 frames
21ED  80 ; jump for 96 frames


So, based on this (the fact that there are no potential movements to the right after climbing to the third girder), it is impossible for Jumpman to get to the 4th girder... correct?

Well, as I stated, after the #15th input, the code "overflows" out of the table data and starts reading inputs from the program code itself. It seems that #16 and #17 make him run to the right for around 220 frames, but that's as far as I've ever seen him survive.

[corey.chambers]

I used the invinsibility cheat along with the always climb cheat. I shut off the always climb while jumpman is half way up Pauline's ladder. He will climb to the top on his own, get to the top and stop. The rom literally stalls, it will not let you play the game.

[f_symbols]

I just realized that this is the only way to actually end the game next to Pauline

[alumbrada]

I used the invinsibility cheat along with the always climb cheat. I shut off the always climb while jumpman is half way up Pauline's ladder. He will climb to the top on his own, get to the top and stop. The rom literally stalls, it will not let you play the game.

A reliable source told me you were on 1.3M pace. Great job!

[ChrisP]

Using the "Always Climb" was a great idea, Corey.

Fascinating that the game stalls. Imagine if it kept going. Somebody could painstakingly put together a kill screen...

We finally got demo-Jumpman through the attract mode! Love it!

[stella_blue]

I used the invinsibility cheat along with the always climb cheat. I shut off the always climb while jumpman is half way up Pauline's ladder. He will climb to the top on his own, get to the top and stop. The rom literally stalls, it will not let you play the game.

How do we end up with 9800 on the Bonus Timer?

[mikegmi2]

Yea 9800 bonus timer, wht da? Is that part of the invincibility cheat, it gives you extra time?

Pretty cool though.

[corey.chambers]

The bonus timer was an additional cheat, infinite bonus timer.

[LMDAVE]

Code: [Select]

21E9  04 80 ; up for 255 frames
21EB  04 60 ; up 128 frames
21ED  80 ; jump for 96 frames


So, based on this (the fact that there are no potential movements to the right after climbing to the third girder), it is impossible for Jumpman to get to the 4th girder... correct?

Well, as I stated, after the #15th input, the code "overflows" out of the table data and starts reading inputs from the program code itself. It seems that #16 and #17 make him run to the right for around 220 frames, but that's as far as I've ever seen him survive.

That is very interesting, at least now I can see there is no programmed input beyond climbing to the 3rd girder, the run to right is strictly an overflow error like you said, but that almost always gets to that point. Maybe the programmer was jsut OK with that happening. But, when he does climb to the 3rd girder, if he ever managers to jump over the barrels, and there is a clear path....he'll run to the right but not fall off, or who knows what the next thing will do after the 220 frames to the right you said.

Would be neat to run the invisibility cheat without "always climb" and wait for him to climb to the 3rd girder and see what happens after the run to the right? You'd have to wait for about 15-20 screens through but should be able to do the fast forward.

BTW, not sure how always climb worked on the attact screen, does he always run in the right direction after climbing?

[f_symbols]

Dave, I believe that Chris said he runs off the right edge of the 3rd girder; apparently most of the "invincible" attract screens end with jump man on the bottom girder, due to survival after falling off the edges...

Perhaps the always climb cheat skips the bottom hammer?  Between the 13 other coded inputs and the overflow, it must've been enough

[stella_blue]

Would be neat to run the invisibility cheat without "always climb" and wait for him to climb to the 3rd girder and see what happens after the run to the right?

How would we know that he's climbed to the 3rd girder, if we can't see him?

Just kidding, Dave.  I know what you meant.   

[LMDAVE]

Dave, I believe that Chris said he runs off the right edge of the 3rd girder; apparently most of the "invincible" attract screens end with jump man on the bottom girder, due to survival after falling off the edges...

Perhaps the always climb cheat skips the bottom hammer?  Between the 13 other coded inputs and the overflow, it must've been enough

You can't run off the edge of the 3 girder to the right.

[JCHarrist]

Now that I know what the numbers do, and since the barrels always do the same thing the first time through in MAME, it should be a simple matter to program Jumpman to make it to the top.