Exploring Atari VCS/2600 Adventure – part 4

See also: part 1, part 2, part 3, part 4 … and more to come…

Objectively speaking

Welcome back to the world of Atari Adventure! After spending some time figuring out how the rooms were drawn, it’s time to look at how the game objects are drawn.

Adventure contains many objects that can be displayed:

  1. Bat
  2. Bridge
  3. Castle gate (or Portcullis – “a strong, heavy grating sliding up and down in vertical grooves, lowered to block a gateway to a fortress or town.”)
  4. Chalice
  5. Created by Warren Robinett (author’s name)
  6. Dot (for getting in to the easter egg room)
  7. Dragon (red, green and yellow)
  8. Key
  9. Magnet
  10. Number “1” (for game select screen)
  11. Number “2”
  12. Number “3”
  13. Sword

Some objects have multiple frames. For instance, the bat has two: wings up, and wings down. The dragon has three: open mouth, closed mouth, dead. The dragons can also be drawn facing left, or facing right.

I also found an entry for something called “Surround,” which appears to be the the square around the player in the invisible mazes.

In the ROM disassembly, it looks like these objects are just stored as bytes that represent them:

GfxChallise:
  .byte $81 ;X      X                                                                  
  .byte $81 ;X      X                                                                  
  .byte $C3 ;XX    XX                                                                  
  .byte $7E ; XXXXXX                                                                   
  .byte $7E ; XXXXXX
  .byte $3C ;  XXXX
  .byte $18 ;   XX
  .byte $18 ;   XX
  .byte $7E ; XXXXXX
  .byte $00 

Above, the game-winning chalice appears to be 8×9.

The dragon is much larger:

GfxDrag0:
  .byte $06 ;     XX 
  .byte $0F ;    XXXX 
  .byte $F3 ;XXXX  XX 
  .byte $FE ;XXXXXXX 
  .byte $0E ;    XXX 
  .byte $04 ;     X
  .byte $04 ;     X
  .byte $1E ;   XXXX
  .byte $3F ;  XXXXXX
  .byte $7F ; XXXXXXX
  .byte $E3 ;XXX   XX
  .byte $C3 ;XX    XX
  .byte $C3 ;XX    XX
  .byte $C7 ;XX   XXX
  .byte $FF ;XXXXXXXX
  .byte $3C ;  XXXX
  .byte $08 ;    X
  .byte $8F ;X   XXXX
  .byte $E1 ;XXX    X
  .byte $3F ;  XXXXXX
  .byte $00

But it is still represented as 8 pixels wide. The code to display it must magnify it to make it larger on screen.

Even the bridge, the widest object displayed in the game, is represented as 8-bits wide. This is the first thing we will need to dig in to… What controls how large these small object representations are drawn?

Also, it appears every object is terminated with a $00 rather than having the length at the start. For example, instead of this:

10 READ N:FOR I=1 TO N:READ A$:PRINT A$:NEXT:END
20 DATA 10,a,b,c,d,e,f,g,h,i,j

…it works like this:

10 READ A$:IF A$="0" THEN END ELSE PRINT A$:GOTO 10:END
20 DATA a,b,c,d,e,f,g,h,i,0

Since both would have taken the same amount of data storage space in the ROM, I am betting the code to parse that data may have been smaller to loop and check for 0 versus loading a size and counting down.

Also, this presents a restriction for the graphics — none can contain an “empty” row in the graphic ($00). Because of this, each line (byte) must have at least one pixel set. This explains the dots in the easter egg signature!

;Object #4 : State FF : Graphic
GfxAuthor:
 .byte $F0    ;XXXX
 .byte $80    ;X
 .byte $80    ;X
 .byte $80    ;X
 .byte $F4    ;XXXX X
 .byte $04    ;     X
 .byte $87    ;X    XXX
 .byte $E5    ;XXX  X X
 .byte $87    ;X    XXX
 .byte $80    ;X
 .byte $05    ;     X X
 .byte $E5    ;XXX  X X
 .byte $A7    ;X X  XXX
 .byte $E1    ;XXX    X
 .byte $87    ;X    XXX
 .byte $E0    ;XXX
 .byte $01    ;       X
 .byte $E0    ;XXX
 .byte $A0    ;X X
 .byte $F0    ;XXXX
 .byte $01    ;       X
 .byte $40    ; X
 .byte $E0    ;XXX
 .byte $40    ; X
 .byte $40    ; X
 .byte $40    ; X
 .byte $01    ;       X
 .byte $E0    ;XXX
 .byte $A0    ;X X
 .byte $E0    ;XXX
 .byte $80    ;X
 .byte $E0    ;XXX
 .byte $01    ;       X
 .byte $20    ;  X
 .byte $20    ;  X
 .byte $E0    ;XXX
 .byte $A0    ;X X
 .byte $E0    ;XXX
 .byte $01    ;       X
 .byte $01    ;       X
 .byte $01    ;       X
 .byte $88    ;   X   X
 .byte $A8    ;X X X
 .byte $A8    ;X X X
 .byte $A8    ;X X X
 .byte $F8    ;XXXXX
 .byte $01    ;       X
 .byte $E0    ;XXX
 .byte $A0    ;X X
 .byte $F0    ;XXXX
 .byte $01    ;       X
 .byte $80    ;X
 .byte $E0    ;XXX
 .byte $8F    ;X   XXXX
 .byte $89    ;X   X  X
 .byte $0F    ;    XXXX
 .byte $8A    ;X   X X
 .byte $E9    ;XXX X  X
 .byte $80    ;X
 .byte $8E    ;X   XXX
 .byte $0A    ;    X X
 .byte $EE    ;XXX XXX
 .byte $A0    ;X X
 .byte $E8    ;XXX X
 .byte $88    ;X   X
 .byte $EE    ;XXX XXX
 .byte $0A    ;    X X
 .byte $8E    ;X   XXX
 .byte $E0    ;XXX
 .byte $A4    ;X X  X
 .byte $A4    ;X X  X
 .byte $04    ;     X
 .byte $80    ;X
 .byte $08    ;    X
 .byte $0E    ;    XXX
 .byte $0A    ;    X X
 .byte $0A    ;    X X
 .byte $80    ;X
 .byte $0E    ;    XXX
 .byte $0A    ;    X X
 .byte $0E    ;    XXX
 .byte $08    ;    X
 .byte $0E    ;    XXX
 .byte $80    ;X
 .byte $04    ;     X
 .byte $0E    ;    XXX
 .byte $04    ;     X
 .byte $04    ;     X
 .byte $04    ;     X
 .byte $80    ;X
 .byte $04    ;     X
 .byte $0E    ;    XXX
 .byte $04    ;     X
 .byte $04    ;     X
 .byte $04    ;     X
 .byte $00

Those rows would have been empty, but having a $00 there would have prevented there rest from being drawn. Interesting!

Draw something

For today’s installment, I’ll be using Color BASIC on a CoCo emulator to write a simple program to plot these objects on the screen. I copied all of the assembly definitions into an editor then did a search and replace to turn those .byte instructions into incredibly inefficient one-number-per-line DATA statements like this:

2830 ' Object #10 : State FF : Graphic  
2840 ' GfxChallise:
2850 DATA &H81 ' X      X   
2860 DATA &H81 ' X      X   
2870 DATA &HC3 ' XX    XX   
2880 DATA &H7E '  XXXXXX    
2890 DATA &H7E '  XXXXXX   
2900 DATA &H3C '   XXXX     
2910 DATA &H18 '    XX      
2920 DATA &H18 '    XX      
2930 DATA &H7E '  XXXXXX    
2940 DATA &H00

Unfortunately, READ/DATA in Color BASIC does not allow that. It will READ the first value, then give an ?SN ERROR when it tries to read the next because the parser doesn’t handle the apostrophe “REM” marker!

So I changed it to this:

2830 ' Object #10 : State FF : Graphic  
2840 ' GfxChallise:
2850 DATA &H81:REM X      X   
2860 DATA &H81:REM X      X   
2870 DATA &HC3:REM XX    XX   
2880 DATA &H7E:REM  XXXXXX    
2890 DATA &H7E:REM  XXXXXX   
2900 DATA &H3C:REM   XXXX     
2910 DATA &H18:REM    XX      
2920 DATA &H18:REM    XX      
2930 DATA &H7E:REM  XXXXXX    
2940 DATA &H00

This also fails — and is a problem I only recently discovered when writing my Base-64 articles.

I guess I can’t have this code be as bulky and inefficient as I wanted. I removed them so it was just the DATA statements:

2830 ' Object #10 : State FF : Graphic  
2840 ' GfxChallise:
2850 DATA &H81
2860 DATA &H81
2870 DATA &HC3
2880 DATA &H7E
2890 DATA &H7E
2900 DATA &H3C
2910 DATA &H18
2920 DATA &H18
2930 DATA &H7E
2940 DATA &H00

Oh well. Who needs comments anyway — especially in BASIC where they just bloat the code and slow it down.

A quick-and-dirty routine allowed me to see it was working:

Atari Adventure graphics rendered on a CoCo :)

…but there was no way I was going to be able to fit the “Created by Warren Robinett” graphic on a low-res 64×32 screen. To solve this, I hacked together a version that used a “high resolution” screen:

Atari Adventure graphics rendered in “high resolution” on a CoCo :)

It looks like the data is pretty straight forward. The dragons would have to be drawn reversed to make them face right, and the bridge looks like it needs an X scaling factor added to make it as wide as it is in the game. But, not bad for a first attempt.

Just for fun, here is the brute-force hacky nasty Color BASIC code I quickly put together to do this:

10 REM AdvObjs.bas
20 POKE 65395,0
30 FOR A=0 TO 7:BT(A)=2^A:NEXT
40 CLS0:XS=1:YS=1:Y=1
50 PMODE 1,1:PCLS:SCREEN 1,0
60 READ V:IF V=-1 THEN 120 ELSE IF V=0 THEN 90
70 X=XS:FOR A=7 TO 0 STEP-1:IF V AND BT(A) THEN PSET(X*2,Y*2,C)
80 X=X+1:NEXT:Y=Y+1:GOTO 60
90 XS=XS+9:IF XS>120 THEN XS=1:YS=YS+32
100 C=C+1:IF C=1 THEN C=2 ELSE IF C>3 THEN C=0
110 Y=YS:GOTO 60
120 GOTO 120
130 ' Object #0A : State FF : Graphic  
140 ' GfxBridge:
150 DATA &HC3
160 DATA &HC3
170 DATA &HC3
180 DATA &HC3
190 DATA &H42
200 DATA &H42
210 DATA &H42
220 DATA &H42
230 DATA &H42
240 DATA &H42
250 DATA &H42
260 DATA &H42
270 DATA &H42
280 DATA &H42
290 DATA &H42
300 DATA &H42
310 DATA &H42
320 DATA &H42
330 DATA &H42
340 DATA &H42
350 DATA &HC3
360 DATA &HC3
370 DATA &HC3
380 DATA &HC3
390 DATA &H00
400 ' Object #5 State #1 Graphic :'1'  
410 ' GfxNum1:
420 DATA &H04
430 DATA &H0C
440 DATA &H04
450 DATA &H04
460 DATA &H04
470 DATA &H04
480 DATA &H0E
490 DATA &H00
500 ' Object #0B : State FF : Graphic  
510 ' GfxKey:
520 DATA &H07
530 DATA &HFD
540 DATA &HA7
550 DATA &H00
560 ' Object #5 State #2 Grphic :
570 ' GfxNum2:
580 DATA &H0E
590 DATA &H11
600 DATA &H01
610 DATA &H02
620 DATA &H04
630 DATA &H08
640 DATA &H1F
650 DATA &H00
660 ' Object #5 State #3 Graphic :'3'  
670 ' GfxNum3:
680 DATA &H0E
690 DATA &H11
700 DATA &H01
710 DATA &H06
720 DATA &H01
730 DATA &H11
740 DATA &H0E
750 DATA &H00
760 ' Object #0E : State 03 : Graphic  
770 ' GfxBat1:
780 DATA &H81
790 DATA &H81
800 DATA &HC3
810 DATA &HC3
820 DATA &HFF
830 DATA &H5A
840 DATA &H66
850 DATA &H00
860 ' Object #0E : State FF : Graphic  
870 ' GfxBat2:
880 DATA &H01
890 DATA &H80
900 DATA &H01
910 DATA &H80
920 DATA &H3C
930 DATA &H5A
940 DATA &H66
950 DATA &HC3
960 DATA &H81
970 DATA &H81
980 DATA &H81
990 DATA &H00
1000 ' Object #6 : State #00 : Graphic  
1010 ' GfxDrag0:
1020 DATA &H06
1030 DATA &H0F
1040 DATA &HF3
1050 DATA &HFE
1060 DATA &H0E
1070 DATA &H04
1080 DATA &H04
1090 DATA &H1E
1100 DATA &H3F
1110 DATA &H7F
1120 DATA &HE3
1130 DATA &HC3
1140 DATA &HC3
1150 DATA &HC7
1160 DATA &HFF
1170 DATA &H3C
1180 DATA &H08
1190 DATA &H8F
1200 DATA &HE1
1210 DATA &H3F
1220 DATA &H00
1230 ' Object 6 : State FF : Graphic    
1240 ' GfxDrag1:
1250 DATA &H80
1260 DATA &H40
1270 DATA &H26
1280 DATA &H1F
1290 DATA &H0B
1300 DATA &H0E
1310 DATA &H1E
1320 DATA &H24
1330 DATA &H44
1340 DATA &H8E
1350 DATA &H1E
1360 DATA &H3F
1370 DATA &H7F
1380 DATA &H7F
1390 DATA &H7F
1400 DATA &H7F
1410 DATA &H3E
1420 DATA &H1C
1430 DATA &H08
1440 DATA &HF8
1450 DATA &H80
1460 DATA &HE0
1470 DATA &H00
1480 ' Object 6 : State 02 : Graphic    
1490 ' GfxDrag2:
1500 DATA &H0C
1510 DATA &H0C
1520 DATA &H0C
1530 DATA &H0E
1540 DATA &H1B
1550 DATA &H7F
1560 DATA &HCE
1570 DATA &H80
1580 DATA &HFC
1590 DATA &HFE
1600 DATA &HFE
1610 DATA &H7E
1620 DATA &H78
1630 DATA &H20
1640 DATA &H6E
1650 DATA &H42
1660 DATA &H7E
1670 DATA &H00
1680 ' Object #9 : State FF : Graphics  
1690 ' GfxSword:
1700 DATA &H20
1710 DATA &H40
1720 DATA &HFF
1730 DATA &H40
1740 DATA &H20
1750 DATA &H00
1760 ' Object #0F : State FF : Graphic  
1770 ' GfxDot:
1780 DATA &H80
1790 DATA &H00
1800 ' Object #4 : State FF : Graphic   
1810 ' GfxAuthor:
1820 DATA &HF0
1830 DATA &H80
1840 DATA &H80
1850 DATA &H80
1860 DATA &HF4
1870 DATA &H04
1880 DATA &H87
1890 DATA &HE5
1900 DATA &H87
1910 DATA &H80
1920 DATA &H05
1930 DATA &HE5
1940 DATA &HA7
1950 DATA &HE1
1960 DATA &H87
1970 DATA &HE0
1980 DATA &H01
1990 DATA &HE0
2000 DATA &HA0
2010 DATA &HF0
2020 DATA &H01
2030 DATA &H40
2040 DATA &HE0
2050 DATA &H40
2060 DATA &H40
2070 DATA &H40
2080 DATA &H01
2090 DATA &HE0
2100 DATA &HA0
2110 DATA &HE0
2120 DATA &H80
2130 DATA &HE0
2140 DATA &H01
2150 DATA &H20
2160 DATA &H20
2170 DATA &HE0
2180 DATA &HA0
2190 DATA &HE0
2200 DATA &H01
2210 DATA &H01
2220 DATA &H01
2230 DATA &H88
2240 DATA &HA8
2250 DATA &HA8
2260 DATA &HA8
2270 DATA &HF8
2280 DATA &H01
2290 DATA &HE0
2300 DATA &HA0
2310 DATA &HF0
2320 DATA &H01
2330 DATA &H80
2340 DATA &HE0
2350 DATA &H8F
2360 DATA &H89
2370 DATA &H0F
2380 DATA &H8A
2390 DATA &HE9
2400 DATA &H80
2410 DATA &H8E
2420 DATA &H0A
2430 DATA &HEE
2440 DATA &HA0
2450 DATA &HE8
2460 DATA &H88
2470 DATA &HEE
2480 DATA &H0A
2490 DATA &H8E
2500 DATA &HE0
2510 DATA &HA4
2520 DATA &HA4
2530 DATA &H04
2540 DATA &H80
2550 DATA &H08
2560 DATA &H0E
2570 DATA &H0A
2580 DATA &H0A
2590 DATA &H80
2600 DATA &H0E
2610 DATA &H0A
2620 DATA &H0E
2630 DATA &H08
2640 DATA &H0E
2650 DATA &H80
2660 DATA &H04
2670 DATA &H0E
2680 DATA &H04
2690 DATA &H04
2700 DATA &H04
2710 DATA &H80
2720 DATA &H04
2730 DATA &H0E
2740 DATA &H04
2750 DATA &H04
2760 DATA &H04
2770 DATA &H00
2780 ' Object #10 : State FF : Graphic  
2790 ' GfxChallise:
2800 DATA &H81
2810 DATA &H81
2820 DATA &HC3
2830 DATA &H7E
2840 DATA &H7E
2850 DATA &H3C
2860 DATA &H18
2870 DATA &H18
2880 DATA &H7E
2890 DATA &H00
2900 ' Object #11 : State FF : Graphic  
2910 ' GfxMagnet:
2920 DATA &H3C
2930 DATA &H7E
2940 DATA &HE7
2950 DATA &HC3
2960 DATA &HC3
2970 DATA &HC3
2980 DATA &HC3
2990 DATA &HC3
3000 DATA &H00
3010 ' Object #1 States 940FF (Graphic)
3020 ' GfxPort01:
3030 DATA &HFE
3040 DATA &HAA
3050 'GfxPort02:
3060 DATA &HFE
3070 DATA &HAA
3080 'GfxPort03:
3090 DATA &HFE
3100 DATA &HAA
3110 'GfxPort04:
3120 DATA &HFE
3130 DATA &HAA
3140 'GfxPort05:
3150 DATA &HFE
3160 DATA &HAA
3170 'GfxPort06:
3180 DATA &HFE
3190 DATA &HAA
3200 'GfxPort07:
3210 DATA &HFE
3220 DATA &HAA
3230 'GfxPort08:
3240 DATA &HFE
3250 DATA &HAA
3260 'GfxPort09:
3270 DATA &H00
3280 DATA -1

It ain’t pretty, but it’s mine!

I suppose next I should get back to looking at how the rooms are connected. And there’s some interesting stuff in here, including rooms that “change” as the game is played.

Stay tuned…

2 thoughts on “Exploring Atari VCS/2600 Adventure – part 4

  1. MiaM

    Sorry if I’m not paying attention, but which disassembly are you using?

    It would be interesting to see the code that displays the graphics.

    I assume that the 0 termination is to make the code simpler. The 6502 only has three registers and they have different meaning (one is the accumulator, two is index registers that are used with different index modes. The index registers can also be used for temp storage and as counters (plus/minus one in one one byte two cycle instruction).

    Reply
    1. Allen Huffman Post author

      It was included in a 2006 zip (I think) from the guy who ported it to C back then. I don’t find it in his current source file, though, so I’m not sure where it first came from. I’ll see if I can find a pointer. I do not know 6502.

      Reply

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