Playfield Variables vs Playfield (and data table)

[KevKelley]

I was trying something out and didn't quite get the results I was thinking would happen so I wanted to get a better understanding as to why.

 

My program's space is limited.  I am trying to finish things up but I have the following space remaining in each of the banks:

 

      1 bytes of ROM space left in bank 1
      41 bytes of ROM space left in bank 2
      6 bytes of ROM space left in bank 3
      6 bytes of ROM space left in bank 4

 

I had thought that instead of using the drawing the entire playfield like this:

 playfield:
 X....X....X....X....X....XXX...X
 X....X....X....X....X....XXX...X
 ................................
 ................................
 .........XX.XXX.XXX.XX..XXX.XXX.
 ..XXXXXXX...X...X.X.X.X..X..X...
 .XX.X.X.X...X...XXX.XX...X...XX.
 .X.X.X.XX...XXX.X.X.X.X..X..XXX.
 ..XXXXXX........................
 ...XXXX..XXXXXXXXXXXXXXXXXXXXXX.
 ..XX..XX........................
end

I could only use certain playfield variables and rewrite a couple of lines and save some space by doing this:

 var16=%00000000 :var17=%01110110 :var18=%11101100 :var19=%01110111 
 var20=%00111111 :var21=%00010001 :var22=%10101010 :var23=%00010010 
 var24=%01101010 :var25=%00010001 :var26=%11101100 :var27=%01100010 
 var28=%01010101 :var29=%01110001 :var30=%10101010 :var31=%01110010 
 var32=%00111111 :var33=%00000000 :var34=%00000000 :var35=%00000000 
 var36=%00011110 :var37=%11111110 :var38=%11111111 :var39=%01111111 
 var40=%00110011 :var41=%00000000 :var42=%00000000 :var43=%00000000 

I had used playfield variables in my game when I needed to change just a small portion of the screen like maybe a couple pixels so I thought this could be an easy way to save space but when I tried to compile I got an error message that listed the remaining bytes as such:

 

segment:  3fd4          eqm       vs current org: 3ffd
      1 bytes of ROM space left in bank 1
      41 bytes of ROM space left in bank 2
      -41 bytes of ROM space left in bank 3

 

The code I was changing was in bank 3 but my question is why would drawing 7 lines of the playfield using playfield variable use more space then drawing the entire thing with "x"s and "."s?

 

Also, when I tried instead using a data table instead I saved space but I couldn't figure out how to read the data, and if that would be a better solution? 

[+Random Terrain]

Have you tried sticking the test code here:

 

https://www.randomterrain.com/atari-2600-memories-batari-basic-commands.html#memory

[KevKelley]

12 minutes ago, Random Terrain said:

Have you tried sticking the test code here:

 

https://www.randomterrain.com/atari-2600-memories-batari-basic-commands.html#memory

Thanks.  I never noticed this before on your page so I just tried it and the results are the same:

 

 Routine 1:  57   (playfield)
 Routine 2:  144  (playfield variables)
 Routine 3:  47    (data table)

 

I had thought that if I just used a couple playfield variables I would have used much less.  The results were surprising. Basically, in my code I had the screens change but I was thinking that since half of the screen stays the same I could save space by just redrawing half the screen so I was wondering about the workings of the code to understand the differences.

[+Karl G]

Normally bB will read playfield data into RAM using a loop, so relatively little code needed to do so. In your case, you have less data, but more space is taken up by your code to populate it into RAM. 

[+Karl G]

Also, try something like this to see if it helps (untested):

 

    for temp5 = 0 to 27
        var17[temp5] = my_screen[temp5]
    next

    data my_screen
    %00000000, %01110110, %11101100, %01110111
    %00111111, %00010001, %10101010, %00010010 
    %01101010, %00010001, %11101100, %01100010 
    %01010101, %01110001, %10101010, %01110010 
    %00111111, %00000000, %00000000, %00000000 
    %00011110, %11111110, %11111111, %01111111 
    %00110011, %00000000, %00000000, %00000000
end

 

Edit: Also, don't forget that the bit order is reversed for the odd-numbered variables, assuming you haven't already taken that into account in your data.

[+Karl G]

Also - I believe bB sticks playfield data in the last bank regardless of where you define it. If you instead populate from a regular data table like in my example, it would take up space wherever you define it instead. 

[KevKelley]

I had thought about setting a temp variable and doing it that way but I don't think I have any left. Thanks for the answer. It is interesting to hear the going on under the hood. I had a similar question last year as I was working on Crossdock but that was when I switch from using pfpixel to the playfield variables so I just assumed the difference would be great. 

 

Thank you for your response but I will definitely play around with this in the future. I was looking for creative ways to try and save space in my program (or future programs).