Trying to figure out an algorithm's purpose. (Z80 coding.)

[MKnightDH]

http://pastebin.com/JSuYJAb7

Note about my notes: 0?,C,B,17 are held in CAA8 through CAAB in order. Ignore the 0?, it's actually there to handle an outside process.

I have been trying to figure out what the algorithm is for, but so far, no luck. Any ideas?

[Pennywise]

Aside from the fact this is in the wrong category, code with no context can be almost anything. Figuring out what the variables correspond to is probably a good start.

[KingMike]

Right, any idea what's the purpose of the value in register DE and RAM $CAAB?

[MKnightDH]

As it turns out, DE is supposed to hold a number. RAM $CAAB is used to determine how many more times the thing loops.

Through a suggestion from the chat room to use a debugger, I managed to stumble upon what I did miss: while I did guess that I was missing something with the Carry Flag, I didn't guess that, as it turns out, 00:2A42's Carry Flag setting is preserved up to the next Rotate instruction. This allows the new BC to actually hold something sooner or later. A little more fiddling around with the debugger and it turns out that the whole thing turns out to be one thing: division. As it turns out, when $CAAB is finally 0, DE should have its final result being what it was divided by old BC, and BC should be holding the remainder.

Well, now this topic is unnecessary. And apologies for posting in the wrong forum. I saw "Script Help" and figured that's where I should post. Ah well.
November 09, 2011, 11:12:08 PM - (Auto Merged - Double Posts are not allowed before 7 days.)
Well, let's do an example, since somebody questioned this on PM:

DE = 100, BC = 20, HL = CAAB, CAA9 = 20, CAAA = 0

(Spoiler tags for length.)

Spoiler

00:2A38 3611    ld [hl],$11 <-- DE = 100, BC = 20, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 17
00:2A3A 010000  ld bc,$0000 <-- DE = 100, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 17
00:2A3D 21ABCA  ld hl,$CAAB <-- DE = 100, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 17
00:2A40 CB13    rl e <-- DE = 200, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 17
00:2A42 CB12    rl d <-- DE = 200, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 17
00:2A44 35      dec [hl] <-- DE = 200, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 16
00:2A45 281A    jr z,$2A61 <-- DE = 200, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 16
00:2A47 CB11    rl c <-- DE = 200, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 16
00:2A49 CB10    rl b <-- DE = 200, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 16
00:2A4B 2B      dec hl <-- DE = 200, BC = 0, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 16
00:2A4C 2B      dec hl <-- DE = 200, BC = 0, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 16
00:2A4D 79      ld a,c <-- A = 0, DE = 200, BC = 0, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 16
00:2A4E 96      sub [hl] <-- A = 236, DE = 200, BC = 0, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A4F 4F      ld c,a <-- A = 236, DE = 200, BC = 236, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A50 23      inc hl <-- A = 236, DE = 200, BC = 236, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A51 78      ld a,b <-- A = 0, DE = 200, BC = 236, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A52 9E      sbc [hl] <-- A = 255, DE = 200, BC = 236, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A53 47      ld b,a <-- A = 255, DE = 200, BC = 65516, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A54 3008    jr nc,$2A5E <-- A = 255, DE = 200, BC = 65516, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A56 2B      dec hl <-- A = 255, DE = 200, BC = 65516, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A57 79      ld a,c <-- A = 236, DE = 200, BC = 65516, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A58 86      add a,[hl] <-- A = 0, DE = 200, BC = 65516, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A59 4F      ld c,a <-- A = 0, DE = 200, BC = 65280, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A5A 23      inc hl <-- A = 0, DE = 200, BC = 65280, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A5B 78      ld a,b <-- A = 255, DE = 200, BC = 65280, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A5C 8E      adc a,[hl] <-- A = 0, DE = 200, BC = 65280, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A5D 47      ld b,a <-- A = 0, DE = 200, BC = 0, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 16 (Carry On)
00:2A5E 3F      ccf <-- A = 0, DE = 200, BC = 0, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 16
00:2A5F 18DC    jr $2A3D <-- A = 0, DE = 200, BC = 0, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 16

[close]

This didn't seem to do anything, but look again. DE has been increased. Let's keep the loop while tracking DE and CAAB:
CAAB = 17, DE = 100
CAAB = 16, DE = 200
CAAB = 15, DE = 400
CAAB = 14, DE = 800
CAAB = 13, DE = 1600
CAAB = 12, DE = 3200
CAAB = 11, DE = 6400
CAAB = 10, DE = 12800
CAAB = 9, DE = 25600
CAAB = 8, DE = 51200

Now let's do the loop again, with that last line, because double 51200 is going to exceed 65535.

Spoiler

00:2A3D 21ABCA  ld hl,$CAAB  <-- A = 0, DE = 51200, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 8
00:2A40 CB13    rl e <-- A = 0, DE = 51200, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 8
00:2A42 CB12    rl d <-- A = 0, DE = 36864, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 8 (Carry On)
00:2A44 35      dec [hl] <-- A = 0, DE = 36864, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A45 281A    jr z,$2A61 <-- A = 0, DE = 36864, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A47 CB11    rl c <-- A = 0, DE = 36864, BC = 1, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 7
00:2A49 CB10    rl b <-- A = 0, DE = 36864, BC = 1, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 7
00:2A4B 2B      dec hl <-- A = 0, DE = 36864, BC = 1, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 7
00:2A4C 2B      dec hl <-- A = 0, DE = 36864, BC = 1, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 7
00:2A4D 79      ld a,c <-- A = 1, DE = 36864, BC = 1, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 7
00:2A4E 96      sub [hl] <-- A = 237, DE = 36864, BC = 1, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A4F 4F      ld c,a <-- A = 237, DE = 36864, BC = 237, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A50 23      inc hl <-- A = 237, DE = 36864, BC = 237, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A51 78      ld a,b <-- A = 0, DE = 36864, BC = 237, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A52 9E      sbc [hl] <-- A = 255, DE = 36864, BC = 237, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A53 47      ld b,a <-- A = 255, DE = 36864, BC = 65517, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A54 3008    jr nc,$2A5E <-- A = 255, DE = 36864, BC = 65517, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A56 2B      dec hl <-- A = 255, DE = 36864, BC = 65517, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A57 79      ld a,c <-- A = 237, DE = 36864, BC = 65517, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A58 86      add a,[hl] <-- A = 1, DE = 36864, BC = 65517, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A59 4F      ld c,a <-- A = 1, DE = 36864, BC = 65281, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A5A 23      inc hl <-- A = 1, DE = 36864, BC = 65281, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A5B 78      ld a,b <-- A = 255, DE = 36864, BC = 65281, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A5C 8E      adc a,[hl] <-- A = 0, DE = 36864, BC = 65281, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A5D 47      ld b,a <-- A = 0, DE = 36864, BC = 1, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 7 (Carry On)
00:2A5E 3F      ccf <-- A = 0, DE = 36864, BC = 1, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 7
00:2A5F 18DC    jr $2A3D <-- A = 0, DE = 36864, BC = 1, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 7

[close]

Well, now BC is changed, so we may actually do something. Let's compare BC, DE, and CAAB for now.
BC = 0, DE = 51200, CAAB = 8
BC = 1, DE = 36864, CAAB = 7
BC = 3, DE = 8192, CAAB = 6
BC = 6, DE = 16384, CAAB = 5
BC = 12, DE = 32768, CAAB = 4

Oh, BC is going to exceed 20, so let's see what happens again.

Spoiler

00:2A3D 21ABCA  ld hl,$CAAB <-- A = 0, DE = 32768, BC = 12, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 4
00:2A40 CB13    rl e <-- A = 0, DE = 32768, BC = 12, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 4
00:2A42 CB12    rl d <-- A = 0, DE = 0, BC = 12, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 4 (Carry On)
00:2A44 35      dec [hl] <-- A = 0, DE = 0, BC = 12, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 3 (Carry On)
00:2A45 281A    jr z,$2A61 <-- A = 0, DE = 0, BC = 12, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 3 (Carry On)
00:2A47 CB11    rl c <-- A = 0, DE = 0, BC = 25, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A49 CB10    rl b <-- A = 0, DE = 0, BC = 25, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A4B 2B      dec hl <-- A = 0, DE = 0, BC = 25, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A4C 2B      dec hl <-- A = 0, DE = 0, BC = 25, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A4D 79      ld a,c <-- A = 25, DE = 0, BC = 25, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A4E 96      sub [hl] <-- A = 5, DE = 0, BC = 25, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A4F 4F      ld c,a <-- A = 5, DE = 0, BC = 5, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A50 23      inc hl <-- A = 5, DE = 0, BC = 5, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A51 78      ld a,b <-- A = 0, DE = 0, BC = 5, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A52 9E      sbc [hl] <-- A = 0, DE = 0, BC = 5, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A53 47      ld b,a <-- A = 0, DE = 0, BC = 5, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A54 3008    jr nc,$2A5E <-- A = 0, DE = 0, BC = 5, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A5E 3F      ccf <-- A = 0, DE = 0, BC = 5, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 3 (Carry On)
00:2A5F 18DC    jr $2A3D <-- A = 0, DE = 0, BC = 5, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 3 (Carry On)

00:2A3D 21ABCA  ld hl,$CAAB <-- A = 0, DE = 0, BC = 5, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 3 (Carry On)
00:2A40 CB13    rl e <-- A = 0, DE = 1, BC = 5, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A42 CB12    rl d <-- A = 0, DE = 1, BC = 5, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 3
00:2A44 35      dec [hl] <-- A = 0, DE = 1, BC = 5, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 2
00:2A45 281A    jr z,$2A61 <-- A = 0, DE = 1, BC = 5, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 2
00:2A47 CB11    rl c <-- A = 0, DE = 1, BC = 10, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 2
00:2A49 CB10    rl b <-- A = 0, DE = 1, BC = 10, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 2
00:2A4B 2B      dec hl <-- A = 0, DE = 1, BC = 10, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 2
00:2A4C 2B      dec hl <-- A = 0, DE = 1, BC = 10, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 2
00:2A4D 79      ld a,c <-- A = 10, DE = 1, BC = 10, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 2
00:2A4E 96      sub [hl] <-- A = 246, DE = 1, BC = 10, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A4F 4F      ld c,a <-- A = 246, DE = 1, BC = 246, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A50 23      inc hl <-- A = 246, DE = 1, BC = 246, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A51 78      ld a,b <-- A = 0, DE = 1, BC = 246, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A52 9E      sbc [hl] <-- A = 255, DE = 1, BC = 246, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A53 47      ld b,a <-- A = 255, DE = 1, BC = 65526, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A54 3008    jr nc,$2A5E <-- A = 255, DE = 1, BC = 65526, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A56 2B      dec hl <-- A = 255, DE = 1, BC = 65526, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A57 79      ld a,c <-- A = 246, DE = 1, BC = 65526, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A58 86      add a,[hl] <-- A = 10, DE = 1, BC = 65526, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A59 4F      ld c,a <-- A = 0, DE = 1, BC = 65290, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A5A 23      inc hl <-- A = 0, DE = 1, BC = 65290, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A5B 78      ld a,b <-- A = 255, DE = 1, BC = 65290, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A5C 8E      adc a,[hl] <-- A = 0, DE = 1, BC = 65290, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A5D 47      ld b,a <-- A = 0, DE = 1, BC = 10, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 2 (Carry On)
00:2A5E 3F      ccf <-- A = 0, DE = 1, BC = 10, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 2
00:2A5F 18DC    jr $2A3D <-- A = 0, DE = 1, BC = 10, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 2

00:2A3D 21ABCA  ld hl,$CAAB <-- A = 0, DE = 1, BC = 10, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 2
00:2A40 CB13    rl e <-- A = 0, DE = 2, BC = 10, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 2
00:2A42 CB12    rl d <-- A = 0, DE = 2, BC = 10, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 2
00:2A44 35      dec [hl] <-- A = 0, DE = 2, BC = 10, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A45 281A    jr z,$2A61 <-- A = 0, DE = 2, BC = 10, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A47 CB11    rl c <-- A = 0, DE = 2, BC = 20, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A49 CB10    rl b <-- A = 0, DE = 2, BC = 20, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A4B 2B      dec hl <-- A = 0, DE = 2, BC = 20, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A4C 2B      dec hl <-- A = 0, DE = 2, BC = 20, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A4D 79      ld a,c <-- A = 20, DE = 2, BC = 20, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A4E 96      sub [hl] <-- A = 0, DE = 2, BC = 20, HL = CAA9, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A4F 4F      ld c,a <-- A = 0, DE = 2, BC = 0, HL = CAA0, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A50 23      inc hl <-- A = 0, DE = 2, BC = 0, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A51 78      ld a,b <-- A = 0, DE = 2, BC = 0, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A52 9E      sbc [hl] <-- A = 0, DE = 2, BC = 0, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A53 47      ld b,a <-- A = 0, DE = 2, BC = 0, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A54 3008    jr nc,$2A5E <-- A = 0, DE = 2, BC = 0, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A5E 3F      ccf <-- A = 0, DE = 2, BC = 0, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 1 (Carry On)
00:2A5F 18DC    jr $2A3D <-- A = 0, DE = 2, BC = 0, HL = CAAA, CAA9 = 20, CAAA = 0, CAAB = 1 (Carry On)

00:2A3D 21ABCA  ld hl,$CAAB <-- A = 0, DE = 2, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 1 (Carry On)
00:2A40 CB13    rl e <-- A = 0, DE = 5, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A42 CB12    rl d <-- A = 0, DE = 5, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 1
00:2A44 35      dec [hl] <-- A = 0, DE = 5, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 0 (Zero On)
00:2A45 281A    jr z,$2A61 <-- A = 0, DE = 5, BC = 0, HL = CAAB, CAA9 = 20, CAAA = 0, CAAB = 1 (Zero On)

[close]

Well there we go. DE and BC were, as expected, 5 and 0 respectively, the rounded down answer and remainder of 100/20.

That should prove my point.