NES Drop-Rate Programming
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ROM AddressOpcodeCommandTranslation
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$8973:A9A9 01LDA #$01Load the accumaltor with a value of $01 (a=1)
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$8974:01
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$8975:A6A6 44LDX $0044 = #$04 Load value found at address $0044 to the X register. (In this case, the game is on Level 4, and $0044 is the RAM address of the current level, so X=$04)
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$8976:44
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$8977:E0E0 1DCPX #$1D Compare X to $1D. (fyi...$1D=29)
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$8978:1D
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$8979:B0B0 03BCS $897E If X is equal to $1D, skip ahead to address $897E. (If you are on Level 29, jump ahead, but in this case, you are on Level 04.)
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$897A:03
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$897B:BDBD 8E 89LDA $898E,X @ $8992 = #$1CLoad value of $8992 to accumulator, accumulator now equals $1C (a=28)
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$897C:8E*From above, note that X = $04, so go to $898E and count four steps to $8992 and find that $1C is the value found there.
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$897D:89
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$897E:8585 AFSTA $00AF = #$1CStore value of accumulator at RAM address $00AF....Now $00AF=$1C (fyi...$1C=28)
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$897F:AF
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$8980:A5A5 45LDA $0045 = #$00Load value found at address $0045 to accumulator. (Note that $0045 is the "fall timer" and it increases by one unit for every frame displayed. So, three frames from now $0045 will read $03, nine frames from now it will read $09, etc)
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$8981:45
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$8982:C5C5 AFCMP $00AF = #$1CCompare the accumulator to $00AF (Basically, it is comparing $0045 to $00AF, so that when 28 frames have passed, $0045 will be equal to $00AF, and the piece will drop one unit)
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$8983:AF
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$8984:1010 D2BPL $8958I don't know about BPL, but $8958 is where the fall timer resets to zero and the piece drops one unit.
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$8985:D2
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$8986:4C4C 72 89JMP $8972Jump to address $8972 (presumably to restart the cycle all over again)
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$8987:72
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$8988:89
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$8989:E6E6 4EINC $004E = #$00I don't know about this one.
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$898A:4E
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$898B:4C4C 72 89JMP $8972Jump to address $8972 (presumably to restart the cycle all over again)
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$898C:72
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$898D:89
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$898E:30(this, of course, is where the speed definitions begin, this being Level 0)
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$898E:2B
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$8990:26
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$8991:21
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$8992:1CThis is Level 04's drop rate.
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$8993:17
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$8994:12
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$8995:0D
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$8996:08
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$8997:06
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$8998:05
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$8999:05
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$899A:05
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$899B:04
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$899C:04
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$899D:04
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$899E:03
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$899F:03
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$89A0:03
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$89A1:02
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$89A2:02
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$89A3:02
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$89A4:02
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$89A5:02
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$89A6:02
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$89A7:02
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$89A8:02
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$89A9:02
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$89AA:02
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$89AB:01
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