Scrolling Sky Disassembly Notes

; Landing site BG tileset lookup
org $88AD9C
    DW B180, B980, C180, C980, D180,

; west ocean   BG tileset lookup
org $88ADA6
    DW B180, B980, C180, C980, D980, E180

; Main ASM Update Screen BG2 (dynamically loads the background)
$88:AFB0 C2 30       REP #$30 ; 16bit
$88:AFB2 AD 15 09    LDA $0915  [$7E:0915] ; Screen Y position
$88:AFB5 85 B7       STA $B7    [$7E:00B7] ; $2110 BG 2 Vertical Scroll Offset
$88:AFB7 AE 30 03    LDX $0330  [$7E:0330] ; D table index

$88:AFBA A9 40 00    LDA #$0040
$88:AFBD 95 D0       STA $D0,x  [$7E:00D7] ; DMA0 size
$88:AFBF 95 D7       STA $D7,x  [$7E:00DE] ; DMA1 size
$88:AFC1 95 DE       STA $DE,x  [$7E:00E5] ; DMA2 size
$88:AFC3 95 E5       STA $E5,x  [$7E:00EC] ; DMA3 size

; Get Source for DMA0-1
$88:AFC5 AD 15 09    LDA $0915  [$7E:0915] ;Screen Y
$88:AFC8 29 F8 07    AND #$07F8 ; #$ 00000111 11111000
$88:AFCB 38          SEC
$88:AFCC E9 10 00    SBC #$0010 ; -10
$88:AFCF 48          PHA        ; PHA
$88:AFD0 29 00 FF    AND #$FF00
$88:AFD3 EB          XBA
$88:AFD4 0A          ASL A      ; LSR 7
$88:AFD5 A8          TAY        ; Y = 0 - E (even)
$88:AFD6 68          PLA        ; PLA
$88:AFD7 29 FF 00    AND #$00FF ; #$ 00000000 11111000
$88:AFDA 0A          ASL A
$88:AFDB 0A          ASL A
$88:AFDC 0A          ASL A      ; ASL 3
$88:AFDD 18          CLC
$88:AFDE 77 00       ADC [$00],y[$88:AF9A] ; ADD to ROM GFX table lookup
$88:AFE0 95 D2       STA $D2,x  [$7E:00D9] ; DMA0 Source
$88:AFE2 18          CLC
$88:AFE3 69 40 00    ADC #$0040            ; Add 40 byte offset
$88:AFE6 95 D9       STA $D9,x  [$7E:00E0] ; DMA1 Source

; Get Source for DMA2-3
$88:AFE8 AD 15 09    LDA $0915  [$7E:0915] ; Screen Y position
$88:AFEB 29 F8 07    AND #$07F8 ; #$ 00000111 11111000
$88:AFEE 18          CLC
$88:AFEF 69 F0 00    ADC #$00F0 ; +F0
$88:AFF2 48          PHA        ; PHA
$88:AFF3 29 00 FF    AND #$FF00
$88:AFF6 EB          XBA
$88:AFF7 0A          ASL A      ; LSR 7
$88:AFF8 A8          TAY        ; Y = 0 - E (even)
$88:AFF9 68          PLA        ; PLA
$88:AFFA 29 FF 00    AND #$00FF ; #$ 00000000 11111000
$88:AFFD 0A          ASL A
$88:AFFE 0A          ASL A
$88:AFFF 0A          ASL A      ; ASL 3
$88:B000 18          CLC
$88:B001 77 00       ADC [$00],y[$88:AD9C] ; ADD to ROM GFX table lookup
$88:B003 95 E0       STA $E0,x  [$7E:00E7] ; DMA2 Source
$88:B005 18          CLC
$88:B006 69 40 00    ADC #$0040            ; Add 40 byte offset
$88:B009 95 E7       STA $E7,x  [$7E:00EE] ; DMA3 Source

$88:B00B E2 20       SEP #$20               ; 8bit
$88:B00D A9 8A       LDA #$8A               ; ROM GFX bank (8A)
$88:B00F 95 D4       STA $D4,x  [$7E:00DB]  ; DMA0 Source Bank
$88:B011 95 DB       STA $DB,x  [$7E:00E2]  ; DMA1 Source Bank
$88:B013 95 E2       STA $E2,x  [$7E:00E9]  ; DMA2 Source Bank
$88:B015 95 E9       STA $E9,x  [$7E:00F0]  ; DMA3 Source Bank

$88:B017 C2 20       REP #$20               ; 16 bit
$88:B019 A5 59       LDA $59    [$7E:0059]  ; $2108 BG 2 Address and Size (aaaaaabb)
$88:B01B 29 FC 00    AND #$00FC             ; #$ 00000000 11111100
$88:B01E EB          XBA                    ; ASL 8
$88:B01F 85 12       STA $12    [$7E:0012]  ; STA 12

$88:B021 AD 15 09    LDA $0915  [$7E:0915]  ; Screen Y position
$88:B024 38          SEC
$88:B025 E9 10 00    SBC #$0010             ; -10
$88:B028 29 F8 01    AND #$01F8             ; #$ 00000001 11111000
$88:B02B 0A          ASL A
$88:B02C 0A          ASL A                  ; ASL 2
$88:B02D 18          CLC
$88:B02E 65 12       ADC $12    [$7E:0012]  ; Add to BG2 Base Address
$88:B030 95 D5       STA $D5,x  [$7E:00DC]  ; DMA0 Destination
$88:B032 18          CLC
$88:B033 69 20 00    ADC #$0020             ; Add 20
$88:B036 95 DC       STA $DC,x  [$7E:00E3]  ; DMA1 Destination

$88:B038 AD 15 09    LDA $0915  [$7E:0915]  ; Screen Y position
$88:B03B 18          CLC
$88:B03C 69 F0 00    ADC #$00F0             ; +F0
$88:B03F 29 F8 01    AND #$01F8             ; #$ 00000001 11111000
$88:B042 0A          ASL A
$88:B043 0A          ASL A                  ; ASL 2
$88:B044 18          CLC
$88:B045 65 12       ADC $12    [$7E:0012]  ; Add to BG2 Base Address
$88:B047 95 E3       STA $E3,x  [$7E:00EA]  ; DMA2 Destination
$88:B049 18          CLC
$88:B04A 69 20 00    ADC #$0020             ; Add 20
$88:B04D 95 EA       STA $EA,x  [$7E:00F1]  ; DMA3 Destination

$88:B04F 8A          TXA
$88:B050 18          CLC
$88:B051 69 1C 00    ADC #$001C
$88:B054 8D 30 03    STA $0330  [$7E:0330]  ; Update D Table head
$88:B057 6B          RTL
}




;pre-Instruction HDMA code (Handles the scrolling effect)
$88:ADC2 E2 30       SEP #$30               ; 8 bit
$88:ADC4 A9 4A       LDA #$4A
$88:ADC6 85 59       STA $59    [$7E:0059]  ; $2108 Screen address
$88:ADC8 C2 30       REP #$30               ; 16 bit
$88:ADCA A0 00 00    LDY #$0000

; Setup HDMA scroll updates
$88:ADCD B9 C7 AE    LDA $AEC7,y[$88:AEC7]  ; Load HDMA Address
$88:ADD0 AA          TAX
$88:ADD1 B9 C3 AE    LDA $AEC3,y[$88:AEC3]  ; Load Row Scroll Speed
$88:ADD4 18          CLC
$88:ADD5 7F 00 00 7E ADC $7E0000,x[$7E:9F80]; Add the scroll speed to scroll
$88:ADD9 9F 00 00 7E STA $7E0000,x[$7E:9F80]; Save new scroll
$88:ADDD B9 C5 AE    LDA $AEC5,y[$88:AEC5]  ; Always 0?
$88:ADE0 7F 02 00 7E ADC $7E0002,x[$7E:9F82]; add 0?
$88:ADE4 9F 02 00 7E STA $7E0002,x[$7E:9F82]; Do nothing?
$88:ADE8 98          TYA
$88:ADE9 18          CLC
$88:ADEA 69 08 00    ADC #$0008             ; Increment Y to next line
$88:ADED A8          TAY
$88:ADEE C9 B8 00    CMP #$00B8
$88:ADF1 30 DA       BMI $DA    [$ADCD]     ; For each line

; I can only assume this sets up the HDMA in some way
$88:ADF3 A9 00 00    LDA #$0000
$88:ADF6 8F D8 9F 7E STA $7E9FD8[$7E:9FD8]
$88:ADFA 8F DA 9F 7E STA $7E9FDA[$7E:9FDA]
$88:ADFE A9 1F 00    LDA #$001F
$88:AE01 8F 00 9F 7E STA $7E9F00[$7E:9F00]
$88:AE05 A9 9E 05    LDA #$059E
$88:AE08 8F 01 9F 7E STA $7E9F01[$7E:9F01]

$88:AE0C AD 15 09    LDA $0915  [$7E:0915]  ; Screen Y position
$88:AE0F 18          CLC
$88:AE10 69 20 00    ADC #$0020
$88:AE13 85 12       STA $12    [$7E:0012]  ; Screen Y + 20 (top viewable)
$88:AE15 18          CLC
$88:AE16 69 C0 00    ADC #$00C0
$88:AE19 85 14       STA $14    [$7E:0014]  ; Screen Y + E0 (bottom viewable)
$88:AE1B A0 00 00    LDY #$0000
$88:AE1E A2 03 00    LDX #$0003

$88:AE21 A5 12       LDA $12    [$7E:0012]
$88:AE23 D9 C1 AE    CMP $AEC1,y[$88:AEC1]
$88:AE26 30 05       BMI $05    [$AE2D]     ; If screen y+20 > curr height
$88:AE28 D9 C9 AE    CMP $AEC9,y[$88:AEC9]
$88:AE2B 30 44       BMI $44    [$AE71]     ; and < next height go to cont
$88:AE2D 98          TYA
$88:AE2E 18          CLC
$88:AE2F 69 08 00    ADC #$0008             ; Increment Y
$88:AE32 A8          TAY
$88:AE33 C9 B8 00    CMP #$00B8
$88:AE36 30 E9       BMI $E9    [$AE21]     ; While not end of lines...

; screen is below the scroll lines (partial or completely)
$88:AE38 A9 FF 05    LDA #$05FF
$88:AE3B 38          SEC
$88:AE3C E5 12       SBC $12    [$7E:0012]

$88:AE3E 85 18       STA $18    [$7E:0018]  ; height of visisble lines
$88:AE40 C9 80 00    CMP #$0080
$88:AE43 30 19       BMI $19    [$AE5E]     ; No more scrolling line visisble
$88:AE45 A9 7F 00    LDA #$007F
$88:AE48 9F 00 9F 7E STA $7E9F00,x[$7E:9F15]
$88:AE4C A9 B5 00    LDA #$00B5
$88:AE4F 9F 01 9F 7E STA $7E9F01,x[$7E:9F16]; HDMA stuff?
$88:AE53 E8          INX
$88:AE54 E8          INX
$88:AE55 E8          INX
$88:AE56 A5 18       LDA $18    [$7E:0018]
$88:AE58 38          SEC
$88:AE59 E9 7F 00    SBC #$007F
$88:AE5C 80 E0       BRA $E0    [$AE3E]     ; Scan again for next line

; finished scanning for scrolling lines on a partial screen
$88:AE5E 9F 00 9F 7E STA $7E9F00,x[$7E:9F1B];HDMA stuff?
$88:AE62 A9 B5 00    LDA #$00B5
$88:AE65 9F 01 9F 7E STA $7E9F01,x[$7E:9F1C]
$88:AE69 A9 00 00    LDA #$0000
$88:AE6C 9F 03 9F 7E STA $7E9F03,x[$7E:9F1E]
$88:AE70 6B          RTL                    ;Finished

; screen is is completely filled with scrolling lines
$88:AE71 B9 C9 AE    LDA $AEC9,y[$88:AED1]  ; bottom of this scrolling line
$88:AE74 38          SEC
$88:AE75 E5 12       SBC $12    [$7E:0012]
$88:AE77 85 18       STA $18    [$7E:0018]  ; height of visible lines
$88:AE79 C9 80 00    CMP #$0080
$88:AE7C 30 19       BMI $19    [$AE97]     ; No more scrolling line visible
$88:AE7E A9 7F 00    LDA #$007F
$88:AE81 9F 00 9F 7E STA $7E9F00,x[$7E:9F06]
$88:AE85 B9 C7 AE    LDA $AEC7,y[$88:AED7]
$88:AE88 1A          INC A
$88:AE89 1A          INC A
$88:AE8A 9F 01 9F 7E STA $7E9F01,x[$7E:9F07]; HDMA stuff
$88:AE8E E8          INX
$88:AE8F E8          INX
$88:AE90 E8          INX
$88:AE91 A5 18       LDA $18    [$7E:0018]
$88:AE93 38          SEC
$88:AE94 E9 7F 00    SBC #$007F             ; Scane again for next line

; finished scanning for scrolling lines on a complete screen
$88:AE97 9F 00 9F 7E STA $7E9F00,x[$7E:9F03]
$88:AE9B B9 C7 AE    LDA $AEC7,y[$88:AECF]
$88:AE9E 18          CLC
$88:AE9F 69 02 00    ADC #$0002
$88:AEA2 9F 01 9F 7E STA $7E9F01,x[$7E:9F04];HDMA stuff
$88:AEA6 A5 18       LDA $18    [$7E:0018]
$88:AEA8 18          CLC
$88:AEA9 65 12       ADC $12    [$7E:0012]
$88:AEAB 85 12       STA $12    [$7E:0012]  ; Update the bottom of the scanned screen
$88:AEAD E8          INX
$88:AEAE E8          INX
$88:AEAF E8          INX
$88:AEB0 A5 12       LDA $12    [$7E:0012]
$88:AEB2 C5 14       CMP $14    [$7E:0014]
$88:AEB4 10 03       BPL $03    [$AEB9]     ; If not done scanning
$88:AEB6 4C 21 AE    JMP $AE21  [$88:AE21]  ; go to partial cover scan

$88:AEB9 A9 00 00    LDA #$0000
$88:AEBC 9F 03 9F 7E STA $7E9F03,x[$7E:9F15]; HDMA stuff
$88:AEC0 6B          RTL                    ; Finish
$88:AEC1

      ;AEC1  ;AEC3  ;AEC5  ;AEC7
      ;Height ;Speed ;??? ;HDMA ADR
    DW $0000, $8000, $0000, $9F80,
    DW $0010, $C000, $0000, $9F84,
    DW $0038, $8000, $0000, $9F88,
    DW $00D0, $C000, $0000, $9F8C,
    DW $00E0, $8000, $0000, $9F90,
    DW $0120, $C000, $0000, $9F94,
    DW $01A0, $8000, $0000, $9F98,
    DW $01D8, $C000, $0000, $9F9C,
    DW $0238, $8000, $0000, $9FA0,
    DW $0268, $C000, $0000, $9FA4,
    DW $02A0, $8000, $0000, $9FA8,
    DW $02E0, $C000, $0000, $9FA0,
    DW $0300, $8000, $0000, $9FB0,
    DW $0320, $C000, $0000, $9FB4,
    DW $0350, $8000, $0000, $9FB8,
    DW $0378, $C000, $0000, $9FBC,
    DW $03C8, $8000, $0000, $9FC0,
    DW $0440, $7000, $0000, $9FC4,
    DW $0460, $C000, $0000, $9FC8,
    DW $0480, $8000, $0000, $9FCC,
    DW $0490, $0000, $0000, $9FD0, ; stationary line?
    DW $04A8, $0000, $0000, $9FD4, ; stationary line?
    DW $04B8, $0000, $0000, $9FD8, ; stationary line?
    DW $0500 ;; extra line to use and hight of previous pand