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⌂ ∞ Elite on the BBC Micro and NES

Combat demo: ScrollTextUpScreen

[NES version, Bank 6]

       Name: ScrollTextUpScreen                                      [Show more]
       Type: Subroutine
   Category: Combat demo
    Summary: Go through the line y-coordinate table at Y1TB, moving each line
             coordinate up the screen by W2Y (i.e. by one full line of text)

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawScrollText calls ScrollTextUpScreen




.ScrollTextUpScreen

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

                        ; We now work our way through every y-coordinate in the
                        ; Y1TB table (so that's the y-coordinate of each line in
                        ; the line coordinate tables), adding 51 to each of them
                        ; to move the scroll text up the screen, and removing
                        ; any lines that move off the top of the scroll text

 LDY #16                ; Set Y as a loop counter so we work our way through
                        ; the y-coordinates in Y1TB, from entry 239 down to
                        ; entry 224

.sups1

 LDA Y1TB+223,Y         ; Set A to the Y-th y-coordinate in this section of the
                        ; Y1TB table

 BEQ sups3              ; If A = 0 then this entry is already empty, so jump to
                        ; sups3 to move on to the next entry

 CLC                    ; Otherwise this is a valid y-coordinate, so add W2Y to
 ADC #(W2Y<<4 + W2Y)    ; the high nibble and W2Y to the low nibble, so we add
                        ; W2Y to both of the y-coordinates stored in this entry

 BCC sups2              ; If the addition overflowed, set A = 0 to remove this
 LDA #0                 ; entry from the table, as the line no longer fits
 CLC                    ; on-screen (we also clear the C flag, though this
                        ; doesn't appear to be necessary)

.sups2

 STA Y1TB+223,Y         ; Store the updated y-coordinate back in the Y1TB table

.sups3

 DEY                    ; Decrement the loop counter in Y

 BNE sups1              ; Loop back until we have processed all the entries in
                        ; this section of the Y1TB table

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDY #32                ; Set Y as a loop counter so we work our way through
                        ; the y-coordinates in Y1TB, from entry 223 down to
                        ; entry 192

.sups4

 LDA Y1TB+191,Y         ; Set A to the Y-th y-coordinate in this section of the
                        ; Y1TB table

 BEQ sups6              ; If A = 0 then this entry is already empty, so jump to
                        ; sups6 to move on to the next entry

 CLC                    ; Otherwise this is a valid y-coordinate, so add W2Y to
 ADC #(W2Y<<4 + W2Y)    ; the high nibble and W2Y to the low nibble, so we add
                        ; W2Y to both of the y-coordinates stored in this entry

 BCC sups5              ; If the addition overflowed, set A = 0 to remove this
 LDA #0                 ; entry from the table, as the line no longer fits
 CLC                    ; on-screen (we also clear the C flag, though this
                        ; doesn't appear to be necessary)

.sups5

 STA Y1TB+191,Y         ; Store the updated y-coordinate back in the Y1TB table

.sups6

 DEY                    ; Decrement the loop counter in Y

 BNE sups4              ; Loop back until we have processed all the entries in
                        ; this section of the Y1TB table

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDY #32                ; Set Y as a loop counter so we work our way through
                        ; the y-coordinates in Y1TB, from entry 191 down to
                        ; entry 160

.sups7

 LDA Y1TB+159,Y         ; Set A to the Y-th y-coordinate in this section of the
                        ; Y1TB table

 BEQ sups9              ; If A = 0 then this entry is already empty, so jump to
                        ; sups9 to move on to the next entry

 CLC                    ; Otherwise this is a valid y-coordinate, so add W2Y to
 ADC #(W2Y<<4 + W2Y)    ; the high nibble and W2Y to the low nibble, so we add
                        ; W2Y to both of the y-coordinates stored in this entry

 BCC sups8              ; If the addition overflowed, set A = 0 to remove this
 LDA #0                 ; entry from the table, as the line no longer fits
 CLC                    ; on-screen (we also clear the C flag, though this
                        ; doesn't appear to be necessary)

.sups8

 STA Y1TB+159,Y         ; Store the updated y-coordinate back in the Y1TB table

.sups9

 DEY                    ; Decrement the loop counter in Y

 BNE sups7              ; Loop back until we have processed all the entries in
                        ; this section of the Y1TB table

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDY #32                ; Set Y as a loop counter so we work our way through
                        ; the y-coordinates in Y1TB, from entry 159 down to
                        ; entry 128

.sups10

 LDA Y1TB+127,Y         ; Set A to the Y-th y-coordinate in this section of the
                        ; Y1TB table

 BEQ sups12             ; If A = 0 then this entry is already empty, so jump to
                        ; sups12 to move on to the next entry

 CLC                    ; Otherwise this is a valid y-coordinate, so add W2Y to
 ADC #(W2Y<<4 + W2Y)    ; the high nibble and W2Y to the low nibble, so we add
                        ; W2Y to both of the y-coordinates stored in this entry

 BCC sups11             ; If the addition overflowed, set A = 0 to remove this
 LDA #0                 ; entry from the table, as the line no longer fits
 CLC                    ; on-screen (we also clear the C flag, though this
                        ; doesn't appear to be necessary)

.sups11

 STA Y1TB+127,Y         ; Store the updated y-coordinate back in the Y1TB table

.sups12

 DEY                    ; Decrement the loop counter in Y

 BNE sups10             ; Loop back until we have processed all the entries in
                        ; this section of the Y1TB table

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDY #32                ; Set Y as a loop counter so we work our way through
                        ; the y-coordinates in Y1TB, from entry 127 down to
                        ; entry 96

.sups13

 LDA Y1TB+95,Y          ; Set A to the Y-th y-coordinate in this section of the
                        ; Y1TB table

 BEQ sups15             ; If A = 0 then this entry is already empty, so jump to
                        ; sups15 to move on to the next entry

 CLC                    ; Otherwise this is a valid y-coordinate, so add W2Y to
 ADC #(W2Y<<4 + W2Y)    ; the high nibble and W2Y to the low nibble, so we add
                        ; W2Y to both of the y-coordinates stored in this entry

 BCC sups14             ; If the addition overflowed, set A = 0 to remove this
 LDA #0                 ; entry from the table, as the line no longer fits
 CLC                    ; on-screen (we also clear the C flag, though this
                        ; doesn't appear to be necessary)
.sups14

 STA Y1TB+95,Y          ; Store the updated y-coordinate back in the Y1TB table

.sups15

 DEY                    ; Decrement the loop counter in Y

 BNE sups13             ; Loop back until we have processed all the entries in
                        ; this section of the Y1TB table

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDY #32                ; Set Y as a loop counter so we work our way through
                        ; the y-coordinates in Y1TB, from entry 95 down to
                        ; entry 64

.sups16

 LDA Y1TB+63,Y          ; Set A to the Y-th y-coordinate in this section of the
                        ; Y1TB table

 BEQ sups18             ; If A = 0 then this entry is already empty, so jump to
                        ; sups18 to move on to the next entry

 CLC                    ; Otherwise this is a valid y-coordinate, so add W2Y to
 ADC #(W2Y<<4 + W2Y)    ; the high nibble and W2Y to the low nibble, so we add
                        ; W2Y to both of the y-coordinates stored in this entry

 BCC sups17             ; If the addition overflowed, set A = 0 to remove this
 LDA #0                 ; entry from the table, as the line no longer fits
 CLC                    ; on-screen (we also clear the C flag, though this
                        ; doesn't appear to be necessary)
.sups17

 STA Y1TB+63,Y          ; Store the updated y-coordinate back in the Y1TB table

.sups18

 DEY                    ; Decrement the loop counter in Y

 BNE sups16             ; Loop back until we have processed all the entries in
                        ; this section of the Y1TB table

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDY #32                ; Set Y as a loop counter so we work our way through
                        ; the y-coordinates in Y1TB, from entry 63 down to
                        ; entry 32

.sups19

 LDA Y1TB+31,Y          ; Set A to the Y-th y-coordinate in this section of the
                        ; Y1TB table

 BEQ sups21             ; If A = 0 then this entry is already empty, so jump to
                        ; sups21 to move on to the next entry

 CLC                    ; Otherwise this is a valid y-coordinate, so add W2Y to
 ADC #(W2Y<<4 + W2Y)    ; the high nibble and W2Y to the low nibble, so we add
                        ; W2Y to both of the y-coordinates stored in this entry

 BCC sups20             ; If the addition overflowed, set A = 0 to remove this
 LDA #0                 ; entry from the table, as the line no longer fits
 CLC                    ; on-screen (we also clear the C flag, though this
                        ; doesn't appear to be necessary)

.sups20

 STA Y1TB+31,Y          ; Store the updated y-coordinate back in the Y1TB table

.sups21

 DEY                    ; Decrement the loop counter in Y

 BNE sups19             ; Loop back until we have processed all the entries in
                        ; this section of the Y1TB table

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDY #32                ; Set Y as a loop counter so we work our way through
                        ; the y-coordinates in Y1TB, from entry 31 down to
                        ; entry 0

.sups22

 LDA Y1TB-1,Y           ; Set A to the Y-th y-coordinate in this section of the
                        ; Y1TB table

 BEQ sups24             ; If A = 0 then this entry is already empty, so jump to
                        ; sups24 to move on to the next entry

 CLC                    ; Otherwise this is a valid y-coordinate, so add W2Y to
 ADC #(W2Y<<4 + W2Y)    ; the high nibble and W2Y to the low nibble, so we add
                        ; W2Y to both of the y-coordinates stored in this entry

 BCC sups23             ; If the addition overflowed, set A = 0 to remove this
 LDA #0                 ; entry from the table, as the line no longer fits
 CLC                    ; on-screen (we also clear the C flag, though this
                        ; doesn't appear to be necessary)

.sups23

 STA Y1TB-1,Y           ; Store the updated y-coordinate back in the Y1TB table

.sups24

 DEY                    ; Decrement the loop counter in Y

 BNE sups22             ; Loop back until we have processed all the entries in
                        ; this section of the Y1TB table

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 RTS                    ; Return from the subroutine