Skip to navigation

⌂ ∞ ▽ Elite on the 6502

Drawing the screen: CLYNS

[NES version, Bank 7]

       Name: CLYNS                                                   [Show more]
       Type: Subroutine
   Category: Drawing the screen
    Summary: Clear the bottom two text rows of the visible screen

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ChangeCmdrName calls CLYNS
             * dockEd calls CLYNS
             * FlightLoop4To16 calls CLYNS
             * HME2 calls CLYNS
             * JMTB calls CLYNS
             * SetSelectedSystem calls CLYNS
             * TT102 calls CLYNS
             * TT147 calls CLYNS
             * YESNO calls CLYNS
             * PrintFlightMessage calls via CLYNS+8




 This routine clears some space at the bottom of the screen and moves the text
 cursor to column 1, row 22.



 Other entry points:

   CLYNS+8              Don't zero DLY and de



.CLYNS

 LDA #0                 ; Set the delay in DLY to 0, to indicate that we are
 STA DLY                ; no longer showing an in-flight message, so any new
                        ; in-flight messages will be shown instantly

 STA de                 ; Clear de, the flag that appends " DESTROYED" to the
                        ; end of the next text token, so that it doesn't

 LDA #%11111111         ; Set DTW2 = %11111111 to denote that we are not
 STA DTW2               ; currently printing a word

 LDA #%10000000         ; Set bit 7 of QQ17 to switch standard tokens to
 STA QQ17               ; Sentence Case

 LDA #22                ; Move the text cursor to row 22, near the bottom of
 STA YC                 ; the screen

 LDA #1                 ; Move the text cursor to column 1
 STA XC

 LDA firstPattern       ; Set the next free pattern number in firstFreePattern
 STA firstFreePattern   ; to the value of firstPattern, which contains the
                        ; number of the first pattern for which we send pattern
                        ; data to the PPU in the NMI handler, so it's also the
                        ; pattern we can start drawing into when we next start
                        ; drawing into tiles

 LDA QQ11               ; If bit 7 of the view type in QQ11 is clear then there
 BPL clyn2              ; is a dashboard, so jump to clyn2 to return from the
                        ; subroutine

 LDA #HI(nameBuffer0+23*32)     ; Set SC(1 0) to the address of the tile in
 STA SC+1                       ; column 0 on tile row 23 in nametable buffer 0
 LDA #LO(nameBuffer0+23*32)
 STA SC

 LDA #HI(nameBuffer1+23*32)     ; Set SC(1 0) to the address of the tile in
 STA SC2+1                      ; column 0 on tile row 23 in nametable buffer 1
 LDA #LO(nameBuffer1+23*32)
 STA SC2

 LDX #2                 ; We want to clear two text rows, row 23 and row 24, so
                        ; set a counter in X to count 2 rows

.CLYL

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

 LDY #2                 ; We are going to clear tiles from column 2 to 30 on
                        ; each row, so set a tile index in Y to start from
                        ; column 2

 LDA #0                 ; Set A = 0 to use as the pattern number for the blank
                        ; background tile

.EE2

 STA (SC),Y             ; Set the Y-th tile on the row in nametable buffer 0 to
                        ; the blank tile

 STA (SC2),Y            ; Set the Y-th tile on the row in nametable buffer 1 to
                        ; the blank tile

 INY                    ; Increment the tile counter to move on to the next tile

 CPY #31                ; Loop back to blank the next tile until we have done
 BNE EE2                ; all the tiles up to column 30

 LDA SC                 ; Add 32 to SC(1 0) to point to the next row, starting
 ADC #31                ; with the low bytes (the ADC #31 adds 32 because the C
 STA SC                 ; flag is set from the comparison above, which resulted
                        ; in equality)

 STA SC2                ; Add 32 to the low byte of SC2(1 0) as well

 BCC clyn1              ; If the addition didn't overflow, jump to clyn1 to skip
                        ; the high bytes

 INC SC+1               ; Increment the high bytes of SC(1 0) and SC2(1 0)
 INC SC2+1              ; to point to the next page in memory

.clyn1

 DEX                    ; Decrement the row counter in X

 BNE CLYL               ; Loop back to blank another row, until we have done the
                        ; number of rows in X

.clyn2

 RTS                    ; Return from the subroutine