; We start by clearing the screen bitmap from $4000 to ; $5FFF by zeroing this part of memory LDA #0 ; Set the low byte of ZP(1 0) to 0 STA ZP TAY ; Set Y = 0 to act as a byte counter LDX #$40 ; Set X = $40 to use as the high byte of ZP(1 0), so the ; next instruction initialises ZP(1 0) to $4000 .LOOP2 STX ZP+1 ; Set the high byte of ZP(1 0) to X .LOOP1 STA (ZP),Y ; Zero the Y-th byte of ZP(1 0) INY ; Increment the byte counter in Y BNE LOOP1 ; Loop back until we have zeroed a whole page at ZP(1 0) LDX ZP+1 ; Set X to the high byte of ZP(1 0) INX ; Increment X to point to the next page in memory CPX #$60 ; Loop back to zero the next page in memory until we BNE LOOP2 ; have zeroed all the way to $5FFF ; We now reset the two banks of screen RAM from $6000 to ; $63FF and $6400 to $67FF, so we can then populate them ; with colour data for the text view ($6000 to $63FF) ; and the space view ($6400 to $67FF) LDA #$10 ; Set A to the colour byte that we want to fill both ; blocks of screen RAM with, which is $10 to set the ; palette to foreground colour 1 (red) and background ; colour 0 (black) ; At this point, X = $60 from above, which we use as the ; high byte of ZP(1 0), and ZP hasn't changed from zero, ; so the next instruction initialises ZP(1 0) to $6000 .LOOP3 STX ZP+1 ; Set the high byte of ZP(1 0) to X .LOOP4 STA (ZP),Y ; Set the Y-th byte of ZP(1 0) to $10 INY ; Increment the byte counter BNE LOOP4 ; Loop back until we have filled a whole page with the ; red/black palette byte LDX ZP+1 ; Set X to the high byte of ZP(1 0) INX ; Increment X to point to the next page in memory CPX #$68 ; Loop back to zero the next page in memory until we BNE LOOP3 ; have zeroed all the way to $67FF ; Next, we populate screen RAM for the space view ($6400 ; to $67FF), starting with the dashboard in the lower ; part of the screen LDA #LO(SCBASE+$2400+$2D0) ; Set ZP(1 0) to the address within the space STA ZP ; view's screen RAM that corresponds to the LDA #HI(SCBASE+$2400+$2D0) ; dashboard (i.e. offset $2D0 within the screen STA ZP+1 ; RAM at SCBASE + $2400, or $6400) LDA #LO(sdump) ; Set (A ZP2) = sdump STA ZP2 LDA #HI(sdump) JSR mvsm ; Call mvsm to copy 280 bytes of data from sdump to the ; dashboard's screen RAM for the space view, so this ; sets the correct colour data for the dashboard (along ; with the data that we copy into colour RAM in part 6) ;LDX #0 ; These instructions are commented out in the original ; ; source ;.LOOP20 ; ;LDA date,X ;STA SCBASE+$7A0,X ;DEX ;BNE LOOP20 ; Now we populate screen RAM for the text view ($6000 ; to $63FF) to set the correct colour for the border box ; around the edges of the screen ; ; The screen borders are four character blocks wide on ; each side of the screen (so the 256-pixel-wide game ; screen gets shown in the middle of the 320-pixel wide ; screen mode) ; ; The outside three character blocks show nothing and ; are plain black, which we achieve by setting both the ; foreground and background colours to black for these ; character blocks ; ; The innermost of the four character blocks on each ; side is used to draw the border box, with the border ; being right up against the game screen, so for this we ; need a palette of yellow on black, so we can draw the ; border box in yellow LDA #0 ; Set ZP(1 0) = $6000 STA ZP ; LDA #$60 ; So ZP(1 0) points to screen RAM for the text view STA ZP+1 LDX #25 ; The text view is 25 character rows high, so set a row ; counter in X .LOOP10 LDA #$70 ; Set A to the colour byte that we want to apply to the ; border box, which is $70 to set the palette to ; foreground colour 7 (yellow) and background colour 0 ; (black) LDY #36 ; Set the colour data for column 36 (i.e. the right edge STA (ZP),Y ; of the border box) to the yellow/black palette LDY #3 ; Set the colour data for column 3 (i.e. the left edge STA (ZP),Y ; of the border box) to the yellow/black palette ; Next, we set the palette to black on black for the ; outside three character blocks on the left side of the ; screen, so they don't show anything at all DEY ; Set Y = 2 to use as a column counter for the three ; character blocks, so we work our way through columns ; 2, 1 and 0 LDA #$00 ; Set A to the colour byte that we want to apply to the ; outer border area, which is $00 to set the palette to ; foreground colour 0 (black) and background colour 0 ; (black) .frogl STA (ZP),Y ; Set the colour data for column Y to the black/black ; palette DEY ; Decrement the column counter BPL frogl ; Loop back until we have set all three character blocks ; on the left edge of this character row to the ; black/black palette ; And now we set the palette to black on black for the ; outside three character blocks on the right side of ; the screen, so they also show nothing LDY #37 ; Set Y = 2 to use as a column counter for the three ; character blocks, so we work our way through columns ; 37, 38 and 39 STA (ZP),Y ; Set the colour data for column 37 to the black/black ; palette INY ; Set the colour data for column 38 to the black/black STA (ZP),Y ; palette INY ; Set the colour data for column 39 to the black/black STA (ZP),Y ; palette LDA ZP ; Set ZP(1 0) = ZP(1 0) + 40 CLC ; ADC #40 ; So ZP(1 0) points to the next character row in screen STA ZP ; RAM (as there are 40 character blocks on each row) BCC P%+4 INC ZP+1 DEX ; Decrement the row counter in X BNE LOOP10 ; Loop back until we have set the colour data for the ; left and right border box edges in the text view ; Now we populate screen RAM for the text view ($6000 ; to $63FF) to set the correct colour for the border box ; around the edges of the space view LDA #0 ; Set ZP(1 0) = $6400 STA ZP ; LDA #$64 ; So ZP(1 0) points to screen RAM for the space view STA ZP+1 LDX #18 ; The space view is 18 character rows high, so set a row ; counter in X .LOOP11 LDA #$70 ; Set A to the colour byte that we want to apply to the ; border box, which is $70 to set the palette to ; foreground colour 7 (yellow) and background colour 0 ; (black) LDY #36 ; Set the colour data for column 36 (i.e. the right edge STA (ZP),Y ; of the border box) to the yellow/black palette LDY #3 ; Set the colour data for column 3 (i.e. the left edge STA (ZP),Y ; of the border box) to the yellow/black palette ; Next, we set the palette to black on black for the ; outside three character blocks on the left side of the ; screen, so they don't show anything at all DEY ; Set Y = 2 to use as a column counter for the three ; character blocks, so we work our way through columns ; 2, 1 and 0 LDA #$00 ; Set A to the colour byte that we want to apply to the ; outer border area, which is $00 to set the palette to ; foreground colour 0 (black) and background colour 0 ; (black) .newtl STA (ZP),Y ; Set the colour data for column Y to the black/black ; palette DEY ; Decrement the column counter BPL newtl ; Loop back until we have set all three character blocks ; on the left edge of this character row to the ; black/black palette ; And now we set the palette to black on black for the ; outside three character blocks on the right side of ; the screen, so they also show nothing LDY #37 ; Set Y = 2 to use as a column counter for the three ; character blocks, so we work our way through columns ; 37, 38 and 39 STA (ZP),Y ; Set the colour data for column 37 to the black/black ; palette INY ; Set the colour data for column 38 to the black/black STA (ZP),Y ; palette INY ; Set the colour data for column 39 to the black/black STA (ZP),Y ; palette LDA ZP ; Set ZP(1 0) = ZP(1 0) + 40 CLC ; ADC #40 ; So ZP(1 0) points to the next character row in screen STA ZP ; RAM (as there are 40 character blocks on each row) BCC P%+4 INC ZP+1 DEX ; Decrement the row counter in X BNE LOOP11 ; Loop back until we have set the colour data for the ; left and right border box edges in the space view ; Finally, we set the colour data for the bottom row in ; the text view, so the bottom of the border box is also ; shown in yellow LDA #$70 ; Set A to the colour byte that we want to apply to the ; border box, which is $70 to set the palette to ; foreground colour 7 (yellow) and background colour 0 ; (black) LDY #31 ; Set a counter in Y to work through the 31 character ; columns in the text view .LOOP16 STA $63C4,Y ; Set the colour data for column Y + 4 on row 24 to ; yellow on black ; ; The address breaks down as follows: ; ; $63C4 = $6000 + 24 * 40 + 4 ; ; So $63C4 + Y is column Y + 4 on row 24 and this loop ; sets the colour for the bottom character row of the ; text view DEY ; Decrement the column counter BPL LOOP16 ; Loop back until we have set the colour for the bottom ; border box in the text viewName: Elite loader (Part 5 of 7) [Show more] Type: Subroutine Category: Loader Summary: Configure the screen bitmap and copy colour data into screen RAMContext: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
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Label LOOP1 is local to this routine
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Label LOOP10 is local to this routine
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Label LOOP11 is local to this routine
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Label LOOP16 is local to this routine
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Label LOOP2 is local to this routine
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Label LOOP3 is local to this routine
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Label LOOP4 is local to this routine
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Configuration variable SCBASE = $4000
The address of the screen bitmap
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Workspace ZP (category: Workspaces)
Important variables used by the loader
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Label frogl is local to this routine
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Subroutine mvsm (category: Loader)
Copy 280 bytes in memory
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Label newtl is local to this routine
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Variable sdump (category: Drawing the screen)
Screen RAM colour data for the dashboard