.SCR1 RTS ; Return from the subroutine .SCAN LDA QQ11 ; If QQ11 is non-zero then this is not the space view BNE SCR1 ; and there is no dashboard on-screen, so jump to SCR1 ; to return from the subroutine LDA INWK+31 ; Fetch the ship's scanner flag from byte #31 AND #%00010000 ; If bit 4 is clear then the ship should not be shown BEQ SCR1 ; on the scanner, so return from the subroutine (as SCR1 ; contains an RTS) LDX TYPE ; Fetch the ship's type from TYPE into X BMI SCR1 ; If this is the planet or the sun, then the type will ; have bit 7 set and we don't want to display it on the ; scanner, so return from the subroutine (as SCR1 ; contains an RTS) LDA scacol,X ; Set A to the scanner colour for this ship type from ; the X-th entry in the scacol table STA COL ; Store the scanner colour in COL so it can be used to ; draw this ship in the correct colour LDA INWK+1 ; If any of x_hi, y_hi and z_hi have a 1 in bit 6 or 7, ORA INWK+4 ; then the ship is too far away to be shown on the ORA INWK+7 ; scanner, so return from the subroutine (as SCR1 AND #%11000000 ; contains an RTS) BNE SCR1 ; If we get here, we know x_hi, y_hi and z_hi are all ; 63 (%00111111) or less ; Now, we convert the x_hi coordinate of the ship into ; the screen x-coordinate of the dot on the scanner, ; using the following: ; ; X1 = 123 + (x_sign x_hi) LDA INWK+1 ; Set A = x_hi CLC ; Clear the C flag so we can do addition below LDX INWK+2 ; Set X = x_sign BPL SC2 ; If x_sign is positive, skip the following EOR #%11111111 ; x_sign is negative, so flip the bits in A and add 1 ADC #1 ; to make it a negative number (bit 7 will now be set ; as we confirmed above that bits 6 and 7 are clear). So ; this gives A the sign of x_sign and gives it a value ; range of -63 (%11000001) to 0 .SC2 ADC #123 ; Set X1 = 123 + (x_sign x_hi) STA X1 ; Next, we convert the z_hi coordinate of the ship into ; the y-coordinate of the base of the ship's stick, ; like this: ; ; SC = 220 - (z_sign z_hi) / 4 ; ; though the following code actually does it like this: ; ; SC = 255 - (35 + z_hi / 4) LDA INWK+7 ; Set A = z_hi / 4 LSR A ; LSR A ; So A is in the range 0-15 CLC ; Clear the C flag for the addition below LDX INWK+8 ; Set X = z_sign BPL SC3 ; If z_sign is positive, skip the following EOR #%11111111 ; z_sign is negative, so flip the bits in A and set the SEC ; C flag. As above, this makes A negative, this time ; with a range of -16 (%11110000) to -1 (%11111111). And ; as we are about to do an ADC, the SEC effectively adds ; another 1 to that value, giving a range of -15 to 0 .SC3 ADC #83 ; Set A = 83 + A to give a number in the range 48 to 98 EOR #%11111111 ; Flip all the bits and store in Y2, so Y2 is in the STA SC ; range 157 to 207, with a higher z_hi giving a lower Y2 ; Now for the stick height, which we calculate using the ; following: ; ; A = - (y_sign y_hi) / 2 LDA INWK+4 ; Set A = y_hi / 2 LSR A CLC ; Clear the C flag LDX INWK+5 ; Set X = y_sign BMI SCD6 ; If y_sign is negative, skip the following, as we ; already have a positive value in A EOR #%11111111 ; y_sign is positive, so flip the bits in A and set the SEC ; C flag. This makes A negative, and as we are about to ; do an ADC below, the SEC effectively adds another 1 to ; that value to implement two's complement negation, so ; we don't need to add another 1 here .SCD6 ; We now have all the information we need to draw this ; ship on the scanner, namely: ; ; X1 = the screen x-coordinate of the ship's dot ; ; SC = the screen y-coordinate of the base of the ; stick ; ; A = the screen height of the ship's stick, with the ; correct sign for adding to the base of the stick ; to get the dot's y-coordinate ; ; First, though, we have to make sure the dot is inside ; the dashboard, by moving it if necessary ADC SC ; Set A = SC + A, so A now contains the y-coordinate of ; the end of the stick, plus the length of the stick, to ; give us the screen y-coordinate of the dot ;BPL ld246 ; This instruction is commented out in the original ; source CMP #146 ; If A >= 146, skip the following instruction, as 146 is BCS P%+4 ; the minimum allowed value of A LDA #146 ; A < 146, so set A to 146, the minimum allowed value ; for the y-coordinate of our ship's dot CMP #199 ; If A < 199, skip the following instruction, as 198 is BCC P%+4 ; the maximum allowed value of A .ld246 LDA #198 ; A >= 199, so set A to 198, the maximum allowed value ; for the y-coordinate of our ship's dot STA Y1 ; Store A in Y1, as it now contains the screen ; y-coordinate for the ship's dot, clipped so that it ; fits within the dashboard SEC ; Set A = A - SC to get the stick length, by reversing SBC SC ; the ADC SC we did above. This clears the C flag if the ; result is negative (i.e. the stick length is negative) ; and sets it if the result is positive (i.e. the stick ; length is negative) ; So now we have the following: ; ; X1 = the screen x-coordinate of the ship's dot, ; clipped to fit into the dashboard ; ; Y1 = the screen y-coordinate of the ship's dot, ; clipped to fit into the dashboard ; ; SC = the screen y-coordinate of the base of the ; stick ; ; A = the screen height of the ship's stick, with the ; correct sign for adding to the base of the stick ; to get the dot's y-coordinate ; ; C = 0 if A is negative, 1 if A is positive ; ; and we can get on with drawing the dot and stick PHP ; Store the flags (specifically the C flag) from the ; above subtraction ;BCS SC48 ; These instructions are commented out in the original ;EOR #$FF ; source. They would negate A if the C flag were set, ;ADC #1 ; which would reverse the direction of all the sticks, ; so you could turn your joystick around. Perhaps one of ; the authors' test sticks were easier to use upside ; down? Who knows... .SC48 PHA ; Store the stick height in A on the stack JSR CPIX4 ; Draw a double-height dot at (X1, Y1). This also leaves ; the following variables set up for the dot's top-right ; pixel, the last pixel to be drawn (as the dot gets ; drawn from the bottom up): ; ; SC(1 0) = screen address of the pixel's character ; block ; ; Y = number of the character row containing the pixel ; ; X = the pixel's number (0-3) in that row ; ; We can use there as the starting point for drawing the ; stick, if there is one LDA CTWOS2+2,X ; Load the same bitmap one-pixel byte that we just used AND COL ; for the top-right pixel, and mask it with the same STA X1 ; colour, storing the result in X1, so we can use it as ; the character row byte for the stick PLA ; Restore the stick height from the stack into A PLP ; Restore the flags from above, so the C flag once again ; reflects the sign of the stick height TAX ; Copy the stick height into X BEQ RTS ; If the stick height is zero, then there is no stick to ; draw, so return from the subroutine (as RTS contains ; an RTS) BCC VL3 ; If the C flag is clear then the stick height in A is ; negative, so jump down to VL3 .VLL1 ; If we get here then the stick length is positive (so ; the dot is below the ellipse and the stick is above ; the dot, and we need to draw the stick upwards from ; the dot) DEY ; We want to draw the stick upwards, so decrement the ; pixel row in Y BPL VL1 ; If Y is still positive then it correctly points at the ; line above, so jump to VL1 to skip the following LDY #7 ; We just decremented Y up through the top of the ; character block, so we need to move it to the last row ; in the character above, so set Y to 7, the number of ; the last row ; We now need to move up into the character block above, ; and each character row in screen memory takes up $140 ; bytes ($100 for the visible part and $20 for each of ; the blank borders on the side of the screen), so ; that's what we need to subtract from SC(1 0) LDA SC ; Set SC(1 0) = SC(1 0) - $140 SEC ; SBC #$40 ; Starting with the low bytes STA SC LDA SC+1 ; And then subtracting the high bytes SBC #$01 STA SC+1 .VL1 LDA X1 ; Set A to the character row byte for the stick, which ; we stored in X1 above, and which has the same pixel ; pattern as the bottom-right pixel of the dot (so the ; stick comes out of the right side of the dot) EOR (SC),Y ; Draw the stick on row Y of the character block using STA (SC),Y ; EOR logic DEX ; Decrement the (positive) stick height in X BNE VLL1 ; If we still have more stick to draw, jump up to VLL1 ; to draw the next pixel .RTS RTS ; Return from the subroutine ; If we get here then the stick length is negative (so ; the dot is above the ellipse and the stick is below ; the dot, and we need to draw the stick downwards from ; the dot) .VL3 INY ; We want to draw the stick downwards, so we first ; increment the row counter so that it's pointing to the ; bottom-right pixel in the dot (as opposed to the top- ; right pixel that the call to CPIX4 finished on) CPY #8 ; If the row number in Y is less than 8, then it BNE VLL2 ; correctly points at the next line down, so jump to ; VLL2 to skip the following LDY #0 ; We just incremented Y down through the bottom of the ; character block, so we need to move it to the first ; row in the character below, so set Y to 0, the number ; of the first row LDA SC ; Otherwise we need to move up into the character block ADC #$3F ; below, so add 320 ($140) to SC(1 0) to move down one STA SC ; pixel line, as there are 320 bytes in each character LDA SC+1 ; row in the screen bitmap ADC #1 ; STA SC+1 ; We know the C flag is set as we just passed through a ; BNE, so we only need to add $13F to get the result .VLL2 INY ; We want to draw the stick itself, heading downwards, ; so increment the pixel row in Y CPY #8 ; If the row number in Y is less than 8, then it BNE VL2 ; correctly points at the next line down, so jump to ; VL2 to skip the following LDY #0 ; We just incremented Y down through the bottom of the ; character block, so we need to move it to the first ; row in the character below, so set Y to 0, the number ; of the first row LDA SC ; Otherwise we need to move up into the character block ADC #$3F ; below, so add 320 ($140) to SC(1 0) to move down one STA SC ; pixel line, as there are 320 bytes in each character LDA SC+1 ; row in the screen bitmap ADC #1 ; STA SC+1 ; We know the C flag is set as we just passed through a ; BNE, so we only need to add $13F to get the result .VL2 LDA X1 ; Set A to the character row byte for the stick, which ; we stored in X1 above, and which has the same pixel ; pattern as the bottom-right pixel of the dot (so the ; stick comes out of the right side of the dot) EOR (SC),Y ; Draw the stick on row Y of the character block using STA (SC),Y ; EOR logic INX ; Increment the (negative) stick height in X BNE VLL2 ; If we still have more stick to draw, jump up to VLL2 ; to draw the next pixel RTS ; Return from the subroutineName: SCAN [Show more] Type: Subroutine Category: Dashboard Summary: Display the current ship on the scanner Deep dive: The 3D scannerContext: See this subroutine in context in the source code References: This subroutine is called as follows: * ESCAPE calls SCAN * Main flight loop (Part 11 of 16) calls SCAN * MVEIT (Part 2 of 9) calls SCAN * MVEIT (Part 9 of 9) calls SCAN * WPSHPS calls SCAN
This is used both to display a ship on the scanner, and to erase it again.
Arguments: INWK The ship's data block
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Subroutine CPIX4 (category: Drawing pixels)
Draw a double-height dot on the dashboard
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Variable CTWOS2 (category: Drawing pixels)
Ready-made single-pixel character row bytes for multicolour bitmap mode
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Label RTS is local to this routine
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Label SC2 is local to this routine
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Label SC3 is local to this routine
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Label SCD6 is local to this routine
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Label SCR1 is local to this routine
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Label VL1 is local to this routine
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Label VL2 is local to this routine
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Label VL3 is local to this routine
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Label VLL1 is local to this routine
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Label VLL2 is local to this routine
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Variable scacol (category: Drawing ships)
Ship colours on the scanner