.scan1 ; If we jump here, Y is set to the offset of the first ; sprite for this ship on the scanner, so the three ; sprites are at addresses ySprite0 + Y, ySprite1 + Y ; and ySprite2 + Y LDA #240 ; Hide sprites Y to Y+2 in the sprite buffer by setting STA ySprite0,Y ; their y-coordinates to 240, which is off the bottom STA ySprite1,Y ; of the screen STA ySprite2,Y ; ; So this removes the ship's scanner sprites from the ; scanner .scan2 RTS ; Return from the subroutine .SCAN LDA QQ11 ; If this is not the space view (i.e. QQ11 is non-zero) BNE scan2 ; then jump to scan2 to return from the subroutine as ; there is no scanner to update LDX TYPE ; Fetch the ship's type from TYPE into X BMI scan2 ; 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 jump to scan2 to return from the ; subroutine as there is nothing to draw LDA INWK+33 ; Set A to ship byte #33, which contains the number of ; this ship on the scanner BEQ scan2 ; If A = 0 then this ship is not being shown on the ; scanner, so jump to scan2 to return from the ; subroutine as there is nothing to draw TAX ; Set X to the number of this ship on the scanner ASL A ; Set Y = (A * 2 + A) * 4 + 44 ADC INWK+33 ; = 44 + A * 3 * 4 ASL A ; ASL A ; This gives us the offset of the first sprite for this ADC #44 ; ship on the scanner within the sprite buffer, as each TAY ; ship has three sprites allocated to it, and there are ; four bytes per sprite in the buffer, and the first ; scanner sprite is sprite 11 (which is at offset 44 in ; the buffer) ; ; We will refer to the sprites that we will use to draw ; the ship on the scanner as sprites Y, Y+1 and Y+2, ; just to keep things simple LDA scannerColour,X ; Set A to the scanner colour for this ship, which was ; set to a sprite palette number in the NWSHP routine ; when the ship was added to the local bubble of ; universe STA attrSprite0,Y ; Set the attributes for sprite Y to the value in A, ; so the sprite's attributes are: ; ; * Bits 0-1 = sprite palette number in A ; * Bit 5 clear = show in front of background ; * Bit 6 clear = do not flip horizontally ; * Bit 7 clear = do not flip vertically ; ; This ensures that the ship appears on the scanner in ; the correct colour for the ship type ; ; We will copy these attributes for use in the other two ; sprites later ; The following algorithm is the same as the FAROF2 ; routine, which calculates the distance from our ship ; to the point (x_hi, y_hi, z_hi) to see if the ship is ; close enough to be visible on the scanner ; ; Note that it actually calculates half the distance to ; the point (i.e. 0.5 * |x y z|) as this will ensure the ; result fits into one byte LDA INWK+1 ; If x_hi >= y_hi, jump to scan3 to skip the following CMP INWK+4 ; instruction, leaving A containing the higher of the BCS scan3 ; two values LDA INWK+4 ; Set A = y_hi, which is the higher of the two values, ; so by this point we have: ; ; A = max(x_hi, y_hi) .scan3 CMP INWK+7 ; If A >= z_hi, jump to scan4 to skip the following BCS scan4 ; instruction, leaving A containing the higher of the ; two values LDA INWK+7 ; Set A = z_hi, which is the higher of the two values, ; so by this point we have: ; ; A = max(x_hi, y_hi, z_hi) .scan4 CMP #64 ; If A >= 64 then at least one of x_hi, y_hi and z_hi is BCS scan1 ; greater or equal to 64, so jump to scan1 to hide the ; ship's scanner sprites and return from the subroutine, ; as the ship is too far away to appear on the scanner STA SC2 ; Otherwise set SC2 = max(x_hi, y_hi, z_hi) ; ; Let's call this max(x, y, z) LDA INWK+1 ; Set A = x_hi + y_hi + z_hi ADC INWK+4 ; ADC INWK+7 ; Let's call this x + y + z ; ; There is a risk that the addition will overflow here, ; but presumably this isn't an issue BCS scan1 ; If the addition overflowed then A > 255, so jump to ; scan1 to hide the ship's scanner sprites and return ; from the subroutine, as the ship is too far away to ; appear on the scanner SEC ; Set SC2+1 = A - SC2 / 4 SBC SC2 ; = (x + y + z - max(x, y, z)) / 4 LSR A LSR A STA SC2+1 LSR A ; Set A = (SC2+1 / 4) + SC2+1 + SC2 LSR A ; = 5/4 * SC2+1 + SC2 ADC SC2+1 ; = 5 * (x + y + z - max(x, y, z)) / (8 * 4) ADC SC2 ; + max(x, y, z) / 2 ; ; If h is the longest of x, y, z, and a and b are the ; other two sides, then we have: ; ; max(x, y, z) = h ; ; x + y + z - max(x, y, z) = a + b + h - h ; = a + b ; ; So: ; ; A = 5 * (a + b) / (8 * 4) + h / 2 ; = 5/32 * a + 5/32 * b + 1/2 * h ; ; This estimates half the length of the (x, y, z) ; vector, i.e. 0.5 * |x y z|, using an approximation ; that estimates the length within 8% of the correct ; value, and without having to do any multiplication ; or take any square roots CMP #64 ; If A >= 64 then jump to scan1 to hide the ship's BCS scan1 ; scanner sprites and return from the subroutine, as the ; ship is too far away to appear on the scanner ; We now calculate the position of the ship on the ; scanner, starting with the x-coordinate (see the deep ; dive on "The 3D scanner" for an explanation of the ; following) 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 scan5 ; If x_sign is positive, skip the following EOR #%11111111 ; x_sign is negative, so flip the bits in A and add ADC #1 ; 1 to make it a negative number .scan5 ADC #124 ; Set SC2 = 124 + (x_sign x_hi) STA SC2 ; ; So this gives us the x-coordinate of the ship on the ; scanner ; Next, we convert the z_hi coordinate of the ship into ; the y-coordinate of the base of the ship's stick, ; like this: ; ; SC2+1 = 199 - (z_sign z_hi) / 4 LDA INWK+7 ; Set A = z_hi / 4 LSR A LSR A CLC ; Clear the C flag for the addition below LDX INWK+8 ; Set X = z_sign BMI scan6 ; If z_sign is negative, skip the following so we add ; the magnitude of x_hi (which is the same as ; subtracting a negative x_hi) EOR #%11111111 ; z_sign is positive, so flip the bits in A and set the SEC ; C flag. This makes A negative using two's complement, ; and as we are about to do an ADC, the SEC effectively ; add the 1 we need, giving A = -x_hi .scan6 ADC #199+YPAL ; Set SC2+1 = 199 + A to give us the y-coordinate of the STA SC2+1 ; base of the ship's stick ; Finally, we calculate Y1 to represent the height of ; stick as: ; ; Y1 = min(y_hi, 47) LDA INWK+4 ; Set A = y_hi CMP #48 ; If A < 48, jump to scan7 to skip the following BCC scan7 ; instruction LDA #47 ; Set A = 47, so A contains y_hi capped to a maximum ; value of 47, i.e. A = min(y_hi, 47) .scan7 LSR A ; Set Y1 = A = min(y_hi, 47) STA Y1 CLC ; Clear the C flag (though this has no effect) ; We now have the following data: ; ; * SC2 is the x-coordinate of the ship on the scanner ; ; * SC2+1 is the y-coordinate of the base of the ; ship's stick ; ; * Y1 is the height of the stick ; ; So now we draw the ship on the scanner, with the first ; step being to work out whether we should draw the ship ; above or below the 3D ellipse (and therefore in front ; of or behind the background tiles that make up the 3D ; ellipse on-screen) BEQ scan8 ; If A = 0 then y_hi must be 0, so the ship is exactly ; on the 3D ellipse on the scanner (not above or below ; it), so jump to scan8 to draw the ship in front of ; the ellipse background LDX INWK+5 ; If y_sign is positive then the ship is above the 3D BPL scan8 ; ellipse on the scanner, so jump to scan8 to draw the ; ship in front of the ellipse background JMP scan12 ; Otherwise the ship is below the 3D ellipse on the ; scanner, so jump to scan12 to draw the ship behind ; the ellipse background .scan8 ; If we get here then we draw the ship in front of the ; 3D ellipse LDA SC2+1 ; Set SC2+1 = SC2+1 - 8 SEC ; SBC #8 ; This subtracts 8 from the y-coordinate of the bottom STA SC2+1 ; of the stick, and ensures that the bottom of the stick ; looks as if it is touching the 3D ellipse ; The ship is drawn on the scanner using up to three ; sprites - sprites Y, Y+1 and Y+2 ; ; Sprite Y is the end of the stick that's furthest from ; the ship dot (i.e. the "bottom" of the stick which ; appears to touch the 3D ellipse) ; ; Sprite Y+1 is the middle part of the stick ; ; Sprite Y+2 is the big ship dot at the end of the stick ; (i.e. the "top" of the stick) ; ; If a stick is full height, then we show all three ; sprites, if a stick is medium height we display ; sprites Y+1 and Y+2, and if a stick is small we only ; display sprite Y+2 ; ; We always draw sprite Y+2, so first we concentrate on ; drawing sprites Y and Y+1, if required LDA Y1 ; If Y1 < 16 then the stick is either medium or small, CMP #16 ; so jump to scan9 to skip the following BCC scan9 ; If we get here then the stick is full height, so we ; draw both sprite Y and sprite Y+1 LDA SC2 ; Set the x-coordinate of sprite Y to SC2 STA xSprite0,Y STA xSprite1,Y ; Set the x-coordinate of sprite Y+1 to SC2 LDA SC2+1 ; Set the y-coordinate of sprite Y to SC2+1, as SC2+1 STA ySprite0,Y ; contains the y-coordinate of the bottom of the stick SEC ; Set the y-coordinate of sprite Y+1 to SC2+1 - 8, which SBC #8 ; is eight pixels higher up the screen then sprite Y STA ySprite1,Y ; ; This stacks the sprites one above the other, as each ; sprite is eight pixels LDA attrSprite0,Y ; Clear bits 2 to 7 of the attributes for sprite Y to AND #%00000011 ; ensure that the following attributes are set: STA attrSprite0,Y ; ; * Bit 5 clear = show in front of background ; * Bit 6 clear = do not flip horizontally ; * Bit 7 clear = do not flip vertically ; ; This makes sure we display the sprite in front of the ; 3D ellipse, as the ship is above the ellipse in space STA attrSprite1,Y ; Set the attributes for sprite Y+1 to the same as those ; for sprite Y LDA SC2+1 ; Set SC2+1 = SC2+1 - 16 SBC #16 ; STA SC2+1 ; This subtracts 16 from the y-coordinate of the bottom ; of the stick to give us the y-coordinate of the top ; sprite in the stick BNE scan11 ; Jump to scan11 to draw sprite Y+2 at the top of the ; stick (this BNE is effectively a JMP as A is never ; zero) .scan9 ; If we get here then the stick is either medium or ; small height CMP #8 ; If Y1 < 8 then the stick is small, so jump to scan10 BCC scan10 ; to skip the following ; If we get here then the stick is medium height, so we ; draw sprite Y+1 and hide sprite Y LDA #240 ; Hide sprite Y by setting its y-coordinate to 240, STA ySprite0,Y ; which moves it off the bottom of the screen LDA SC2 ; Set the x-coordinate of sprite Y+1 to SC2 STA xSprite1,Y LDA SC2+1 ; Set the y-coordinate of sprite Y+1 to SC2+1, as SC2+1 STA ySprite1,Y ; contains the y-coordinate of the bottom of the stick ; (which is the middle sprite in a medium height stick) LDA attrSprite0,Y ; Clear bits 2 to 7 of the attributes for sprite Y+1 to AND #%00000011 ; ensure that the following attributes are set: STA attrSprite1,Y ; ; * Bit 5 clear = show in front of background ; * Bit 6 clear = do not flip horizontally ; * Bit 7 clear = do not flip vertically ; ; This makes sure we display the sprite in front of the ; 3D ellipse, as the ship is above the ellipse in space ; ; Note that we do this by copying the attributes we set ; up for sprite Y into sprite Y+1, clearing the bits as ; we do so LDA SC2+1 ; Set SC2+1 = SC2+1 - 8 SBC #8 ; STA SC2+1 ; This subtracts 8 from the y-coordinate of the bottom ; of the stick to give us the y-coordinate of the top ; sprite in the stick BNE scan11 ; Jump to scan11 to draw sprite Y+2 at the top of the ; stick (this BNE is effectively a JMP as A is never ; zero) .scan10 ; If we get here then the stick is small, so we hide ; sprites Y and Y+1, leaving just sprite Y+2 visible on ; the scanner LDA #240 ; Hide sprites Y and Y+1 from the scanner by setting STA ySprite0,Y ; their y-coordinates to 240, which moves them off the STA ySprite1,Y ; bottom of the screen .scan11 ; We now draw sprite Y+2, which contains the ship dot at ; the end of the stick LDA Y1 ; Set A = Y1 mod 8 AND #7 ; ; This gives us the height of the top part of the stick, ; as there are eight pixels in each sprite making up the ; stick, so this is the remainder after sprites Y and ; Y+1 are taken away CLC ; Set the pattern number for sprite Y+2 to 219 + A ADC #219 ; STA pattSprite2,Y ; Sprites 219 to 226 contain ship dots with trailing ; sticks, starting with the dot at the bottom of the ; pattern (in pattern 219) up to the dot at the top of ; the pattern (in pattern 226), so this sets the tile ; pattern for sprite Y+2 to the dot height given in A, ; which is the correct pattern for the top of the ship's ; stick LDA attrSprite0,Y ; Clear bits 2 to 7 of the attributes for sprite Y+1 to AND #%00000011 ; ensure that the following attributes are set: STA attrSprite2,Y ; ; * Bit 5 clear = show in front of background ; * Bit 6 clear = do not flip horizontally ; * Bit 7 clear = do not flip vertically ; ; This makes sure we display the sprite in front of the ; 3D ellipse, as the ship is above the ellipse in space ; ; Note that we do this by copying the attributes we set ; up for sprite Y into sprite Y+2, clearing the bits as ; we do so LDA SC2 ; Set the x-coordinate of sprite Y+2 to SC2 STA xSprite2,Y LDA SC2+1 ; Set the y-coordinate of sprite Y+2 to SC2+1, which by STA ySprite2,Y ; now contains the y-coordinate of the top of the stick, RTS ; Return from the subroutine .scan12 ; If we get here then we draw the ship behind the 3D ; ellipse ; ; We jump here with A set to Y1, the height of the ; stick, which we now need to clip to ensure it doesn't ; fall off the bottom of the screen CLC ; Set A = Y1 + SC2+1 ADC SC2+1 CMP #220+YPAL ; If A < 220, jump to scan13 to skip the following BCC scan13 ; instruction LDA #220+YPAL ; Set A = 220, so A is a maximum of 220 .scan13 SEC ; Set Y1 = A - SC2+1 SBC SC2+1 ; STA Y1 ; So this leaves Y1 alone unless Y1 + SC2+1 >= 220, in ; which case Y1 is clipped so that Y1 + SC2+1 = 220 (so ; this moves the "top" of the stick so that the ship dot ; doesn't go off the bottom of the screen) ; The ship is drawn on the scanner using up to three ; sprites - sprites Y, Y+1 and Y+2 ; ; Sprite Y is the end of the stick that's furthest from ; the ship dot (i.e. the "bottom" of the stick which ; appears to touch the 3D ellipse, though as we are ; drawing the stick below the ellipse, this is the part ; of the stick that is highest up the screen) ; ; Sprite Y+1 is the middle part of the stick ; ; Sprite Y+2 is the big ship dot at the end of the stick ; (i.e. the "top" of the stick, though as we are ; drawing the stick below the ellipse, this is the part ; of the stick that is furthest down the screen) ; ; If a stick is full height, then we show all three ; sprites, if a stick is medium height we display ; sprites Y+1 and Y+2, and if a stick is small we only ; display sprite Y+2 ; ; We always draw sprite Y+2, so first we concentrate on ; drawing sprites Y and Y+1, if required CMP #16 ; If Y1 < 16 then the stick is either medium or small, BCC scan14 ; so jump to scan14 to skip the following ; If we get here then the stick is full height, so we ; draw both sprite Y and sprite Y+1 LDA SC2 ; Set the x-coordinate of sprite Y to SC2 STA xSprite0,Y STA xSprite1,Y ; Set the x-coordinate of sprite Y+1 to SC2 LDA SC2+1 ; Set the y-coordinate of sprite Y to SC2+1, as SC2+1 STA ySprite0,Y ; contains the y-coordinate of the "bottom" of the stick CLC ; Set the y-coordinate of sprite Y+1 to SC2+1 + 8, which ADC #8 ; is eight pixels lower down the screen then sprite Y STA ySprite1,Y ; ; This stacks the sprites one below the other, as each ; sprite is eight pixels LDA attrSprite0,Y ; Set bit 5 of the attributes for sprite Y to ensure ORA #%00100000 ; that the following attribute is set: STA attrSprite0,Y ; ; * Bit 5 set = show behind background ; ; This makes sure we display the sprite behind the ; 3D ellipse, as the ship is below the ellipse in space STA attrSprite1,Y ; Set the attributes for sprite Y+1 to the same as those ; for sprite Y LDA SC2+1 ; Set SC2+1 = SC2+1 + 16 CLC ; ADC #16 ; This adds 16 to the y-coordinate of the "bottom" of STA SC2+1 ; the stick to give us the y-coordinate of the "top" ; sprite in the stick BNE scan16 ; Jump to scan16 to draw sprite Y+2 at the "top" of the ; stick (this BNE is effectively a JMP as A is never ; zero) .scan14 ; If we get here then the stick is either medium or ; small height CMP #8 ; If Y1 < 8 then the stick is small, so jump to scan15 BCC scan15 ; to skip the following ; If we get here then the stick is medium height, so we ; draw sprite Y+1 and hide sprite Y LDA #240 ; Hide sprite Y by setting its y-coordinate to 240, STA ySprite0,Y ; which moves it off the bottom of the screen LDA SC2 ; Set the x-coordinate of sprite Y+1 to SC2 STA xSprite1,Y LDA SC2+1 ; Set the y-coordinate of sprite Y+1 to SC2+1, as SC2+1 STA ySprite1,Y ; contains the y-coordinate of the bottom of the stick ; (which is the middle sprite in a medium height stick) LDA attrSprite0,Y ; Set bit 5 of the attributes for sprite Y to ensure ORA #%00100000 ; that the following attribute is set: STA attrSprite1,Y ; ; * Bit 5 set = show behind background ; ; This makes sure we display the sprite behind the ; 3D ellipse, as the ship is below the ellipse in space ; ; Note that we do this by copying the attributes we set ; up for sprite Y into sprite Y+1, clearing the bits as ; we do so LDA SC2+1 ; Set SC2+1 = SC2+1 + 8 ADC #7 ; STA SC2+1 ; This adds 8 to the y-coordinate of the "bottom" of the ; stick to give us the y-coordinate of the "top" sprite ; in the stick ; ; The addition works as we know the C flag is set, as we ; passed through a BCS above, so the ADC #7 actually ; adds 8 BNE scan16 ; Jump to scan16 to draw sprite Y+2 at the "top" of the ; stick (this BNE is effectively a JMP as A is never ; zero) .scan15 ; If we get here then the stick is small, so we hide ; sprites Y and Y+1, leaving just sprite Y+2 visible on ; the scanner LDA #240 ; Hide sprites Y and Y+1 from the scanner by setting STA ySprite0,Y ; their y-coordinates to 240, which moves them off the STA ySprite1,Y ; bottom of the screen .scan16 ; We now draw sprite Y+2, which contains the ship dot at ; the end of the stick LDA Y1 ; Set A = Y1 mod 8 AND #7 ; ; This gives us the height of the top part of the stick, ; as there are eight pixels in each sprite making up the ; stick, so this is the remainder after sprites Y and ; Y+1 are taken away CLC ; Set the pattern number for sprite Y+2 to 219 + A ADC #219 ; STA pattSprite2,Y ; Sprites 219 to 226 contain ship dots with trailing ; sticks, starting with the dot at the bottom of the ; pattern (in pattern 219) up to the dot at the top of ; the pattern (in pattern 226), so this sets the tile ; pattern for sprite Y+2 to the dot height given in A, ; which is the correct pattern for the top of the ship's ; stick LDA attrSprite0,Y ; Set bits 5 to 7 of the attributes for sprite Y to ORA #%11100000 ; ensure that the following attributes are set: STA attrSprite2,Y ; ; * Bit 5 set = show behind background ; * Bit 6 set = flip horizontally ; * Bit 7 set = flip vertically ; ; This makes sure we display the sprite behind the ; 3D ellipse, as the ship is below the ellipse in space, ; and that the dot end of the stick is at the bottom of ; the sprite, not the top ; ; Note that we do this by copying the attributes we set ; up for sprite Y into sprite Y+1, clearing the bits as ; we do so LDA SC2 ; Set the x-coordinate of sprite Y+2 to SC2 STA xSprite2,Y LDA SC2+1 ; Set the y-coordinate of sprite Y+2 to SC2+1, which by STA ySprite2,Y ; now contains the y-coordinate of the top of the stick, RTS ; Return from the subroutineName: SCAN [Show more] Type: Subroutine Category: Dashboard Summary: Display the current ship on the scanner Deep dive: Sprite usage in NES EliteContext: See this subroutine in context in the source code References: This subroutine is called as follows: * SCAN_b1 calls SCAN
This routine does a similar job to the routine of the same name in the BBC Master version of Elite, but the code is significantly different.
Arguments: INWK The ship's data block
[X]
Configuration variable YPAL = 6 AND _PAL
A margin of 6 pixels that is applied to a number of y-coordinates for the PAL version only (as the PAL version has a taller screen than NTSC)
[X]
Variable attrSprite0 in workspace Sprite buffer
Attributes for sprite 0
[X]
Variable attrSprite1 in workspace Sprite buffer
Attributes for sprite 1
[X]
Variable attrSprite2 in workspace Sprite buffer
Attributes for sprite 2
[X]
Variable pattSprite2 in workspace Sprite buffer
Pattern number for sprite 2
[X]
Label scan1 is local to this routine
[X]
Label scan10 is local to this routine
[X]
Label scan11 is local to this routine
[X]
Label scan12 is local to this routine
[X]
Label scan13 is local to this routine
[X]
Label scan14 is local to this routine
[X]
Label scan15 is local to this routine
[X]
Label scan16 is local to this routine
[X]
Label scan2 is local to this routine
[X]
Label scan3 is local to this routine
[X]
Label scan4 is local to this routine
[X]
Label scan5 is local to this routine
[X]
Label scan6 is local to this routine
[X]
Label scan7 is local to this routine
[X]
Label scan8 is local to this routine
[X]
Label scan9 is local to this routine
[X]
Variable scannerColour in workspace WP
The colour of each ship number on the scanner, stored as the sprite palette number for that ship's three scanner sprites
[X]
Variable xSprite0 in workspace Sprite buffer
Screen x-coordinate for sprite 0
[X]
Variable xSprite1 in workspace Sprite buffer
Screen x-coordinate for sprite 1
[X]
Variable xSprite2 in workspace Sprite buffer
Screen x-coordinate for sprite 2
[X]
Variable ySprite0 in workspace Sprite buffer
Screen y-coordinate for sprite 0
[X]
Variable ySprite1 in workspace Sprite buffer
Screen y-coordinate for sprite 1
[X]
Variable ySprite2 in workspace Sprite buffer
Screen y-coordinate for sprite 2