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BBC Micro Elite

Dashboard: SCAN (6502SP version)

Name: SCAN [View in context] Type: Subroutine Category: Dashboard Summary: Display the current ship on the scanner Deep dive: The 3D scanner
This is used both to display a ship on the scanner, and to erase it again. Arguments: INWK The ship's data block
.SCAN 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 SC5 \ on the scanner, so return from the subroutine (as SC5 \ contains an RTS) LDX TYPE \ Fetch the ship's type from TYPE into X BMI SC5 \ 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 SC5 \ 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 SCANcol \ Store the scanner colour in SCANcol so it can be sent \ to the I/O processor with the #onescan command 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 SC5 AND #%11000000 \ contains an RTS) BNE SC5 \ 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 (see the deep dive on "The 3D \ scanner" for an explanation): \ \ X1 = 123 + (x_sign x_hi) LDA INWK+1 \ Set 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 subtract ADC #1 \ 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 A = 123 + x_hi STA SCANx1 \ Store the x-coordinate in SCANx1 so it can be sent \ to the I/O processor with the #onescan command \ Next, we convert the z_hi coordinate of the ship into \ the y-coordinate of the base of the ship's stick, \ like this (see the deep dive on "The 3D scanner" for \ an explanation): \ \ 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 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 #35 \ Set A = 35 + A to give a number in the range 20 to 50 EOR #%11111111 \ Flip all the bits and store in SC, so SC is in the STA SC \ range 205 to 235, with a higher z_hi giving a lower SC \ Now for the stick height, which we calculate using the \ following (see the deep dive on "The 3D scanner" for \ an explanation): \ \ 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 FIXIT \ If the result has bit 0 clear, then the result has \ overflowed and is bigger than 256, so jump to FIXIT to \ set A to the maximum allowed value of 246 (this \ instruction isn't required as we test both the maximum \ and minimum below, but it might save a few cycles) CMP #194 \ If A >= 194, skip the following instruction, as 194 is BCS P%+4 \ the minimum allowed value of A LDA #194 \ A < 194, so set A to 194, the minimum allowed value \ for the y-coordinate of our ship's dot CMP #247 \ If A < 247, skip the following instruction, as 246 is BCC P%+4 \ the maximum allowed value of A .FIXIT LDA #246 \ A >= 247, so set A to 246, the maximum allowed value \ for the y-coordinate of our ship's dot STA SCANy1 \ Store the y-coordinate in SCANy1 so it can be sent \ to the I/O processor with the #onescan command 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 STA SCANlen \ Store the stick height in SCANlen so it can be sent \ to the I/O processor with the #onescan command ROR SCANflg \ Rotate the C flag into bit 7 of SCANflg, so bit 7 is \ the sign bit of the stick length .SC48 LDX #LO(SCANpars) \ Set (Y X) to point to the SCANpars parameter block LDY #HI(SCANpars) LDA #onescan \ Send a #onescan command to the I/O processor to draw JMP OSWORD \ the ship on the scanner, returning from the subroutine \ using a tail call