Skip to navigation

BBC Micro Elite

Drawing planets: PL9 (Part 3 of 3)

Name: PL9 (Part 3 of 3) [View in context] Type: Subroutine Category: Drawing planets Summary: Draw the planet's crater
Draw the planet's crater. Arguments: K(1 0) The planet's radius K3(1 0) Pixel x-coordinate of the centre of the planet K4(1 0) Pixel y-coordinate of the centre of the planet INWK The planet's ship data block
.PL26 LDA INWK+20 \ Set A = roofv_z_hi BMI PL20 \ If A is negative, the crater is on the far side of the \ planet, so return from the subroutine (as PL2 \ contains an RTS) LDX #15 \ Set X = 15, so the following call to PLS3 operates on \ roofv JSR PLS3 \ Call PLS3 to calculate: \ \ (Y A P) = 222 * roofv_x / z \ \ to give the x-coordinate of the crater offset and \ increment X to point to roofv_y for the next call CLC \ Calculate: ADC K3 \ STA K3 \ K3(1 0) = (Y A) + K3(1 0) \ = 222 * roofv_x / z + x-coordinate of planet \ centre \ \ starting with the high bytes TYA \ And then doing the low bytes, so now K3(1 0) contains ADC K3+1 \ the x-coordinate of the crater offset plus the planet STA K3+1 \ centre to give the x-coordinate of the crater's centre JSR PLS3 \ Call PLS3 to calculate: \ \ (Y A P) = 222 * roofv_y / z \ \ to give the y-coordinate of the crater offset STA P \ Calculate: LDA K4 \ SEC \ K4(1 0) = K4(1 0) - (Y A) SBC P \ = 222 * roofv_x / z - y-coordinate of planet STA K4 \ centre \ \ starting with the low bytes STY P \ And then doing the low bytes, so now K4(1 0) contains LDA K4+1 \ the y-coordinate of the crater offset plus the planet SBC P \ centre to give the y-coordinate of the crater's centre STA K4+1 LDX #9 \ Set X = 9, so the following call to PLS1 operates on \ nosev JSR PLS1 \ Call PLS1 to calculate the following: \ \ (Y A) = nosev_x / z \ \ and increment X to point to nosev_y for the next call LSR A \ Set (XX16 K2) = (Y A) / 2 STA K2 STY XX16 JSR PLS1 \ Call PLS1 to calculate the following: \ \ (Y A) = nosev_y / z \ \ and increment X to point to nosev_z for the next call LSR A \ Set (XX16+1 K2+1) = (Y A) / 2 STA K2+1 STY XX16+1 LDX #21 \ Set X = 21, so the following call to PLS1 operates on \ sidev JSR PLS1 \ Call PLS1 to calculate the following: \ \ (Y A) = sidev_x / z \ \ and increment X to point to sidev_y for the next call LSR A \ Set (XX16+2 K2+2) = (Y A) / 2 STA K2+2 STY XX16+2 JSR PLS1 \ Call PLS1 to calculate the following: \ \ (Y A) = sidev_y / z \ \ and increment X to point to sidev_z for the next call LSR A \ Set (XX16+3 K2+3) = (Y A) / 2 STA K2+3 STY XX16+3 LDA #64 \ Set TGT = 64, so we draw a full circle in the call to STA TGT \ PLS22 below LDA #0 \ Set CNT2 = 0 as we are drawing a full circle, so we STA CNT2 \ don't need to apply an offset JMP PLS22 \ Jump to PLS22 to draw the crater, returning from the \ subroutine with a tail call }