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Drawing planets: PL9 (Part 3 of 3)

[NES version, Bank 1]

Name: PL9 (Part 3 of 3) [Show more] Type: Subroutine Category: Drawing planets Summary: Draw the planet's crater Deep dive: Drawing craters
Context: See this subroutine in context in the source code References: No direct references to this subroutine in this source file

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_y / 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 ellipse in the call to STA TGT ; PLS22 below LDA #0 ; Set CNT2 = 0 as we are drawing a full ellipse, so we STA CNT2 ; don't need to apply an offset JMP PLS22 ; Jump to PLS22 to draw the crater, returning from the ; subroutine using a tail call