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BBC Micro Elite

Drawing suns: SUN (Part 1 of 4)

Name: SUN (Part 1 of 4) [View in context] Type: Subroutine Category: Drawing suns Summary: Draw the sun: Set up all the variables needed Deep dive: Drawing the sun
Draw a new sun with radius K at pixel coordinate (K3, K4), removing the old sun if there is one. This routine is used to draw the sun, as well as the star systems on the Short-range Chart. The first part sets up all the variables needed to draw the new sun. Arguments: K The new sun's radius K3(1 0) Pixel x-coordinate of the centre of the new sun K4(1 0) Pixel y-coordinate of the centre of the new sun SUNX(1 0) The x-coordinate of the vertical centre axis of the old sun (the one currently on-screen) Other entry points: RTS2 Contains an RTS
JMP WPLS \ Jump to WPLS to remove the old sun from the screen. We \ only get here via the BCS just after the SUN entry \ point below, when there is no new sun to draw .PLF3 \ This is called from below to negate X and set A to \ &FF, for when the new sun's centre is off the bottom \ of the screen (so we don't need to draw its bottom \ half) TXA \ Negate X using two's complement, so X = ~X + 1 EOR #%11111111 \ CLC \ We do this because X is negative at this point, as it ADC #1 \ is calculated as 191 - the y-coordinate of the sun's TAX \ centre, and the centre is off the bottom of the \ screen, past 191. So we negate it to make it positive .PLF17 \ This is called from below to set A to &FF, for when \ the new sun's centre is right on the bottom of the \ screen (so we don't need to draw its bottom half) LDA #&FF \ Set A = &FF JMP PLF5 \ Jump to PLF5 .SUN LDA #1 \ Set LSX = 1 to indicate the sun line heap is about to STA LSX \ be filled up JSR CHKON \ Call CHKON to check whether any part of the new sun's \ circle appears on-screen, and of it does, set P(2 1) \ to the maximum y-coordinate of the new sun on-screen BCS PLF3-3 \ If CHKON set the C flag then the new sun's circle does \ not appear on-screen, so jump to WPLS (via the JMP at \ the top of this routine) to remove the sun from the \ screen, returning from the subroutine using a tail \ call LDA #0 \ Set A = 0 LDX K \ Set X = K = radius of the new sun CPX #96 \ If X >= 96, set the C flag and rotate it into bit 0 ROL A \ of A, otherwise rotate a 0 into bit 0 CPX #40 \ If X >= 40, set the C flag and rotate it into bit 0 ROL A \ of A, otherwise rotate a 0 into bit 0 CPX #16 \ If X >= 16, set the C flag and rotate it into bit 0 ROL A \ of A, otherwise rotate a 0 into bit 0 \ By now, A contains the following: \ \ * If radius is 96-255 then A = %111 = 7 \ \ * If radius is 40-95 then A = %11 = 3 \ \ * If radius is 16-39 then A = %1 = 1 \ \ * If radius is 0-15 then A = %0 = 0 \ \ The value of A determines the size of the new sun's \ ragged fringes - the bigger the sun, the bigger the \ fringes .PLF18 STA CNT \ Store the fringe size in CNT \ We now calculate the highest pixel y-coordinate of the \ new sun, given that P(2 1) contains the 16-bit maximum \ y-coordinate of the new sun on-screen LDA #2*Y-1 \ #Y is the y-coordinate of the centre of the mode 4 \ space view, so this sets Y to the y-coordinate of the \ bottom of the space view, i.e. 191 LDX P+2 \ If P+2 is non-zero, the maximum y-coordinate is off BNE PLF2 \ the bottom of the screen, so skip to PLF2 with A = 191 CMP P+1 \ If A < P+1, the maximum y-coordinate is underneath the BCC PLF2 \ the dashboard, so skip to PLF2 with A = 191 LDA P+1 \ Set A = P+1, the low byte of the maximum y-coordinate \ of the sun on-screen BNE PLF2 \ If A is non-zero, skip to PLF2 as it contains the \ value we are after LDA #1 \ Otherwise set A = 1, the top line of the screen .PLF2 STA TGT \ Set TGT to A, the maximum y-coordinate of the sun on \ screen \ We now calculate the number of lines we need to draw \ and the direction in which we need to draw them, both \ from the centre of the new sun LDA #2*Y-1 \ Set (A X) = y-coordinate of bottom of screen - K4(1 0) SEC \ SBC K4 \ Starting with the low bytes TAX LDA #0 \ And then doing the high bytes, so (A X) now contains SBC K4+1 \ the number of lines between the centre of the sun and \ the bottom of the screen. If it is positive then the \ centre of the sun is above the bottom of the screen, \ if it is negative then the centre of the sun is below \ the bottom of the screen BMI PLF3 \ If A < 0, then this means the new sun's centre is off \ the bottom of the screen, so jump up to PLF3 to negate \ the height in X (so it becomes positive), set A to &FF \ and jump down to PLF5 BNE PLF4 \ If A > 0, then the new sun's centre is at least a full \ screen above the bottom of the space view, so jump \ down to PLF4 to set X = radius and A = 0 INX \ Set the flags depending on the value of X DEX BEQ PLF17 \ If X = 0 (we already know A = 0 by this point) then \ jump up to PLF17 to set A to &FF before jumping down \ to PLF5 CPX K \ If X < the radius in K, jump down to PLF5, so if BCC PLF5 \ X >= the radius in K, we set X = radius and A = 0 .PLF4 LDX K \ Set X to the radius LDA #0 \ Set A = 0 .PLF5 STX V \ Store the height in V STA V+1 \ Store the direction in V+1 LDA K \ Set (A P) = K * K JSR SQUA2 STA K2+1 \ Set K2(1 0) = (A P) = K * K LDA P STA K2 \ By the time we get here, the variables should be set \ up as shown in the header for part 3 below