.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) ; ; This happens when the y-coordinate of the centre of ; the sun is bigger than the y-coordinate of the bottom ; of the space view TXA ; Negate X using two's complement, so A = ~X + 1 EOR #%11111111 CLC ADC #1 CMP K ; If A >= K then the centre of the sun is further BCS PL40 ; off-screen than the radius of the sun in K, which ; means the sun is too far away from the screen to be ; visible and there is nothing to draw, to jump to PL40 ; to return from the subroutine TAX ; Set X to the negated value in A, so X = ~X + 1 .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 nmiCounter ; Set the random number seed to a fairly random state STA RAND ; that's based on the NMI counter (which increments ; every VBlank, so will be pretty random) JSR CHKON ; Call CHKON to check whether any part of the new sun's ; circle appears on-screen, and if it does, set P(2 1) ; to the maximum y-coordinate of the new sun on-screen BCS PL40 ; If CHKON set the C flag then the new sun's circle does ; not appear on-screen, which means there is nothing to ; draw, so jump to PL40 to return from the subroutine LDA #0 ; Set A = 0 LDX K ; Set X = K = radius of the new sun BEQ PL40 ; If the radius of the new sun is zero then there is ; nothing to draw, so jump to PL40 to return from the ; subroutine 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 Yx2M1 ; Set Y to the y-coordinate of the bottom of the space ; view 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 set ; to the y-coordinate of the bottom of the space view CMP P+1 ; If A < P+1, the maximum y-coordinate is underneath the BCC PLF2 ; dashboard, so skip to PLF2 with A set to the ; y-coordinate of the bottom of the space view 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 Yx2M1 ; 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 the PLFL subroutineName: SUN (Part 1 of 2) [Show more] Type: Subroutine Category: Drawing suns Summary: Draw the sun: Set up all the variables needed to draw the sun Deep dive: Drawing the sunContext: See this subroutine in context in the source code References: This subroutine is called as follows: * PLANET calls SUN * SUN_b1 calls 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)
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Subroutine CHKON (category: Drawing circles)
Check whether any part of a circle appears on the extended screen
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Label PLF17 is local to this routine
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Label PLF2 is local to this routine
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Label PLF3 is local to this routine
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Label PLF4 is local to this routine
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Label PLF5 is local to this routine
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Subroutine SQUA2 (category: Maths (Arithmetic))
Calculate (A P) = A * A
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Variable nmiCounter in workspace WP
A counter that increments every VBlank at the start of the NMI handler