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BBC Micro Elite

Stardust: STARS1

Name: STARS1 [View in context] Type: Subroutine Category: Stardust Summary: Process the stardust for the front view
This moves the stardust towards us according to our speed (so the dust rushes past us), and applies our current pitch and roll to each particle of dust, so the stardust moves correctly when we steer our ship. When a stardust particle rushes past us and falls off the side of the screen, its memory is recycled as a new particle that's positioned randomly on-screen.
.STARS1 { LDY NOSTM \ Set Y to the current number of stardust particles, so \ we can use it as a counter through all the stardust \ In the following, we're going to refer to the 16-bit \ space coordinates of the current particle of stardust \ (i.e. the Y-th particle) like this: \ \ x = (x_hi x_lo) \ y = (y_hi y_lo) \ z = (z_hi z_lo) \ \ These values are stored in (SX+Y SXL+Y), (SY+Y SYL+Y) \ and (SZ+Y SZL+Y) respectively .^STL1 JSR DV42 \ Call DV42 to set the following: \ \ (P R) = 256 * DELTA / z_hi \ = 256 * speed / z_hi \ \ The maximum value returned is P = 2 and R = 128 (see \ DV42 for an explanation) LDA R \ Set A = R, so now: \ \ (P A) = 256 * speed / z_hi LSR P \ Rotate (P A) right by 2 places, which sets P = 0 (as P ROR A \ has a maximum value of 2) and leaves: LSR P \ ROR A \ A = 64 * speed / z_hi ORA #1 \ Make sure A is at least 1, and store it in Q, so we STA Q \ now have result 1 above: \ \ Q = 64 * speed / z_hi LDA SZL,Y \ We now calculate the following: SBC DELT4 \ STA SZL,Y \ (z_hi z_lo) = (z_hi z_lo) - DELT4(1 0) \ \ starting with the low bytes LDA SZ,Y \ And then we do the high bytes STA ZZ \ SBC DELT4+1 \ We also set ZZ to the original value of z_hi, which we STA SZ,Y \ use below to remove the existing particle \ \ So now we have result 2 above: \ \ z = z - DELT4(1 0) \ = z - speed * 64 JSR MLU1 \ Call MLU1 to set: \ \ Y1 = y_hi \ \ (A P) = |y_hi| * Q \ \ So Y1 contains the original value of y_hi, which we \ use below to remove the existing particle \ We now calculate: \ \ (S R) = YY(1 0) = (A P) + y STA YY+1 \ First we do the low bytes with: LDA P \ ADC SYL,Y \ YY+1 = A STA YY \ R = YY = P + y_lo STA R \ \ so we get this: \ \ (? R) = YY(1 0) = (A P) + y_lo LDA Y1 \ And then we do the high bytes with: ADC YY+1 \ STA YY+1 \ S = YY+1 = y_hi + YY+1 STA S \ \ so we get our result: \ \ (S R) = YY(1 0) = (A P) + (y_hi y_lo) \ = |y_hi| * Q + y \ \ which is result 3 above, and (S R) is set to the new \ value of y LDA SX,Y \ Set X1 = A = x_hi STA X1 \ \ So X1 contains the original value of x_hi, which we \ use below to remove the existing particle JSR MLU2 \ Set (A P) = |x_hi| * Q \ We now calculate: \ \ XX(1 0) = (A P) + x STA XX+1 \ First we do the low bytes: LDA P \ ADC SXL,Y \ XX(1 0) = (A P) + x_lo STA XX LDA X1 \ And then we do the high bytes: ADC XX+1 \ STA XX+1 \ XX(1 0) = XX(1 0) + (x_hi 0) \ \ so we get our result: \ \ XX(1 0) = (A P) + x \ = |x_hi| * Q + x \ \ which is result 4 above, and we also have: \ \ A = XX+1 = (|x_hi| * Q + x) / 256 \ \ i.e. A is the new value of x, divided by 256 EOR ALP2+1 \ EOR with the flipped sign of the roll angle alpha, so \ A has the opposite sign to the flipped roll angle \ alpha, i.e. it gets the same sign as alpha JSR MLS1 \ Call MLS1 to calculate: \ \ (A P) = A * ALP1 \ = (x / 256) * alpha JSR ADD \ Call ADD to calculate: \ \ (A X) = (A P) + (S R) \ = (x / 256) * alpha + y \ = y + alpha * x / 256 STA YY+1 \ Set YY(1 0) = (A X) to give: STX YY \ \ YY(1 0) = y + alpha * x / 256 \ \ which is result 5 above, and we also have: \ \ A = YY+1 = y + alpha * x / 256 \ \ i.e. A is the new value of y, divided by 256 EOR ALP2 \ EOR A with the correct sign of the roll angle alpha, \ so A has the opposite sign to the roll angle alpha JSR MLS2 \ Call MLS2 to calculate: \ \ (S R) = XX(1 0) \ = x \ \ (A P) = A * ALP1 \ = -y / 256 * alpha JSR ADD \ Call ADD to calculate: \ \ (A X) = (A P) + (S R) \ = -y / 256 * alpha + x STA XX+1 \ Set XX(1 0) = (A X), which gives us result 6 above: STX XX \ \ x = x - alpha * y / 256 LDX BET1 \ Fetch the pitch magnitude into X LDA YY+1 \ Set A to y_hi and set it to the flipped sign of beta EOR BET2+1 JSR MULTS-2 \ Call MULTS-2 to calculate: \ \ (A P) = X * A \ = -beta * y_hi STA Q \ Store the high byte of the result in Q, so: \ \ Q = -beta * y_hi / 256 JSR MUT2 \ Call MUT2 to calculate: \ \ (S R) = XX(1 0) = x \ \ (A P) = Q * A \ = (-beta * y_hi / 256) * (-beta * y_hi / 256) \ = (beta * y / 256) ^ 2 ASL P \ Double (A P), store the top byte in A and set the C ROL A \ flag to bit 7 of the original A, so this does: STA T \ \ (T P) = (A P) << 1 \ = 2 * (beta * y / 256) ^ 2 LDA #0 \ Set bit 7 in A to the sign bit from the A in the ROR A \ calculation above and apply it to T, so we now have: ORA T \ \ (A P) = (A P) * 2 \ = 2 * (beta * y / 256) ^ 2 \ \ with the doubling retaining the sign of (A P) JSR ADD \ Call ADD to calculate: \ \ (A X) = (A P) + (S R) \ = 2 * (beta * y / 256) ^ 2 + x STA XX+1 \ Store the high byte A in XX+1 TXA STA SXL,Y \ Store the low byte X in x_lo \ So (XX+1 x_lo) now contains: \ \ x = x + 2 * (beta * y / 256) ^ 2 \ \ which is result 7 above LDA YY \ Set (S R) = YY(1 0) = y STA R LDA YY+1 \JSR MAD \ These instructions are commented out in the original \STA S \ source \STX R STA S LDA #0 \ Set P = 0 STA P LDA BETA \ Set A = -beta, so: EOR #%10000000 \ \ (A P) = (-beta 0) \ = -beta * 256 JSR PIX1 \ Call PIX1 to calculate the following: \ \ (YY+1 y_lo) = (A P) + (S R) \ = -beta * 256 + y \ \ i.e. y = y - beta * 256, which is result 8 above \ \ PIX1 also draws a particle at (X1, Y1) with distance \ ZZ, which will remove the old stardust particle, as we \ set X1, Y1 and ZZ to the original values for this \ particle during the calculations above \ We now have our newly moved stardust particle at \ x-coordinate (XX+1 x_lo) and y-coordinate (YY+1 y_lo) \ and distance z_hi, so we draw it if it's still on \ screen, otherwise we recycle it as a new bit of \ stardust and draw that LDA XX+1 \ Set X1 and x_hi to the high byte of XX in XX+1, so STA X1 \ the new x-coordinate is in (x_hi x_lo) and the high STA SX,Y \ byte is in X1 AND #%01111111 \ If |x_hi| >= 120 then jump to KILL1 to recycle this CMP #120 \ particle, as it's gone off the side of the screen, BCS KILL1 \ and re-join at STC1 with the new particle LDA YY+1 \ Set Y1 and y_hi to the high byte of YY in YY+1, so STA SY,Y \ the new x-coordinate is in (y_hi y_lo) and the high STA Y1 \ byte is in Y1 AND #%01111111 \ If |y_hi| >= 120 then jump to KILL1 to recycle this CMP #120 \ particle, as it's gone off the top or bottom of the BCS KILL1 \ screen, and re-join at STC1 with the new particle LDA SZ,Y \ If z_hi < 16 then jump to KILL1 to recycle this CMP #16 \ particle, as it's so close that it's effectively gone BCC KILL1 \ past us, and re-join at STC1 with the new particle STA ZZ \ Set ZZ to the z-coordinate in z_hi .STC1 JSR PIXEL2 \ Draw a stardust particle at (X1,Y1) with distance ZZ, \ i.e. draw the newly moved particle at (x_hi, y_hi) \ with distance z_hi DEY \ Decrement the loop counter to point to the next \ stardust particle BEQ P%+5 \ If we have just done the last particle, skip the next \ instruction to return from the subroutine JMP STL1 \ We have more stardust to process, so jump back up to \ STL1 for the next particle RTS \ Return from the subroutine .KILL1 \ Our particle of stardust just flew past us, so let's \ recycle that particle, starting it at a random \ position that isn't too close to the centre point JSR DORND \ Set A and X to random numbers ORA #4 \ Make sure A is at least 4 and store it in Y1 and y_hi, STA Y1 \ so the new particle starts at least 4 pixels above or STA SY,Y \ below the centre of the screen JSR DORND \ Set A and X to random numbers ORA #8 \ Make sure A is at least 8 and store it in X1 and x_hi, STA X1 \ so the new particle starts at least 8 pixels either STA SX,Y \ side of the centre of the screen JSR DORND \ Set A and X to random numbers ORA #144 \ Make sure A is at least 144 and store it in ZZ and STA SZ,Y \ z_hi so the new particle starts in the far distance STA ZZ LDA Y1 \ Set A to the new value of y_hi. This has no effect as \ STC1 starts with a jump to PIXEL2, which starts with a \ LDA instruction JMP STC1 \ Jump up to STC1 to draw this new particle }