Skip to navigation

BBC Micro Elite

Stardust: STARS6

Name: STARS6 [View in context] Type: Subroutine Category: Stardust Summary: Process the stardust for the rear view
This routine is very similar to STARS1, which processes stardust for the front view. The main difference is that the direction of travel is reversed, so the signs in the calculations are different, as well as the order of the first batch of calculations. When a stardust particle falls away into the far distance, it is removed from the screen and its memory is recycled as a new particle, positioned randomly along one of the four edges of the screen. See STARS1 for an explanation of the maths used in this routine. The calculations are as follows: 1. q = 64 * speed / z_hi 2. x = x - |x_hi| * q 3. y = y - |y_hi| * q 4. z = z + speed * 64 5. y = y - alpha * x / 256 6. x = x + alpha * y / 256 7. x = x - 2 * (beta * y / 256) ^ 2 8. y = y + beta * 256
.STARS6 LDY NOSTM \ Set Y to the current number of stardust particles, so \ we can use it as a counter through all the stardust .STL6 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 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) = x - (A P) STA XX+1 \ First we do the low bytes: LDA SXL,Y \ SBC P \ XX(1 0) = x_lo - (A P) STA XX LDA X1 \ And then we do the high bytes: SBC XX+1 \ STA XX+1 \ XX(1 0) = (x_hi 0) - XX(1 0) \ \ so we get our result: \ \ XX(1 0) = x - (A P) \ = x - |x_hi| * Q \ \ which is result 2 above, and we also have: 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) = y - (A P) STA YY+1 \ First we do the low bytes with: LDA SYL,Y \ SBC P \ YY+1 = A STA YY \ R = YY = y_lo - P STA R \ \ so we get this: \ \ (? R) = YY(1 0) = y_lo - (A P) LDA Y1 \ And then we do the high bytes with: SBC YY+1 \ STA YY+1 \ S = YY+1 = y_hi - YY+1 STA S \ \ so we get our result: \ \ (S R) = YY(1 0) = (y_hi y_lo) - (A P) \ = y - |y_hi| * Q \ \ which is result 3 above, and (S R) is set to the new \ value of y LDA SZL,Y \ We now calculate the following: ADC 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 \ ADC 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 4 above: \ \ z = z + DELT4(1 0) \ = z + speed * 64 LDA XX+1 \ EOR x with the correct sign of the roll angle alpha, EOR ALP2 \ so A has the opposite sign to the roll angle 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+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 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 LDA YY+1 \ Set A to y_hi and set it to the flipped sign of beta EOR BET2+1 LDX BET1 \ Fetch the pitch magnitude into X 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 LDA XX+1 \ Set S = x_hi STA S EOR #%10000000 \ Flip the sign of A, so A now contains -x JSR MUT1 \ Call MUT1 to calculate: \ \ R = XX = x_lo \ \ (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) = -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 STA S \EOR #128 \ These instructions are commented out in the original \JSR MAD \ source \STA S \STX R LDA #0 \ Set P = 0 STA P LDA BETA \ Set A = beta, so (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 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| >= 110 then jump to KILL6 to recycle this CMP #110 \ particle, as it's gone off the top or bottom of the BCS KILL6 \ screen, and re-join at STC6 with the new particle LDA SZ,Y \ If z_hi >= 160 then jump to KILL6 to recycle this CMP #160 \ particle, as it's so far away that it's too far to BCS KILL6 \ see, and re-join at STC1 with the new particle STA ZZ \ Set ZZ to the z-coordinate in z_hi .STC6 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 ST3 \ If we have just done the last particle, skip the next \ instruction to return from the subroutine JMP STL6 \ We have more stardust to process, so jump back up to \ STL6 for the next particle .ST3 RTS \ Return from the subroutine .KILL6 JSR DORND \ Set A and X to random numbers AND #%01111111 \ Clear the sign bit of A to get |A| ADC #10 \ Make sure A is at least 10 and store it in z_hi and STA SZ,Y \ ZZ, so the new particle starts close to us STA ZZ LSR A \ Divide A by 2 and randomly set the C flag BCS ST4 \ Jump to ST4 half the time LSR A \ Randomly set the C flag again LDA #252 \ Set A to either +126 or -126 (252 >> 1) depending on ROR A \ the C flag, as this is a sign-magnitude number with \ the C flag rotated into its sign bit STA X1 \ Set x_hi and X1 to A, so this particle starts on STA SX,Y \ either the left or right edge of the screen JSR DORND \ Set A and X to random numbers STA Y1 \ Set y_hi and Y1 to random numbers, so the particle STA SY,Y \ starts anywhere along either the left or right edge JMP STC6 \ Jump up to STC6 to draw this new particle .ST4 JSR DORND \ Set A and X to random numbers STA X1 \ Set x_hi and X1 to random numbers, so the particle STA SX,Y \ starts anywhere along the x-axis LSR A \ Randomly set the C flag LDA #230 \ Set A to either +115 or -115 (230 >> 1) depending on ROR A \ the C flag, as this is a sign-magnitude number with \ the C flag rotated into its sign bit STA Y1 \ Set y_hi and Y1 to A, so the particle starts anywhere STA SY,Y \ along either the top or bottom edge of the screen BNE STC6 \ Jump up to STC6 to draw this new particle (this BNE is \ effectively a JMP as A will never be zero)