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Elite on the BBC Micro

Flight: DCS1 (Master version)

Name: DCS1 [View in context] Type: Subroutine Category: Flight Summary: Calculate the vector from the ideal docking position to the ship
This routine is called by the docking computer routine in DOCKIT. It works out the vector between the ship and the ideal docking position, which is straight in front of the docking slot, but some distance away. Specifically, it calculates the following: * K3(2 1 0) = K3(2 1 0) - nosev_x_hi * 4 * K3(5 4 3) = K3(5 4 3) - nosev_y_hi * 4 * K3(8 7 6) = K3(8 7 6) - nosev_x_hi * 4 where K3 is the vector from the station to the ship, and nosev is the nose vector for the space station. The nose vector points from the centre of the station through the slot, so -nosev * 4 is the vector from a point in front of the docking slot, but some way from the station, back to the centre of the station. Adding this to the vector from the station to the ship gives the vector from the point in front of the station to the ship. In practice, this routine is called twice, so the ideal docking position is actually at a distance of 8 unit vectors from the centre of the station. Back in DOCKIT, we flip this vector round to get the vector from the ship to the point in front of the station slot. Arguments: K3 The vector from the station to the ship Returns: K3 The vector from the ship to the ideal docking position (4 unit vectors from the centre of the station for each call to DCS1, so two calls will return the vector to a point that's 8 unit vectors from the centre of the station)
.DCS1 JSR P%+3 \ Run the following routine twice, so the subtractions \ are all * 4 LDA K%+NI%+10 \ Set A to the space station's byte #10, nosev_x_hi LDX #0 \ Set K3(2 1 0) = K3(2 1 0) - A * 2 JSR TAS7 \ = K3(2 1 0) - nosev_x_hi * 2 LDA K%+NI%+12 \ Set A to the space station's byte #12, nosev_y_hi LDX #3 \ Set K3(5 4 3) = K3(5 4 3) - A * 2 JSR TAS7 \ = K3(5 4 3) - nosev_y_hi * 2 LDA K%+NI%+14 \ Set A to the space station's byte #14, nosev_z_hi LDX #6 \ Set K3(8 7 6) = K3(8 7 6) - A * 2 \ = K3(8 7 6) - nosev_x_hi * 2 .TAS7 \ This routine subtracts A * 2 from one of the K3 \ coordinates, as determined by the value of X: \ \ * X = 0, set K3(2 1 0) = K3(2 1 0) - A * 2 \ \ * X = 3, set K3(5 4 3) = K3(5 4 3) - A * 2 \ \ * X = 6, set K3(8 7 6) = K3(8 7 6) - A * 2 \ \ Let's document it for X = 0, i.e. K3(2 1 0) ASL A \ Shift A left one place and move the sign bit into the \ C flag, so A = |A * 2| STA R \ Set R = |A * 2| LDA #0 \ Rotate the sign bit of A from the C flag into the sign ROR A \ bit of A, so A is now just the sign bit from the \ original value of A. This also clears the C flag EOR #%10000000 \ Flip the sign bit of A, so it has the sign of -A EOR K3+2,X \ Give A the correct sign of K3(2 1 0) * -A BMI TS71 \ If the sign of K3(2 1 0) * -A is negative, jump to \ TS71, as K3(2 1 0) and A have the same sign \ If we get here then K3(2 1 0) and A have different \ signs, so we can add them to do the subtraction LDA R \ Set K3(2 1 0) = K3(2 1 0) + R ADC K3,X \ = K3(2 1 0) + |A * 2| STA K3,X \ \ starting with the low bytes BCC TS72 \ If the above addition didn't overflow, we have the \ result we want, so jump to TS72 to return from the \ subroutine INC K3+1,X \ The above addition overflowed, so increment the high \ byte of K3(2 1 0) .TS72 RTS \ Return from the subroutine .TS71 \ If we get here, then K3(2 1 0) and A have the same \ sign LDA K3,X \ Set K3(2 1 0) = K3(2 1 0) - R SEC \ = K3(2 1 0) - |A * 2| SBC R \ STA K3,X \ starting with the low bytes LDA K3+1,X \ And then the high bytes SBC #0 STA K3+1,X BCS TS72 \ If the subtraction didn't underflow, we have the \ result we want, so jump to TS72 to return from the \ subroutine LDA K3,X \ Negate the result in K3(2 1 0) by flipping all the EOR #%11111111 \ bits and adding 1, i.e. using two's complement to ADC #1 \ give it the opposite sign, starting with the low STA K3,X \ bytes LDA K3+1,X \ Then doing the high bytes EOR #%11111111 ADC #0 STA K3+1,X LDA K3+2,X \ And finally, flipping the sign bit EOR #%10000000 STA K3+2,X JMP TS72 \ Jump to TS72 to return from the subroutine