Skip to navigation

BBC Micro Elite

Drawing planets: PLS1

Name: PLS1 [View in context] Type: Subroutine Category: Drawing planets Summary: Calculate (Y A) = nosev_x / z
Calculate the following division of a specified value from one of the orientation vectors (in this example, nosev_x): (Y A) = nosev_x / z where z is the z-coordinate of the planet from INWK. The result is an 8-bit magnitude in A, with maximum value 254, and just a sign bit (bit 7) in Y. Arguments: X Determines which of the INWK orientation vectors to divide: * X = 9, 11, 13: divides nosev_x, nosev_y, nosev_z * X = 15, 17, 19: divides roofv_x, roofv_y, roofv_z * X = 21, 23, 25: divides sidev_x, sidev_y, sidev_z INWK The planet's ship data block Returns: A The result as an 8-bit magnitude with maximum value 254 Y The sign of the result in bit 7 K+3 Also the sign of the result in bit 7 X X gets incremented by 2 so it points to the next coordinate in this orientation vector (so consecutive calls to the routine will start with x, then move onto y and then z)
.PLS1 LDA INWK,X \ Set P = nosev_x_lo STA P LDA INWK+1,X \ Set P+1 = |nosev_x_hi| AND #%01111111 STA P+1 LDA INWK+1,X \ Set A = sign bit of nosev_x_lo AND #%10000000 JSR DVID3B2 \ Call DVID3B2 to calculate: \ \ K(3 2 1 0) = (A P+1 P) / (z_sign z_hi z_lo) LDA K \ Fetch the lowest byte of the result into A LDY K+1 \ Fetch the second byte of the result into Y BEQ P%+4 \ If the second byte is 0, skip the next instruction LDA #254 \ The second byte is non-zero, so the result won't fit \ into one byte, so set A = 254 as our maximum one-byte \ value to return LDY K+3 \ Fetch the sign of the result from K+3 into Y INX \ Add 2 to X so the index points to the next coordinate INX \ in this orientation vector (so consecutive calls to \ the routine will start with x, then move onto y and z) RTS \ Return from the subroutine