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Elite on the BBC Micro

Drawing planets: PLS6 [6502SP version]

Name: PLS6 [Show more] Type: Subroutine Category: Drawing planets Summary: Calculate (X K) = (A P) / (z_sign z_hi z_lo)
Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * PROJ calls PLS6 * CHKON calls entry point PL44

Calculate the following: (X K) = (A P) / (z_sign z_hi z_lo) returning an overflow in the C flag if the result is >= 1024. Arguments: INWK The planet or sun's ship data block Returns: C flag Set if the result >= 1024, clear otherwise Other entry points: PL44 Clear the C flag and return from the subroutine
.PLS6 JSR DVID3B2 \ Call DVID3B2 to calculate: \ \ K(3 2 1 0) = (A P+1 P) / (z_sign z_hi z_lo) LDA K+3 \ Set A = |K+3| OR K+2 AND #%01111111 ORA K+2 BNE PL21 \ If A is non-zero then the two high bytes of K(3 2 1 0) \ are non-zero, so jump to PL21 to set the C flag and \ return from the subroutine \ We can now just consider K(1 0), as we know the top \ two bytes of K(3 2 1 0) are both 0 LDX K+1 \ Set X = K+1, so now (X K) contains the result in \ K(1 0), which is the format we want to return the \ result in CPX #4 \ If the high byte of K(1 0) >= 4 then the result is BCS PL6 \ >= 1024, so return from the subroutine with the C flag \ set to indicate an overflow (as PL6 contains an RTS) LDA K+3 \ Fetch the sign of the result from K+3 (which we know \ has zeroes in bits 0-6, so this just fetches the sign) BPL PL6 \ If the sign bit is clear and the result is positive, \ then the result is already correct, so return from \ the subroutine with the C flag clear to indicate \ success (as PL6 contains an RTS) LDA K \ Otherwise we need to negate the result, which we do EOR #%11111111 \ using two's complement, starting with the low byte: ADC #1 \ STA K \ K = ~K + 1 TXA \ And then the high byte: EOR #%11111111 \ ADC #0 \ X = ~X TAX .PL44 CLC \ Clear the C flag to indicate success .PL6 RTS \ Return from the subroutine