Maths: Negate16Bit

       Name: Negate16Bit                                             [Show more]
       Type: Subroutine
   Category: Maths
    Summary: Negate a 16-bit number
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * AddScaled calls Negate16Bit
             * ApplyAerodynamics (Part 3 of 3) calls Negate16Bit
             * ApplyFlightControl calls Negate16Bit
             * ProcessLanding (Part 7 of 7) calls Negate16Bit


 Arguments:

   (G W)                The 16-bit value to be negated

   C flag               Must be set on entry to get the correct result


 Returns:

   (G W)                The original value, negated, i.e. -(G W)

   A                    The high byte of the negated result, (G)


.Negate16Bit

 LDA #0                 \ Negate (G W) by setting (G W) = 0 - (G W), starting
 SBC W                  \ with the low byte
 STA W

 LDA #0                 \ And then the high byte
 SBC G
 STA G

 RTS                    \ Return from the subroutine