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⌂ ∞ ▽ Elite on the BBC Micro and NES

Charts: TT123

[Elite-A, Flight]

       Name: TT123                                                   [Show more]
       Type: Subroutine
   Category: Charts
    Summary: Move galactic coordinates by a signed delta

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * TT16 calls TT123
             * TT105 calls via TT180




 Move an 8-bit galactic coordinate by a certain distance in either direction
 (i.e. a signed 8-bit delta), but only if it doesn't cause the coordinate to
 overflow. The coordinate is in a single axis, so it's either an x-coordinate
 or a y-coordinate.



 Arguments:

   A                    The galactic coordinate to update

   QQ19+3               The delta (can be positive or negative)



 Returns:

   QQ19+4               The updated coordinate after moving by the delta (this
                        will be the same as A if moving by the delta overflows)



 Other entry points:

   TT180                Contains an RTS



.TT123

 STA QQ19+4             \ Store the original coordinate in temporary storage at
                        \ QQ19+4

 CLC                    \ Set A = A + QQ19+3, so A now contains the original
 ADC QQ19+3             \ coordinate, moved by the delta

 LDX QQ19+3             \ If the delta is negative, jump to TT124
 BMI TT124

 BCC TT125              \ If the C flag is clear, then the above addition didn't
                        \ overflow, so jump to TT125 to return the updated value

 RTS                    \ Otherwise the C flag is set and the above addition
                        \ overflowed, so do not update the return value

.TT124

 BCC TT180              \ If the C flag is clear, then because the delta is
                        \ negative, this indicates the addition (which is
                        \ effectively a subtraction) underflowed, so jump to
                        \ TT180 to return from the subroutine without updating
                        \ the return value

.TT125

 STA QQ19+4             \ Store the updated coordinate in QQ19+4

.TT180

 RTS                    \ Return from the subroutine