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Charts: TT123

[BBC Master version]

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