Skip to navigation

Elite on the BBC Micro

Charts: TT105 [Master version]

Name: TT105 [Show more] Type: Subroutine Category: Charts Summary: Draw crosshairs on the Short-range Chart, with clipping
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: * TT103 calls TT105

Check whether the crosshairs are close enough to the current system to appear on the Short-range Chart, and if so, draw them.
.TT105 LDA QQ9 \ Set A = QQ9 - QQ0, the horizontal distance between the SEC \ crosshairs (QQ9) and the current system (QQ0) SBC QQ0 BCS P%+6 \ If the subtraction didn't underflow, skip the next two \ instructions EOR #&FF \ The subtraction underflowed, so negate the result ADC #1 \ using two's complement so that it is positive, i.e. \ A = |QQ9 - QQ0|, the absolute horizontal distance CMP #29 \ If the absolute horizontal distance in A >= 29, then BCS TT180 \ the crosshairs are too far from the current system to \ appear in the Short-range Chart, so jump to TT180 to \ return from the subroutine (as TT180 contains an RTS) LDA QQ9 \ Set A = QQ9 - QQ0, the horizontal distance between the SEC \ crosshairs (QQ9) and the current system (QQ0) SBC QQ0 BPL TT179 \ If the horizontal distance in A is positive, then skip \ the next two instructions CMP #233 \ If the horizontal distance in A < -23, then the BCC TT180 \ crosshairs are too far from the current system to \ appear in the Short-range Chart, so jump to TT180 to \ return from the subroutine (as TT180 contains an RTS) .TT179 ASL A \ Set QQ19 = 104 + A * 4 ASL A \ CLC \ 104 is the x-coordinate of the centre of the chart, ADC #104 \ so this sets QQ19 to the screen pixel x-coordinate JSR SCALEY2 \ STA QQ19 \ The call to SCALEY2 has no effect as it only contains \ an RTS, but having this call instruction here would \ enable different scaling to be applied by altering \ the SCALE routines. This code is left over from the \ conversion to other platforms, where the scale factor \ might need to be different LDA QQ10 \ Set A = QQ10 - QQ1, the vertical distance between the SEC \ crosshairs (QQ10) and the current system (QQ1) SBC QQ1 BCS P%+6 \ If the subtraction didn't underflow, skip the next two \ instructions EOR #&FF \ The subtraction underflowed, so negate the result ADC #1 \ using two's complement so that it is positive, i.e. \ A = |QQ10 - QQ0|, the absolute vertical distance CMP #35 \ If the absolute vertical distance in A >= 35, then BCS TT180 \ the crosshairs are too far from the current system to \ appear in the Short-range Chart, so jump to TT180 to \ return from the subroutine (as TT180 contains an RTS) LDA QQ10 \ Set A = QQ10 - QQ1, the vertical distance between the SEC \ crosshairs (QQ10) and the current system (QQ1) SBC QQ1 ASL A \ Set QQ19+1 = 90 + A * 2 CLC \ ADC #90 \ 90 is the y-coordinate of the centre of the chart, JSR SCALEY2 \ so this sets QQ19+1 to the screen pixel x-coordinate STA QQ19+1 \ of the crosshairs \ \ The call to SCALEY2 has no effect as it only contains \ an RTS, but having this call instruction here would \ enable different scaling to be applied by altering \ the SCALE routines. This code is left over from the \ conversion to other platforms, where the scale factor \ might need to be different LDA #8 \ Set QQ19+2 to 8 denote crosshairs of size 8 STA QQ19+2 LDA #GREEN \ Switch to stripe 3-1-3-1, which is white/yellow in the STA COL \ chart view JMP TT15 \ Jump to TT15 to draw crosshairs of size 8 at the \ crosshairs coordinates, returning from the subroutine \ using a tail call