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Dashboard: UpdateIndicator (Part 2 of 15)

Name: UpdateIndicator (Part 2 of 15) [Show more] Type: Subroutine Category: Dashboard Summary: Calculations for the compass (indicator 0) Deep dive: Hard-coded division in the dashboard routines
Context: See this subroutine in context in the source code References: No direct references to this subroutine in this source file

This section takes the compass heading from yRotationHi and reduces it to the range 0 to 73, before passing it to the DrawIndicatorHand to update the on-screen compass.
.uind1 \ If we get here then the indicator number in X is 0 LDA yRotationHi \ Set T = yRotationHi STA T \ We now calculate A = T * n / 256 with a hardcoded n, \ using unrolled shift-and-add multiplication \ We don't need an LDA T instruction as A already \ contains the same value as T LSR A \ Bit 0 of n is 0 LSR A \ Bit 1 of n is 0 CLC \ Bit 2 of n is 1 ADC T ROR A LSR A \ Bit 3 of n is 0 LSR A \ Bit 4 of n is 0 CLC \ Bit 5 of n is 1 ADC T ROR A LSR A \ Bit 6 of n is 0 \ Bit 7 of n is 0 and the final right shift is missing \ So by now, A is in the range 0 to 73 - here's why: \ \ From the above, n = %00100100 (36), so we just \ calculated: \ \ A = (T * n / 256) << 1 \ = (T * 36 / 256) << 1 \ = T * 72 / 256 \ \ which takes the compass heading in the range 0 to 255 \ and reduces it to the range 0 to 73 JMP DrawIndicatorHand \ Apply min and max limits to the value in A and update \ the indicator on-screen, returning from the subroutine \ using a tail call