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Elite on the BBC Micro

Dashboard: DIALS (Part 2 of 4) [Elite-A, Parasite]

Name: DIALS (Part 2 of 4) [Show more] Type: Subroutine Category: Dashboard Summary: Update the dashboard: pitch and roll indicators Deep dive: The dashboard indicators
Context: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
LDA #0 \ Set R = P = 0 for the low bytes in the call to the ADD STA R \ routine below STA P LDA #8 \ Set S = 8, which is the value of the centre of the STA S \ roll indicator LDA ALP1 \ Fetch the roll angle alpha as a value between 0 and LSR A \ 31, and divide by 4 to get a value of 0 to 7 LSR A ORA ALP2 \ Apply the roll sign to the value, and flip the sign, EOR #%10000000 \ so it's now in the range -7 to +7, with a positive \ roll angle alpha giving a negative value in A JSR ADD \ We now add A to S to give us a value in the range 1 to \ 15, which we can pass to DIL2 to draw the vertical \ bar on the indicator at this position. We use the ADD \ routine like this: \ \ (A X) = (A 0) + (S 0) \ \ and just take the high byte of the result. We use ADD \ rather than a normal ADC because ADD separates out the \ sign bit and does the arithmetic using absolute values \ and separate sign bits, which we want here rather than \ the two's complement that ADC uses JSR DIL2 \ Draw a vertical bar on the roll indicator at offset A \ and increment SC to point to the next indicator (the \ pitch indicator) LDA BETA \ Fetch the pitch angle beta as a value between -8 and \ +8 LDX BET1 \ Fetch the magnitude of the pitch angle beta, and if it BEQ P%+5 \ is 0 (i.e. we are not pitching), skip the next two \ instructions SEC \ The C flag is set by the call to DIL2 above, so this \ instruction has no effect SBC #1 \ The pitch angle beta is non-zero, so set A = A - 1. \ This gives us a value of A from -7 to +7 because these \ are magnitude-based numbers with sign bits, rather \ than two's complement numbers JSR ADD \ We now add A to S to give us a value in the range 1 to \ 15, which we can pass to DIL2 to draw the vertical \ bar on the indicator at this position (see the JSR ADD \ above for more on this) JSR DIL2 \ Draw a vertical bar on the pitch indicator at offset A \ and increment SC to point to the next indicator (the \ four energy banks)