Elite on the BBC Micro and NES

# Version analysis of DIALS (Part 2 of 4)

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

Name: DIALS (Part 2 of 4) Type: Subroutine Category: Dashboard Summary: Update the dashboard: pitch and roll indicators Deep dive: The dashboard indicators

Code variation 1 of 3Minor and very low-impact

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LDA #0 \ Set R = P = 0 for the low bytes in the call to the ADD STA R \ routine below STA P
STZ R \ Set R = P = 0 for the low bytes in the call to the ADD STZ P \ routine below
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

Code variation 2 of 3A variation in the labels only

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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 ADDK \ 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%+4               \ is 0 (i.e. we are not pitching), skip the next
\ instruction

SBC #1                 \ The pitch angle beta is non-zero, so set A = A - 1
\ (the C flag is set by the call to DIL2 above, so we
\ don't need to do a SEC). 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

Code variation 3 of 3A variation in the labels only

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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 ADDK \ 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)