The REDU2 subroutine - Elite on the BBC Micro and NES

       Name: REDU2                                                   [Show more]
       Type: Subroutine
   Category: Dashboard
    Summary: Reduce the value of the pitch or roll dashboard indicator
    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:
             * DOKEY calls REDU2
             * BUMP2 calls via RE3+2


 Reduce X by A, where X is either the current rate of pitch or the current
 rate of roll.

 The rate of pitch or roll ranges from 1 to 255 with 128 as the centre point.
 This is the amount by which the pitch or roll is currently changing, so 1
 means it is decreasing at the maximum rate, 128 means it is not changing,
 and 255 means it is increasing at the maximum rate. These values correspond
 to the line on the DC or RL indicators on the dashboard, with 1 meaning full
 left, 128 meaning the middle, and 255 meaning full right.

 If reducing X would bring it below 1, then X is set to 1.

 If keyboard auto-recentre is configured and the result is greater than 128, we
 reduce X down to the mid-point, 128. This is the equivalent of having a roll
 or pitch in the right half of the indicator, when decreasing the roll or pitch
 should jump us straight to the mid-point.


 Other entry points:

   RE3+2                Auto-recentre the value in X, if keyboard auto-recentre
                        is configured


.REDU2

 STA T                  \ Store argument A in T so we can restore it later

 TXA                    \ Copy argument X into A

 SEC                    \ Set the C flag so we can do subtraction without the
                        \ C flag affecting the result

 SBC T                  \ Set X = A = argument X - argument A
 TAX

 BCS RE3                \ If the C flag is set, then we didn't underflow, so
                        \ jump to RE3 to auto-recentre and return the result

 LDX #1                 \ We have an underflow, so set X to the minimum possible
                        \ value, 1

.RE3

 BPL RE2+2              \ If X has bit 7 clear (i.e. the result < 128), then
                        \ jump to RE2+2 above to return the result as is,
                        \ because the result is on the left side of the centre
                        \ point of 128, so we don't need to auto-centre

                        \ Jumps to RE3+2 end up here

                        \ If we get here, then we need to apply auto-recentre,
                        \ if it is configured

 LDA DJD                \ If keyboard auto-recentre is disabled, then
 BNE RE2+2              \ jump to RE2+2 to restore A and return

 LDX #128               \ If we get here then keyboard auto-recentre is enabled,
 BMI RE2+2              \ so set X to 128 (the middle of our range) and jump to
                        \ RE2+2 to restore A and return from the subroutine
                        \ (this BMI is effectively a JMP as bit 7 of X is always
                        \ set)
Dashboard: REDU2

[Acorn Electron version]
