The DrawSpaceViewInNMI (Bank 0) subroutine - Elite on the BBC Micro and NES

       Name: DrawSpaceViewInNMI                                      [Show more]
       Type: Subroutine
   Category: Drawing the screen
    Summary: Configure the NMI handler to draw the space view
  Deep dive: Drawing vector graphics using NES tiles
             Splitting the main loop in the NES version
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


.DrawSpaceViewInNMI

 LDA QQ11               ; If this is not the space view, jump to spvw3 to skip
 BNE spvw3              ; the following, as we only need to send the drawing
                        ; bitplane to the PPU and update the dashboard in the
                        ; space view

 JSR SetupPPUForIconBar ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0
                        ;
                        ; If bit 7 of setupPPUForIconBar is set, then this also
                        ; affects the C flag as follows:
                        ;
                        ;   * If bit 6 of PPU_STATUS is clear then the C flag
                        ;     is set to bit 7 of PPU_STATUS (which is set if
                        ;     VBlank has started, clear otherwise)
                        ;
                        ;   * If bit 6 of PPU_STATUS is set (sprite 0 has been
                        ;     hit) then the C flag is cleared
                        ;
                        ; So the C flag is set if we are in VBlank, or if we
                        ; aren't in VBlank but the PPU has not yet reached the
                        ; icon bar (i.e. it hasn't started drawing the
                        ; dashboard yet)
                        ;
                        ; In other words, the C flag is clear if the PPU is
                        ; currently drawing the dashboard, and set if it is not

 LDA drawingBitplane    ; If the drawing bitplane is 1, jump to spvw1 to send
 BNE spvw1              ; the drawing plane to the PPU without updating the
                        ; whole dashboard first

                        ; If we get here then the drawing bitplane is 0, so now
                        ; we do various checks to determine whether to update
                        ; the whole dashboard before sending to the PPU (as
                        ; updating the whole dashboard takes time)

 LDA sendDashboardToPPU ; Flip the value of sendDashboardToPPU between 0 and $FF
 EOR #$FF               ; so it flips every time we run the main loop with the
 STA sendDashboardToPPU ; drawing bitplane set to 0 (so we only update the whole
                        ; dashboard every other iteration that the drawing
                        ; bitplane is set to 0)

 BMI spvw2              ; If bit 7 of the result is set (so sendDashboardToPPU
                        ; is now $FF), jump to spvw2 to update the whole
                        ; dashboard before sending it to the PPU

 LDA KY1                ; If the B button is being pressed with either the up or
 ORA KY2                ; down button (to change our speed), or the C flag is
 ROR A                  ; set, jump to spvw2 to update the whole dashboard
 BNE spvw2              ; before sending it to the PPU
                        ;
                        ; This makes the speed indicator react more quickly to
                        ; speed changes, as it triggers an update of the whole
                        ; dashboard when we are changing speed, and it also
                        ; ensures we do not update the whole dashboard if the
                        ; PPU is currently in the process of drawing it

.spvw1

                        ; If we get here then either the drawing bitplane is 1,
                        ; or it's 0 and the following are true:
                        ;
                        ;   * sendDashboardToPPU has just flipped to 0
                        ;
                        ;   * The change speed buttons are not being pressed
                        ;
                        ;   * The C flag is clear, so the PPU is currently
                        ;     drawing the dashboard
                        ;
                        ; So we can get away with not updating the dashboard
                        ; for two iterations out of four (i.e. when the drawing
                        ; bitplane is 1), plus an extra iteration (i.e. every
                        ; other iteration with bitplane 0) but only if the speed
                        ; buttons aren't being pressed or the PPU is currently
                        ; drawing the dashboard

 JSR DrawBitplaneInNMI  ; Configure the NMI to send the drawing bitplane to the
                        ; PPU after drawing the box edges and setting the next
                        ; free tile number

 JSR COMPAS             ; Call COMPAS to update the compass

 JMP DrawPitchRollBars  ; Update the pitch and roll bars on the dashboard,
                        ; returning from the subroutine using a tail call

.spvw2

 LDA #%10001000         ; Set the bitplane flags for the drawing bitplane to the
 JSR SetDrawPlaneFlags  ; following:
                        ;
                        ;   * Bit 2 clear = send tiles up to configured numbers
                        ;   * Bit 3 set   = clear buffers after sending data
                        ;   * Bit 4 clear = we've not started sending data yet
                        ;   * Bit 5 clear = we have not yet sent all the data
                        ;   * Bit 6 clear = only send pattern data to the PPU
                        ;   * Bit 7 set   = send data to the PPU
                        ;
                        ; Bits 0 and 1 are ignored and are always clear
                        ;
                        ; This configures the NMI to send pattern data for the
                        ; drawing bitplane to the PPU during VBlank

 JSR COMPAS             ; Call COMPAS to update the compass

 JSR DrawPitchRollBars  ; Update the pitch and roll bars on the dashboard

 JSR DIALS_b6           ; Call DIALS to update the dashboard

 LDX drawingBitplane    ; Set X to the drawing bitplane

 LDA bitplaneFlags,X    ; Set bit 6 of the flags for the drawing bitplane, so
 ORA #%01000000         ; we send both nametable and pattern table data for
 STA bitplaneFlags,X    ; bitplane X to the PPU in the NMI handler

 RTS                    ; Return from the subroutine

.spvw3

                        ; We jump here if this is not the space view, with the
                        ; view type in A

 CMP #$98               ; If this is not the Status Mode screen, jump to spvw6
 BNE spvw6              ; to skip the following, as we can only flash the
                        ; commander image background when it's on-screen in the
                        ; Status Mode view

 JSR GetStatusCondition ; Set X to our ship's status condition

 CPX previousCondition  ; If our condition hasn't changed, jump to spvw4 to
 BEQ spvw4              ; skip the following instruction

 JSR STATUS             ; Call STATUS to refresh the Status Mode screen, so our
                        ; status updates to show the new condition

.spvw4

 LDX previousCondition  ; Set X to the previous status condition

 CPX #3                 ; If the previous status condition was not red, jump to
 BNE spvw5              ; spvw5 to show the alert colour for the previous
                        ; condition

 LDA nmiCounter         ; If nmiCounter div 32 is odd (which will happen half
 AND #32                ; the time, and for 32 VBlanks in a row), jump to spvw5
 BNE spvw5              ; to skip the following

                        ; We get here if the previous condition was red, but
                        ; only for every other block of 32 VBlanks, so this
                        ; flashes the commander image background on and off with
                        ; a period of 32 VBlanks

 INX                    ; Increment X to 4, which will make the background of
                        ; the commander image flash between the top two alert
                        ; colours (i.e. light red and dark red)

.spvw5

 LDA alertColours,X     ; Change the palette so the visible colour is set to the
 STA visibleColour      ; alert colour for our status condition

.spvw6

 RTS                    ; Return from the subroutine

[X]

Subroutine COMPAS (category: Dashboard)

Update the compass

[X]

Subroutine DIALS_b6 (category: Dashboard)

Call the DIALS routine in ROM bank 6

[X]

Subroutine DrawBitplaneInNMI (category: Drawing the screen)

Configure the NMI to send the drawing bitplane to the PPU after drawing the box edges and setting the next free tile number

[X]

Subroutine DrawPitchRollBars (category: Dashboard)

Update the pitch and roll bars on the dashboard

[X]

Subroutine GetStatusCondition (category: Status)

Calculate our ship's status condition

[X]

Variable KY1 in workspace WP

One pilot is configured and the down and B buttons are both being pressed on controller 1

[X]

Variable KY2 in workspace WP

One pilot is configured and the up and B buttons are both being pressed on controller 1

[X]

Variable QQ11 in workspace ZP

This contains the type of the current view (or, if we are changing views, the type of the view we are changing to)

[X]

Subroutine STATUS (category: Status)

Show the Status Mode screen

[X]

Subroutine SetDrawPlaneFlags (category: Drawing the screen)

Set the drawing bitplane flags to the specified value, draw the box edges and set the next free tile number

[X]

Subroutine SetupPPUForIconBar (category: PPU)

If the PPU has started drawing the icon bar, configure the PPU to use nametable 0 and pattern table 0, while preserving A

[X]

Variable alertColours (category: Status)

Colours for the background of the commander image to show the status condition when we are not looking at the space view

[X]

Variable bitplaneFlags in workspace WP

Flags for bitplane 0 that control the sending of data for this bitplane to the PPU during VBlank in the NMI handler

[X]

Variable drawingBitplane in workspace ZP

Flipped manually by calling FlipDrawingPlane, controls whether we are showing nametable/palette buffer 0 or 1

[X]

Variable nmiCounter in workspace WP

A counter that increments every VBlank at the start of the NMI handler

[X]

Variable previousCondition in workspace WP

Used to store the ship's previous status condition (i.e. docked, green, yellow or red), so we can tell how the situation is changing

[X]

Variable sendDashboardToPPU in workspace WP

A flag that controls whether we send the dashboard to the PPU during the main loop

[X]

Label spvw1 is local to this routine

[X]

Label spvw2 is local to this routine

[X]

Label spvw3 is local to this routine

[X]

Label spvw4 is local to this routine

[X]

Label spvw5 is local to this routine

[X]

Label spvw6 is local to this routine

[X]

Variable visibleColour in workspace ZP

Contains the colour to use for pixels that are visible in palette 0, e.g. $2C for cyan

Drawing the screen: DrawSpaceViewInNMI

[NES version, Bank 0]