Dashboard: DIL2

       Name: DIL2                                                    [Show more]
       Type: Subroutine
   Category: Dashboard
    Summary: Implement the draw_angle command (update the roll or pitch
             indicator on the dashboard)
  Deep dive: The dashboard indicators
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * tube_table calls DIL2


 This routine is run when the parasite sends a draw_angle command. It updates
 the roll or pitch indicator on the dashboard.

 The indicator can show a vertical bar in 16 positions, with a value of 8
 showing the bar in the middle of the indicator.

 In practice this routine is only ever called with A in the range 1 to 15, so
 the vertical bar never appears in the leftmost position (though it does appear
 in the rightmost).


 Arguments:

   A                    The offset of the vertical bar to show in the indicator,
                        from 0 at the far left, to 8 in the middle, and 15 at
                        the far right


 Returns:

   C flag               The C flag is set


.DIL2

 JSR tube_get           \ Get the parameters from the parasite for the command:
 STA angle_1            \
 JSR tube_get           \   draw_angle(value, screen_low, screen_high)
 STA SC                 \
 JSR tube_get           \ and store them as follows:
 STA SC+1               \
                        \   * angle_1 = the value to display in the indicator
                        \
                        \   * SC(1 0) = the screen address of the indicator

 LDY #1                 \ We want to start drawing the vertical indicator bar on
                        \ the second line in the indicator's character block, so
                        \ set Y to point to that row's offset

                        \ We are now going to work our way along the indicator
                        \ on the dashboard, from left to right, working our way
                        \ along one character block at a time. Y will be used as
                        \ a pixel row counter to work our way through the
                        \ character blocks, so each time we draw a character
                        \ block, we will increment Y by 8 to move on to the next
                        \ block (as each character block contains 8 rows)

.DLL10

 SEC                    \ Set A = angle_1 - 4, so that A contains the offset of
 LDA angle_1            \ the vertical bar from the start of this character
 SBC #4                 \ block

 BCS DLL11              \ If angle_1 >= 4 then the character block we are
                        \ drawing does not contain the vertical indicator bar,
                        \ so jump to DLL11 to draw a blank character block

 LDA #&FF               \ Set A to a high number (and &FF is as high as they go)

 LDX angle_1            \ Set X to the offset of the vertical bar, which we know
                        \ is within this character block

 STA angle_1            \ Set angle_1 to a high number (&FF, why not) so we will
                        \ keep drawing blank characters after this one until we
                        \ reach the end of the indicator row

 LDA CTWOS,X            \ CTWOS is a table of ready-made 1-pixel mode 5 bytes,
                        \ just like the TWOS and TWOS2 tables for mode 4 (see
                        \ the PIXEL routine for details of how they work). This
                        \ fetches a mode 5 1-pixel byte with the pixel position
                        \ at X, so the pixel is at the offset that we want for
                        \ our vertical bar

 AND #&F0               \ The 4-pixel mode 5 colour byte &F0 represents four
                        \ pixels of colour %10 (3), which is yellow in the
                        \ normal dashboard palette and white if we have an
                        \ escape pod fitted. We AND this with A so that we only
                        \ keep the pixel that matches the position of the
                        \ vertical bar (i.e. A is acting as a mask on the
                        \ 4-pixel colour byte)

 JMP DLL12              \ Jump to DLL12 to skip the code for drawing a blank,
                        \ and move on to drawing the indicator

.DLL11

                        \ If we get here then we want to draw a blank for this
                        \ character block

 STA angle_1            \ Update angle_1 with the new offset of the vertical
                        \ bar, so it becomes the offset after the character
                        \ block we are about to draw

 LDA #0                 \ Change the mask so no bits are set, so all of the
                        \ character blocks we display from now on will be blank
.DLL12

 STA (SC),Y             \ Draw the shape of the mask on pixel row Y of the
                        \ character block we are processing

 INY                    \ Draw the next pixel row, incrementing Y
 STA (SC),Y

 INY                    \ And draw the third pixel row, incrementing Y
 STA (SC),Y

 INY                    \ And draw the fourth pixel row, incrementing Y
 STA (SC),Y

 TYA                    \ Add 5 to Y, so Y is now 8 more than when we started
 CLC                    \ this loop iteration, so Y now points to the address
 ADC #5                 \ of the first line of the indicator bar in the next
 TAY                    \ character block (as each character is 8 bytes of
                        \ screen memory)

 CPY #30                \ If Y < 30 then we still have some more character
 BCC DLL10              \ blocks to draw, so loop back to DLL10 to display the
                        \ next one along

 RTS                    \ Return from the subroutine