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⌂ ∞ ▽ Elite on the 6502

Version analysis of DIL2

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

Code variations between these versions are shown below.

       Name: DIL2
       Type: Subroutine
   Category: Dashboard
    Summary: Update the roll or pitch indicator on the dashboard
  Deep dive: The dashboard indicators



 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

 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

 STA Q                  \ Store the offset of the vertical bar to draw in Q

                        \ 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

Code variation 1 of 4Related to the screen mode

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked, Electron
6502SP, Master
 SEC                    \ Set A = Q - 4, so that A contains the offset of the
 LDA Q                  \ vertical bar from the start of this character block
 SBC #4

 BCS DLL11              \ If Q >= 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

 SEC                    \ Set A = Q - 2, so that A contains the offset of the
 LDA Q                  \ vertical bar from the start of this character block
 SBC #2

 BCS DLL11              \ If Q >= 2 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 Q                  \ Set X to the offset of the vertical bar, which we know
                        \ is within this character block

 STA Q                  \ Set Q 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

Code variation 2 of 4Related to the screen mode

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked
6502SP, Master
Electron
 LDA CTWOS,X            \ CTWOS is a table of ready-made one-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 one-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 four-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
                        \ four-pixel colour byte)

 LDA CTWOS,X            \ CTWOS is a table of ready-made one-pixel mode 2 bytes,
                        \ just like the TWOS and TWOS2 tables for mode 1 (see
                        \ the PIXEL routine for details of how they work). This
                        \ fetches a mode 2 one-pixel byte with the pixel
                        \ position at X, so the pixel is at the offset that we
                        \ want for our vertical bar

 AND #WHITE2            \ The two-pixel mode 2 byte in #WHITE2 represents two
                        \ pixels of colour %0111 (7), which is white in both
                        \ dashboard palettes. 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
                        \ two-pixel colour byte)

 LDA CTWOS,X            \ CTWOS is a table of ready-made two-pixel mode 4 bytes,
                        \ similar to the TWOS and TWOS2 tables, but laid out in
                        \ a similar way to the mode 5 pixel bytes in the other
                        \ versions (see the PIXEL routine for details of how
                        \ they work). This fetches a two-pixel mode 4 byte with
                        \ the pixel position at 2 * X, so the pixel is at the
                        \ offset that we want for our vertical bar

Code variation 3 of 4Minor and very low-impact

Tap on a block to expand it, and tap it again to revert.

Cassette, 6502SP, Master, Electron
Flight, Docked
 BNE DLL12              \ Jump to DLL12 to skip the code for drawing a blank,
                        \ and move on to drawing the indicator (this BNE is
                        \ effectively a JMP as A is always non-zero)

 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 Q                  \ Update Q 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)

Code variation 4 of 4Related to the screen mode

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked
6502SP, Master
Electron
 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

 INC SC+1               \ Increment the high byte of SC to point to the next
                        \ character row on-screen (as each row takes up exactly
                        \ one page of 256 bytes) - so this sets up SC to point
                        \ to the next indicator, i.e. the one below the one we
                        \ just drew

 RTS                    \ Return from the subroutine

 CPY #60                \ If Y < 60 then we still have some more character
 BCC DLL10              \ blocks to draw, so loop back to DLL10 to display the
                        \ next one along

 INC SC+1               \ Increment the high byte of SC to point to the next
 INC SC+1               \ character row on-screen (as each row takes up exactly
                        \ two pages of 256 bytes) - so this sets up SC to point
                        \ to the next indicator, i.e. the one below the one we
                        \ just drew

 RTS                    \ Return from the subroutine

 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

 JMP NEXTR              \ Jump to NEXTR with the C flag set to move the screen
                        \ address in SC(1 0) down by one character row,
                        \ returning from the subroutine using a tail call