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⌂ ∞ ▽ Elite on the 6502

Version analysis of PZW

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

Code variations between these versions are shown below.

       Name: PZW
       Type: Subroutine
   Category: Dashboard
    Summary: Fetch the current dashboard colours, to support flashing



 Set A and X to the colours we should use for indicators showing dangerous and
 safe values respectively. This enables us to implement flashing indicators,
 which is one of the game's configurable options.

Code variation 1 of 3A variation in the comments only

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

Cassette, Flight, Docked
6502SP, Master
 If flashing is enabled, the colour returned in A (dangerous values) will be
 red for 8 iterations of the main loop, and yellow/white for the next 8, before
 going back to red. If we always use PZW to decide which colours we should use
 when updating indicators, flashing colours will be automatically taken care of
 for us.

 The values returned are &F0 for yellow/white and &0F for red. These are mode 5
 bytes that contain 4 pixels, with the colour of each pixel given in two bits,
 the high bit from the first nibble (bits 4-7) and the low bit from the second
 nibble (bits 0-3). So in &F0 each pixel is %10, or colour 2 (yellow or white,
 depending on the dashboard palette), while in &0F each pixel is %01, or colour
 1 (red).

 If flashing is enabled, the colour returned in A (dangerous values) will be
 red for 8 iterations of the main loop, and green for the next 8, before
 going back to red. If we always use PZW to decide which colours we should use
 when updating indicators, flashing colours will be automatically taken care of
 for us.

 The values returned are #GREEN2 for green and #RED2 for red. These are mode 2
 bytes that contain 2 pixels, with the colour of each pixel given in four bits.




 Returns:

   A                    The colour to use for indicators with dangerous values

   X                    The colour to use for indicators with safe values



.PZW

Code variation 2 of 3Related to the screen mode

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

Cassette, Flight, Docked
6502SP, Master
 LDX #&F0               \ Set X to dashboard colour 2 (yellow/white)

 LDX #STRIPE            \ Set X to the dashboard stripe colour, which is stripe
                        \ 5-1 (magenta/red)

 LDA MCNT               \ A will be non-zero for 8 out of every 16 main loop
 AND #%00001000         \ counts, when bit 4 is set, so this is what we use to
                        \ flash the "danger" colour

 AND FLH                \ A will be zeroed if flashing colours are disabled

Code variation 3 of 3Related to the screen mode

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

Cassette, Flight, Docked
6502SP, Master
 BEQ P%+4               \ If A is zero, skip to the LDA instruction below

 TXA                    \ Otherwise flashing colours are enabled and it's the
                        \ main loop iteration where we flash them, so set A to
                        \ colour 2 (yellow/white) and use the BIT trick below to
                        \ return from the subroutine

 EQUB &2C               \ Skip the next instruction by turning it into
                        \ &2C &A9 &0F, or BIT &0FA9, which does nothing apart
                        \ from affect the flags

 LDA #&0F               \ Set A to dashboard colour 1 (red)

 BEQ P%+5               \ If A is zero, skip the next two instructions

 LDA #GREEN2            \ Otherwise flashing colours are enabled and it's the
 RTS                    \ main loop iteration where we flash them, so set A to
                        \ dashboard colour 2 (green) and return from the
                        \ subroutine

 LDA #RED2              \ Set A to dashboard colour 1 (red)

 RTS                    \ Return from the subroutine