Avalon: Routine at 58896

58896: Intro sequence - toggle sky colour between blue/yellow

Used by the routine at 49274.
This routine alternates the INK and PAPER colours of the sky above the landscape to flash between yellow and blue. It checks the INK and PAPER colours of the byte (1 or 6) and toggles between them. The effect doesn't filter out squares in the left/right border - because a section on the left has yellow in it, it gets affected - see Bugs - Intro sequence - lightning effect colour bleeds into border.
58896	LD B,255	255 bytes covers a couple of rows more than the first third of the screen.
58898	LD HL,22592	Screen attribute address - top left of the screen, then two rows down below the top border
Determine INK colour in screen attribute byte:
58901	LD A,(HL)	Get screen attribute byte
58902	LD D,A	Store in D register
58903	AND 7	Get the INK value from the screen attribute byte (0-7)
58905	CP 1	Is it blue (INK 1)?
58907	JR NZ,58913	Is it blue (INK 1)?
58909	LD A,6	If so, set the INK colour to yellow (6) and skip next check
58911	JR 58919	If so, set the INK colour to yellow (6) and skip next check
58913	CP 6	Is the INK colour yellow (6)?
58915	JR NZ,58919	Is the INK colour yellow (6)?
58917	LD A,1	If so, Set the INK colour to blue
Determine PAPER colour to paint sky - alternating between yellow and blue:
58919	LD E,A	Copy INK colour into E register - blue (1) or yellow (6)
58920	LD A,D	Get screen attribute byte
58921	AND 56	Filter out everything but the PAPER colour (bits 3-5)
58923	CP 8	Is it blue (PAPER 1)?
58925	JR NZ,58931	Is it blue (PAPER 1)?
58927	LD A,48	If so, set to yellow (PAPER 6) and skip next check
58929	JR 58937	If so, set to yellow (PAPER 6) and skip next check
58931	CP 48	Is it yellow (PAPER 6)?
58933	JR NZ,58937	Is it yellow (PAPER 6)?
58935	LD A,8	If so, set to blue (PAPER 1)
Combine calculated INK and PAPER parts of attribute byte:
58937	OR E	Combine PAPER value with INK value calculated earlier
58938	LD (HL),A	...and print onto screen
58939	INC HL	Move right one square
58940	AND 16	This instruction has no effect, as the A register is assigned a new value within the next couple of instructions
58942	DJNZ 58901	Repeat x 255, covering the top third of the screen (and a bit more).
58944	RET	Return to the routine at 49274