Avalon: Routine at E610

E610: Intro sequence - toggle sky colour between blue/yellow

Used by the routine at C07A.
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.
E610	LD B,$FF	255 bytes covers a couple of rows more than the first third of the screen.
E612	LD HL,$5840	Screen attribute address - top left of the screen, then two rows down below the top border
Determine INK colour in screen attribute byte:
E615	LD A,(HL)	Get screen attribute byte
E616	LD D,A	Store in D register
E617	AND $07	Get the INK value from the screen attribute byte (0-7)
E619	CP $01	Is it blue (INK 1)?
E61B	JR NZ,$E621	Is it blue (INK 1)?
E61D	LD A,$06	If so, set the INK colour to yellow (6) and skip next check
E61F	JR $E627	If so, set the INK colour to yellow (6) and skip next check
E621	CP $06	Is the INK colour yellow (6)?
E623	JR NZ,$E627	Is the INK colour yellow (6)?
E625	LD A,$01	If so, Set the INK colour to blue
Determine PAPER colour to paint sky - alternating between yellow and blue:
E627	LD E,A	Copy INK colour into E register - blue (1) or yellow (6)
E628	LD A,D	Get screen attribute byte
E629	AND $38	Filter out everything but the PAPER colour (bits 3-5)
E62B	CP $08	Is it blue (PAPER 1)?
E62D	JR NZ,$E633	Is it blue (PAPER 1)?
E62F	LD A,$30	If so, set to yellow (PAPER 6) and skip next check
E631	JR $E639	If so, set to yellow (PAPER 6) and skip next check
E633	CP $30	Is it yellow (PAPER 6)?
E635	JR NZ,$E639	Is it yellow (PAPER 6)?
E637	LD A,$08	If so, set to blue (PAPER 1)
Combine calculated INK and PAPER parts of attribute byte:
E639	OR E	Combine PAPER value with INK value calculated earlier
E63A	LD (HL),A	...and print onto screen
E63B	INC HL	Move right one square
E63C	AND $10	This instruction has no effect, as the A register is assigned a new value within the next couple of instructions
E63E	DJNZ $E615	Repeat x 255, covering the top third of the screen (and a bit more).
E640	RET	Return to the routine at C07A