Alien 8: Routine at ADC9

ADC9: Take a light year off

Used by the routine at RUN_CLOCK. Used by RUN_CLOCK, but only when the last digit has finished rolling (its count is 0). Takes one off the last digit, borrowing from the digits above it as a 0 becomes a 9; each digit that changes is given a count of 7, to roll into place.
CLOCK_BORROW	ADC9	LD HL,CLOCK_LAST	the last digit still rolling: nothing
	ADCC	LD B,$04
	ADCE	LD A,(HL)
	ADCF	AND $07
	ADD1	RET NZ
CLOCK_BORROW_0	ADD2	LD A,(HL)	not 0: one less, and roll
	ADD3	LD C,A
	ADD4	AND $F0
	ADD6	JR Z,CLOCK_BORROW_1
	ADD8	SUB $10
	ADDA	OR $07
	ADDC	LD (HL),A
	ADDD	RET
CLOCK_BORROW_1	ADDE	LD A,$97	0: 9, rolling, and borrow from the next digit up
	ADE0	LD (HL),A
	ADE1	DEC HL
	ADE2	DJNZ CLOCK_BORROW_0
	ADE4	RET