THE 'CL-ATTR' SUBROUTINE
This subroutine has two separate functions.

i.  For a given display area address the appropriate attribute address is returned in the DE register pair. Note that the value on entry points to the 'ninth' line of a character.

ii. For a given line number, in the B register, the number of character areas in the display from the start of that line onwards is returned in the BC register pair.

0E88	CL-ATTR	LD	A,H	Fetch the high byte.
		RRCA		Multiply this value by
		RRCA		thirty two.
		RRCA
		DEC	A	Go back to the 'eight' line.
		OR	+50	Address the attribute area.
		LD	H,A	Restore to the high byte and
		EX	DE,HL	transfer the address to DE.
		LD	H,C	This is always zero.
		LD	L,B	The line number.
		ADD	HL,HL	Multiply by thirty two.
		ADD	HL,HL
		ADD	HL,HL
		ADD	HL,HL
		ADD	HL,HL
		LD	B,H	Move the result to the
		LD	C,L	BC register pair before
		RET		returning.

THE 'CL-ADDR' SUBROUTINE
For a given line number, in the B register, the appropriate display file address is formed in the HL register pair.

0E9B	CL-ADDR	LD	A,+18	The line number has to be
		SUB	B	reversed.
		LD	D,A	The result is saved in D.
		RRCA		In effect '(A mod 8) * 32'.
		RRCA		In a 'third' of the display
		RRCA		the low byte for the:
		AND	+E0	1st. line = +00,
				2nd. line = +20, etc.
		LD	L,A	The low byte goes into L.
		LD	A,D	The true line number is fetched.
		AND	+18	In effect '64 +8 * INT (A/8)'
		OR	+40	For the upper 'third' of the
				display the high byte = +40,
				middle 'third' = +48, and the
				lower 'third' = +50.
		LD	H,A	The high byte goes to H.
		RET		Finished.

THE 'COPY' COMMAND ROUTINE
The one hundred and seventy six pixel lines of the display are dealt with one by one.

0EAC	COPY	DI		The maskable interrupt is
				disabled during COPY.
		LD	B,+B0	The '176' lines.
		LD	HL,+4000	The base address of the display.

The following loop is now entered.

0EB2	COPY-1	PUSH	HL	Save the base address and
		PUSH	BC	the number of the line.
		CALL	0EF4,COPY-LINE	It is called '176' times.
		POP	BC	Fetch the line number and
		POP	HL	the base address.
		INC	H	The base address is updated by
				'256' locations for each line of
				pixels.