INC	HL	frame counter are incremented
		LD	(FRAMES),HL	every 20ms. (U.K.) The highest
		LD	A,H	byte of the frame counter is
		OR	L	only incremented when the
		JR	NZ,0048,KEY-INT	value of the lower two bytes
		INC	(FRAMES-3)	is zero.
0048	KEY-INT	PUSH	BC	Save the current values held
		PUSH	DE	in these registers.
		CALL	02BF,KEYBOARD	Now scan the keyboard.
		POP	DE	Restore the values.
		POP	BC
		POP	HL
		POP	AF
		EI		The maskable interrupt is en-
		RET		abled before returning.

THE 'ERROR-2' ROUTINE
The return address to the interpreter points to the 'DEFB' that signifies which error has occurred. This 'DEFB' is fetched and transferred to ERR-NR. The machine stack is cleared before jumping forward to clear the calculator stack.
0053	ERROR-2	POP	HL	The address on the stack points
		LD	L,(HL)	to the error code.
0055	ERROR-3	LD	(ERR-NR),L	It is transferred to ERR-NR.
		LD	SP,(ERR-SP)	The machine is cleared before
		JP	16C5,SET-STK	exiting via SET-STK.
		DEFB	+FF,+FF,+FF,+FF	Unused locations.
		DEFB	+FF,+FF,+FF

THE 'NON-MASKABLE INTERRUPT' ROUTINE
This routine is not used in the standard Spectrum but the code allows for a system reset to occur following activation of the NMI line. The system variable at 5CB0, named here NMIADD, has to have the value zero for the reset to occur.
0066	RESET	PUSH	AF	Save the current values held
		PUSH	HL	in these registers.
		LD	HL,(NMIADD)	The two bytes of NMIADD
		LD	A,H	must both be zero for the reset
		OR	L	to occur.
		JR	NZ,0070,NO-RESET	Note: This should have been
				'JR Z'!
		JP	(HL)	Jump to START.
0070	NO-RESET	POP	HL	Restore the current values to
		POP	AF	these registers and return.
		RETN

THE 'CH-ADD+1' SUBROUTINE
The address held in CH-ADD is fetched, incremented and restored. The contents of the location now addressed by CH-ADD is fetched. The entry points of TEMP-PTR1 and TEMP-PTR2 are used to set CH-ADD for a temporary period.
0074	CH-ADD+1	LD	HL,(CH-ADD)	Fetch the address.
0077	TEMP-PTR1	INC	HL	Increment the pointer.
0078	TEMP-PTR2	LD	(CH-ADD),HL	Set CH-ADD.
		LD	A,(HL)	Fetch he addressed value and
		RET		then return.

THE 'SKIP-OVER' SUBROUTINE
The value brought to the subroutine in the A register is tested to see if it is printable. Various special codes lead to HL being incremented once, or twice, and CH-ADD amended accordingly.
007D	SKIP-OVER	CP	+21	Return with the carry flag reset
		RET	NC	if ordinary character code.
		CP	+0D	Return if the end of the line
		RET	Z	has been reached.
		CP	+10	Return with codes +00 to +0F