LD	B,+00	B is set to +00 hex for 'negate'.

'ABS' enters here.

3474	NEG-TEST	LD	A,(HL)	If the first byte is zero, the
		AND	A	jump is made to deal with a
		JR	Z,3483,INT-CASE	'small integer'.
		INC	HL	Point to the second byte.
		LD	A,B	Get +FF for 'abs', +00 for
				'negate'.
		AND	+80	Now +80 for 'abs', +00 for
				'negate'.
		OR	(HL)	This sets bit 7 for 'abs', but
				changes nothing for 'negate'.
		RLA		Now bit 7 is changed, leading to
		CCF		bit 7 of byte 2 reset for 'abs',
		RRA		and simply changed for 'negate'.
		LD	(HL),A	The new second byte is stored.
		DEC	HL	HL points to the first byte
				again.
		RET		Finished.

The 'integer case' does a similar operation with the sign byte.

3483	INT-CASE	PUSH	DE	Save STKEND in DE.
		PUSH	HL	Save pointer to the number in
				HL.
		CALL	2D7F,INT-FETCH	Fetch the sign in C, the number
				in DE.
		POP	HL	Restore the pointer to the
				number in HL.
		LD	A,B	Get +FF for 'abs', +00 for
				'negate'.
		OR	C	Now +FF for 'abs', no change for
				'negate'
		CPL		Now +00 for 'abs', and a changed byte
		LD	C,A	for 'negate': store it in C.
		CALL	2D8E,INT-STORE	Store result on the stack.
		POP	DE	Return STKEND to DE.
		RET		

THE 'SIGNUM' FUNCTION
(Offset 29: 'sgn')

This subroutine handles the function SGN X and therefore returns a 'last value' of 1 if X is positive, zero if X is zero and -1 if X is negative.

3492	sgn	CALL 34E9,TEST-ZERO	If X is zero, just return with
		RET	C	zero as the 'last value'.
		PUSH	DE	Save the pointer to STKEND.
		LD	DE,+0001	Store 1 in DE.
		INC	HL	Point to the second byte of X.
		RL	(HL)	Rotate bit 7 into the carry flag.
		DEC	HL	Point to the destination again.
		SBC	A,A	Set C to zero for positive X and
		LD	C,A	to FF hex for negative X.
		CALL	2D8E,INT-STORE	Stack 1 or -1 as required.
		POP	DE	Restore the pointer to
				STKEND'
		RET		Finished.

THE 'IN' FUNCTION
(Offset 2C: 'in')

This subroutine handles the function IN X. It inputs at processor level from port X, loading BC with X and performing the instruction IN A,(C).