CPL		complemented (OVER 1) or
				1 (OVER 0).
2303	PLOT-END	LD	(HL),A	The byte is entered. Its other
				bits are unchanged in every case.
		JP	OBDB,PO_ATTR	Exit, setting attribute byte.

THE 'STK-TO-BC' SUBROUTINE
This subroutine loads two floating point numbers into the BC register pair. It is thus used to pick up parameters in the range +00-+FF. It also obtains in DE the 'diagonal move' values (+/-1,+/-1) which are used in the line drawing subroutine of DRAW.

2307	STK-TO-BC	CALL	2314,STK-TO-A	First number to A.
		LD	B,A	Hence to B.
		PUSH 	BC	Save it briefly.
		CALL	2314,STK-TO-A	Second number to A.
		LD	E,C	Its sign indicator to E.
		POP	BC	Restore first number.
		LD	D,C	Its signs indicator to D.
		LD	C,A	Second number to C.
		RET		BC, DE are now as required.

THE 'STK-TO-A' SUBROUTINE
This subroutine loads the A register with the floating point number held at the top of the calculator stack. The number must be in the range 00-FF.

2314	STK-TO-A	CALL	2DD5,FP-TO-A	Modulus of rounded last value to
		JP	C,24F9,REPORT-B	A if possible; else, report error.
		LD	C,+01	One to C for positive last value.
		RET	Z	Return if value was positive.
		LD	C,+FF	Else change C to +FF (i.e. minus
		RET		one). Finished.

THE 'CIRCLE' COMMAND ROUTINE
This routine draws an approximation to the circle with centre co-ordinates X and Y and radius Z. These numbers are rounded to the nearest integer before use. Thus Z must be less than 87.5, even when (X,Y) is in the centre of the screen. The method used is to draw a series of arcs approximated by straight lines. It is illustrated in the BASIC program in the appendix. The notation of that program is followed here.
     CIRCLE has four parts:
  i. Tests the radius. If its modulus is less than 1, just plot X,Y;
 ii. Calls CD-PRMS-1 at 2470-24B6, which is used to set the initial parameters for both CIRCLE and DRAW;
iii. Sets up the remaining parameters for CIRCLE, including the initial displacement for the first 'arc' (a straight line in fact);
 iv. Jumps into DRAW to use the arc-drawing loop at 2420-24FA.
Parts i. to iii. will now be explained in turn.
i.   2320-23AA. The radius, say Z', is obtained from the calculator stack. Its modulus Z is formed and used from now on. If Z is less than 1, it is deleted from the stack and the  point X,Y is plotted by a jump to PLOT.

2320	CIRCLE	RST	0017,GET-CHAR	Get the present character.
		CP	+2C	Test for comma.
		JP	NZ,1C8A,REPORT-C	If not so, report the error.
		RST	0020,NEXT-CHAR	Get next character (the radius).
		CALL	1C82,EXPT-1NUM	Radius to calculator stack.
		CALL	1BEE,CHECK-END	Move to consider next statement
				if checking syntax.
		RST	0028,FP-CALC	Use calculator: the stack holds:
		DEFB	+2A,abs	X, Y, Z
		DEFB	+3D,re-stack	Z is re-stacked; its exponent
		DEFB	+38,end-calc	is therefore available.
		LD	A,(HL)	Get exponent of radius.