Data update

Task/Fibonacci-sequence/8080-Assembly/fibonacci-sequence-2.8080

@@ -2,59 +2,63 @@
 	; useful equates
 	;-------------------------------------------------------
 bdos	equ 	5	; BDOS entry
-cr	    equ 	13	; ASCII carriage return
-lf	    equ 	10	; ASCII line feed
-space	equ	    32	; ASCII space char
+cr	equ 	13	; ASCII carriage return
+lf	equ 	10	; ASCII line feed
+space	equ	32	; ASCII space char
 conout	equ 	2	; BDOS console output function
-putstr	equ	    9	; BDOS print string function
+putstr	equ	9	; BDOS print string function
+nterms	equ	20	; number of terms (max=24) to display
 	;-------------------------------------------------------
-	; program code begins here
+	; main code begins here
 	;-------------------------------------------------------
-	org	    100h    ; load point under CP/M
-	lxi	    h,0		; save command processor's stack
-	dad	    sp
+	org	100h
+	lxi	h,0		; save CP/M's stack
+	dad	sp
 	shld	oldstk
-	lxi	    sp,stack	; set our own stack
-	lxi	    d,signon	; print signon message
-	mvi	    c,putstr
+	lxi	sp,stack	; set our own stack
+	lxi	d,signon	; print signon message
+	mvi	c,putstr
 	call	bdos
-	mvi 	a,1		; test by displaying first 20
-mloop:	push	a		; save our count (putdec will trash)	
+	mvi 	a,0		; start with Fib(0)
+mloop:	push	a		; save our count	
 	call 	fib
 	call	putdec
-	mvi	    a,space		; separate the numbers
+	mvi	a,space		; separate the numbers
 	call	putchr
-	pop	    a		; get our count back
-	inr	    a		; increase it by one
-	cpi	    20+1		; have we reached our limit?
-	jnz	    mloop		; not yet; do another
-	lhld	oldstk		; we're done - get CP/M's stack back
+	pop	a		; get our count back
+	inr	a		; increment it
+	cpi	nterms+1	; have we reached our limit?
+	jnz	mloop		; no, keep going
+	lhld	oldstk		; all done; get CP/M's stack back
 	sphl			; restore it
-	ret			    ; back to command processor
+	ret			; back to command processor
 	;-------------------------------------------------------
-	;  calculate nth Fibonacci number
+	;  calculate nth Fibonacci number (max n = 24)
 	;  entry: A = n
 	;  exit:  HL = Fib(n)
 	;-------------------------------------------------------
 fib:	mov 	c,a	; C holds n
-	lxi 	d,0	; initial value for Fib(n-2)
-	lxi 	h,1	; intiial value for Fib(n-1)
+	lxi 	d,0	; DE holds Fib(n-2)
+	lxi 	h,1	; HL holds Fib(n-1)
+	ana	a	; Fib(0) is a special case
+	jnz	fib2	; n > 0 so calculate normally
+	xchg		; otherwise return with HL=0
+	ret
 fib2:	dcr	c
 	jz	fib3	; we're done
 	push 	h	; save Fib(n-1)
-	dad 	d	; HL holds Fib(n)
-	pop 	d	; Former Fib(n-1) now Fib(n-2)
-	jmp 	fib2	; loop until done
+	dad 	d	; HL = Fib(n), soon to be Fib(n-1)
+	pop 	d	; DE = old F(n-1), now Fib(n-2)
+	jmp 	fib2	; ready for next pass
 fib3:	ret
 	;-------------------------------------------------------
 	; console output of char in A register
 	;-------------------------------------------------------
-putchr:	
-    push	h
+putchr:	push	h
 	push	d
 	push	b
-	mov	    e,a
-	mvi	    c,conout
+	mov	e,a
+	mvi	c,conout
 	call	bdos
 	pop	b
 	pop	d
@@ -67,29 +71,29 @@ putchr:
 putdec: push	b
 	push	d
 	push	h
-	lxi	    b,-10
-	lxi	    d,-1
+	lxi	b,-10
+	lxi	d,-1
 putdec2:
-	dad	    b
-	inx	    d
-	jc	    putdec2
-	lxi	    b,10
-	dad	    b
+	dad	b
+	inx	d
+	jc	putdec2
+	lxi	b,10
+	dad	b
 	xchg
-	mov	    a,h
-	ora	    l
-	cnz	    putdec		; recursive call
-	mov	    a,e
-	adi	    '0'
+	mov	a,h
+	ora	l
+	cnz	putdec		; recursive call
+	mov	a,e
+	adi	'0'
 	call	putchr
 	pop	h
 	pop	d
 	pop	b
 	ret
 	;-------------------------------------------------------
-	; message and data area
+	; data area
 	;-------------------------------------------------------
-signon: db	'First 20 Fibonacci numbers:',cr,lf,'$'
+signon: db	'Fibonacci number sequence:',cr,lf,'$'
 oldstk:	dw	0
 stack	equ	$+128		; 64-level stack
 	;