Is fast stepping used in the 1541? (2)

ruud.baltissen_at_apg.nl
Date: 2008-07-31 15:22:04

Hallo allemaal,
 
 
Looking a bit further to find out how all those little routines are
used, I ran in another mistery.

StepHead					;
[FA05]
		lda	STEPS
		bpl	B_FA0E

		eor	#$FF
		clc
		adc	#$01
B_FA0E
		cmp	MINSTP		; fast stepping?
		bcs	B_FA1C		; yes, ->

		lda	#<(E_FA3B)
		sta	NXTSTzp
		lda	#>(E_FA3B)		; pointer $62/$63 to
$FA3B
		sta	NXTSTzp+1

		bne	J_FA2E  		; always ->
 
 
If the drive decides to go slow-stepping, the interrupt routine is
tweaked to $FA3B, see above.


J_FA2E	lda	STEPS			; step counter for head
transport
		bpl	B_FA63

		inc	STEPS			; increment
		ldx	PortB2
		dex
		jmp	J_FA69


;**  Move stepper motor short distance
E_FA3B	lda	STEPS			; step counter for head
transport
		bne	J_FA2E		; not yet zero?

....

B_FA63	dec	STEPS			; step counter for head
transport
		ldx	PortB2
		inx
J_FA69
		txa
		and	#$03
		sta	TMP

		lda	PortB2
		and	#$FC
		ora	TMP
		sta	PortB2

		jmp	J_FABE


As long as there has to be stepped, at $FA3B the routine goes to $FA2E.
There the output of the 6522 is updated. At the next interrupt, $FA3B is
called again and again the 6522 is updated. My mistery: WHERE IS THE
DELAY BETWEEN THE STEPS ????? I'm sure I missed something but what and
where?


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