Questions about 8250 and 1541

Hallo allemaal,


I'm busy disassembling the 8250 and I found out that the book isn't that
good commented as I thought. Fortunately lots of the 1541 is the same as
the 8250 so I could copy a lot of the comment directly from the 1541
into the 8250. But the book gets some credit as well: occasionally I
found comment missing in the equivalent parts of the 1541. 

The 1541 has this part which always had made me wonder:

;**  TALK
B_E680
		jsr	OpenChan4Read	; open channel for reading
		nop				
		nop
		nop
		bne	J_E68E

;**  LISTEN
B_E688
		jsr	OpenChan4Write	; open channel for writing
		nop				
		nop
		nop
J_E68E
		jsr	CheckREL		; verify file type
		cmp	#$04			; file type REL or
Direct access?
		bcs	B_E698		; yes, ->


The 8250 equivalent:

;**  TALK
B_F0E9					;
[F0E9]
		jsr	OpenChan4Read	;
[D83B]

		lda	PAD2
		ora	#$10			; clear DAV
		sta	PAD2
		bne	J_F103		; always ->
  
;**  LISTEN
B_F0F6					;
[F0F6]
		jsr	OpenChan4Write	;
[D856]

		lda	#$04			; clear NRFD 
		ora	PAD2
		and	#$FE			; set ATN ???
		sta	PAD2
J_F103					;
[F103]
		jsr	Check4REL		;
[D873]
		cmp	#$04			; REL or Direct access
file?
		bcs	B_F10D		; yes, ->
 

The interesting part is the line 'bne J_E68E' in the 1541 part. In the
8250 the equivalent branch will always happen due to the 'ora #$10'
instruction. But this is missing in the 1541 and that caused my to study
the 'Open a channel for writing' subroutine. The last part of the
'OpenChan4Write' subroutine:

		sec				; flag for channel
already allocated
		lda	LINTAB,X		; already in use?
		bmi	B_D106		; no, ->

		and	#$0F
		sta	CURCHN

		tax
		clc				; flag for ok
B_D106
		rts

Question: what will happen if the channel is zero ???



The book mentions a diagnostic clip that causes the 8250 to autostart a
program. I never heard of this clip, anyone of you?

;**  Check if the diagnostic clip has been connected
DiagnosticClip				;
[F1A3]
		lda	IeeeDI		; data = 0 ?
		bne	B_F19B		; no, -> no clip
  
		lda	PAD2
		ora	#$10			; clear DAV
		sta	PAD2

		lda	PBD2
		and	#$80			; NRFD (L)?
		beq	B_F19B		; yes, ->
  
		lda	PAD2
		and	#$EF			; set DAV (L)
		sta	PAD2

		lda	PBD2
		and	#$80			; NRFD (L)?
		bne	B_F19B		; no, ->

;* Clip found, start first USR program
B_F1C6					;
[F1C6]
		lda	IeeeDI
		beq	B_F1C6		; wait until clip is removed
  

IMHO the signal DAV is connected to NRFD and one or more data lines
through an inverter. Any comment is welcome!


--
     ___
    / __|__
   / /  |_/     Groetjes, Ruud
   \ \__|_\
    \___|       URL: Ruud.C64.org


 
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