From: Grósz Attila (gyros_at_freemail.hu)
Date: 2007-04-04 14:10:39
Hi,
Anders Carlsson <anders.carlsson@sfks.se> írta:
> If it helps, I might try to read them a few times more,
perhaps
> at a lower speed and confirm that I really get consistent
output.
Sure that'd be great...!
> The fact it sets NTSC mode is in line with the channel
switch on
> the RF modulator.
At a second glance it looks as if the zero page variable $7F
(MSB) determines the actual video mode set... the reset sequence
starts by turning off the TED (so-called TED off bit):
. ed2c a9 20 LDA #$20 ; ted off
. ed2e 8d 07 ff STA $ff07
. ed31 a9 ff LDA #$ff
. ed33 8d 02 fe STA $fe02
. ed36 a9 00 LDA #$00
. ed38 8d 31 04 STA $0431
In freeze mode, the TED is forced to single clock and is doing
RAM refreshes only. (It is mentioned in the preliminary TED data
sheet, as well). Later it is turned back on and the
appropriate video
mode is set:
. c76c bd de c7 LDA $c7de,X
. c76f c5 7f CMP $7f
. c771 f0 5c BEQ $c7cf
. c773 85 7f STA $7f
. c78f ad 07 ff LDA $ff07
. c792 29 ef AND #$ef ; clearing NTSC mode bit
. c794 24 7f BIT $7f
. c796 10 02 BPL $c79a
. c798 09 10 ORA #$10 ; setting NTSC mode bit
. c79a 8d 07 ff STA $ff07
Regards,
Attila
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