OVL signal (Overlay ROM aver RAM)

whitebird · 27 July 2011, 00:24

Hello,

I know this signal exists for the A500 but I didn't find it for the 1200.

This signal alters the address decoding so that at early boot stage (after cold reset) the ROM appears for the 68000 at address 0000h, setting a proper reset vector because the ROM content is well defined.

After that the OVL signal is cleared by the kickstart so that RAM is seen at address 0000h.

Does anyone know if this signal still exists on the 1200 and what is the kickstart procedure to clear it?

Thanks

Toni Wilen · 27 July 2011, 08:23

Gayle implements the signal internally, CIA OVL pin is unused. Basically reset sets it, first CPU CIA access disables it.

It is explained in Gayle datasheet (check eab file server)

whitebird · 27 July 2011, 23:45

Thanks a lot Toni.

I think that as OVL signal is no more related to the CIA, the only reason to continue to write to the CIA (to disable OVL), is for compatibility with software that has been written for models where OVL signal is generated through CIA.

I didn't know about those documents available on eab server.

What a precious data collection!

whitebird · 29 July 2011, 22:51

Isn't there a mistake in the GAYLE specification? It says to negate OVL there should be a write to $BFD000-$BFDFFF, but I actually write to $BFE001, which is an address in the ODD CIA space, and OVL signal is connected on the ODD CIA...

Toni Wilen · 29 July 2011, 23:39

This datasheet most likely describes pre-production/beta Gayle version, not final production version. There is also other minor differences, like PCMCIA address space.

27 July 2011, 00:24	#1
whitebird Registered User Join Date: May 2006 Location: France Age: 48 Posts: 211	OVL signal (Overlay ROM aver RAM) Hello, I know this signal exists for the A500 but I didn't find it for the 1200. This signal alters the address decoding so that at early boot stage (after cold reset) the ROM appears for the 68000 at address 0000h, setting a proper reset vector because the ROM content is well defined. After that the OVL signal is cleared by the kickstart so that RAM is seen at address 0000h. Does anyone know if this signal still exists on the 1200 and what is the kickstart procedure to clear it? Thanks

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
AmigaOS 3.9 BB ROM Update Error & Ram-Handler Not Updating 44.24	crazyegg	support.Apps	23	10 November 2010 21:03
Rom burning - Which 4KB ram reserve module	gulliver	support.Hardware	0	30 October 2010 01:53
Possible to map 3.1 ROM on a Standard1200+8MB RAM?	ancalimon	support.Apps	3	12 December 2009 16:17
Amiga 1200 6 MB RAM 170 MB Hard Disk CD ROM + Zip drive	cd32.co.uk	MarketPlace	4	10 October 2005 14:20
need a 2MB RAM expansion for A-570 CD-ROM ?	Overdoc	Amiga scene	0	22 March 2003 01:34

27 July 2011, 08:23	#2
Toni Wilen WinUAE developer Join Date: Aug 2001 Location: Hämeenlinna/Finland Age: 49 Posts: 26,502	Gayle implements the signal internally, CIA OVL pin is unused. Basically reset sets it, first CPU CIA access disables it. It is explained in Gayle datasheet (check eab file server)

27 July 2011, 23:45	#3
whitebird Registered User Join Date: May 2006 Location: France Age: 48 Posts: 211	Thanks a lot Toni. I think that as OVL signal is no more related to the CIA, the only reason to continue to write to the CIA (to disable OVL), is for compatibility with software that has been written for models where OVL signal is generated through CIA. I didn't know about those documents available on eab server. What a precious data collection!

29 July 2011, 22:51	#4
whitebird Registered User Join Date: May 2006 Location: France Age: 48 Posts: 211	Isn't there a mistake in the GAYLE specification? It says to negate OVL there should be a write to $BFD000-$BFDFFF, but I actually write to $BFE001, which is an address in the ODD CIA space, and OVL signal is connected on the ODD CIA...

29 July 2011, 23:39	#5
Toni Wilen WinUAE developer Join Date: Aug 2001 Location: Hämeenlinna/Finland Age: 49 Posts: 26,502	This datasheet most likely describes pre-production/beta Gayle version, not final production version. There is also other minor differences, like PCMCIA address space.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)