Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!cica!tut.cis.ohio-state.edu!brutus.cs.uiuc.edu!apple!oliveb!amiga!cbmvax!daveh From: daveh@cbmvax.UUCP (Dave Haynie) Newsgroups: comp.sys.amiga.tech Subject: Re: OVL line in Amiga 2000 Message-ID: <7698@cbmvax.UUCP> Date: 19 Aug 89 01:51:24 GMT References: <8908181206.AA07159@jade.berkeley.edu> Organization: Commodore Technology, West Chester, PA Lines: 25 in article <8908181206.AA07159@jade.berkeley.edu>, U211344@HNYKUN11.BITNET (Olaf 'Rhialto' Seibert) says: > This makes me wonder. The 2000 doesn't need a boot rom, does it? The > start of the kickstart ROM has the same structure as the boot rom > in the 1000. (I know; I disassembled it). Does the OVL line maybe > map a PART of the kickstart rom to location zero? In fact. The base of the ROM ($00FC0000) is overlaid at location 0 when the OVL line is asserted. The same thing happens in A1000s; this overlay tells the system where the start of code is, in standard 68000 fashion. One interesting side effect of this ROM overlay, plus the way autoconfig works, is a noticable lack of RAM after RESET. Most of the time that's not a problem, but if you ever need to write a program that resets the machine and then does something other than jump to the start of ROM, it's a problem. I'd personally be interested in seeing a routine that would reliably do that on a 68000 machine; I don't think it's possible.... > Astounding News! By Express Via Norfolk! OS+2 is a trademark of > The Atlantic Crossed in Three Days! Olaf 'Rhialto' Seibert > [E.A. Poe, 1844] U211344@hnykun11.bitnet -- Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: hazy BIX: hazy We have no choice. We are, after all, professionals.