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From: root@zswamp.uucp (Geoffrey Welsh)
Newsgroups: comp.sys.cbm
Subject: Re: QuickSilver 128
Message-ID: <14.2824F701@zswamp.uucp>
Date: 2 May 91 17:02:38 GMT
Organization: Izot's Swamp BBS (FidoNet), Kitchener, Ontario
Lines: 58

In a letter to All, J. Luce (luce@aurs01.UUCP ) wrote:

 >-The clip is required in order to swap the internal ROM with 
 >the C64 ROM.
 >-It does not affect C128 usage as far as I know, since that 
 >ROM is
 >-already in the machine at all times.

 >Bzzzzz!Timeout! What do you mean by the 'ROM is already in 
 >the machine'?

 >I got no ROM with this (how does one get into the little 
 >black box
 >surrounding the midsection of the QS128?)... do I need to 
 >call Sykes? If
 >so, what's their phone number?

   Perhaps I should explain why the wire is needed for the 64 in the first 
place.

   The C64 wedge works by "replacing" the internal C64 Kernal ROM - or, at 
least, part of it, with an external ROM which knows how to access the IEEE 
I/O ports.  In order to do this, the external cartridge decodes the address 
lines and, if it decides that the CPU is talking to the ROM, it will assert 
the -GAME line on the cartridge port, reconfiguring the 64 as a MAX machine 
(in which mode, the 8K at $E000-$FFFF is replaced by an external ROM).  This 
permits an external cartridge to override the Kernal ROM whenever it wants to 
by switching into and out of MAX mode.

   This is all fine & dandy, but there's a hitch: the cartridge asserts the 
GAME line based on the values of the address lines only; it doesn't know 
whather it's overriding the Kernal ROM or the RAM underneath it (which is 
banked in by the HIROM line, conmtrolled by the I/O port on the 6510).  This 
is a real problem for software which uses that upper 8K of RAM, such as 
PaperClip, because suddenly there's a "hole" in the RAM where the external 
cartridge overrides what it thinks is Kernal ROM.

   The clip is meant to be attached to the 6510 CPU or 82100 PLA and feed the 
HIRAM signal out to the cartridge; a low on that line would indicate 
that RAM is banked in and prevent the cartridge from overriding it.

   That explains why we need the clip for C64 mode, but it doesn't explain 
why we don't for C128 mode... to tell you the truth, I'm not sure why it 
wouldn't be needed (RTC's IEEE 128 adapter for its C64-Link II actually has 
*more* clips...) but I suspect that the cartridge could program the MMU 
however it pleases so that it doesn't have to pull stunts like the C64 
cartridge does...

   Matt (who's on the phone as I type) also agrees that a well-designed 128 
cartridge should be able to program its way through, rather than use the brute 
force.
 

--  
Geoffrey Welsh - Operator, Izot's Swamp BBS (FidoNet 1:221/171)
root@zswamp.uucp or ..uunet!watmath!xenitec!zswamp!root
602-66 Mooregate Crescent, Kitchener, ON, N2M 5E6 Canada (519)741-9553
"He who claims to know everything can't possibly know much" -me