[Coco] SCS - $FF40 behavior

[Chad H]

Roger that.  However, the 'banking' scheme done by the CoCo 3 requires the GIME functions and.  These boards will be using a universally compatible scheme that should work on ALL CoCo's.  We already have a 25 byte assembly loader that can precede .BIN data on a EEPROM and load the .BIN at power-up.  The problem is, if it goes over 8K it can crash into the CoCo 3 ROM space.  On a CoCo 2, if it goes over 15.75K, it can crash into I/O vectors.

Solution I'm planning on using to break the .BIN size limit (will still be limited by capacity of the EEPROM) is to modify the assembly to send a signal to the microcontroller that is switching the banks on my boards at the appropriate time.  In order to maintain a good copy of the assembly loader, a copy will have to exist in each bank.   Every bank except for the primary bank will have a byte marker at the beginning (&HC000) to identify it.


** Assembly flow **

 1.  Read 8k (or until end of .BIN data)
 2.  When 8k (&HDFFF) is reached, trip the SCS line  (Microcontroller will 'switch' to the next 8K bank segment)
 3.  Read &HC000 for bank identifier byte.  If not expected bank then go back to #2.
 4.  Bank has switched, reset pointer back to beginning of .BIN data and resume (#1).
   

- Chad H
http://sites.google.com/site/cbhlab101/

-----Original Message-----
From: Coco [mailto:coco-bounces at maltedmedia.com] On Behalf Of RETRO Innovations
Sent: Friday, October 16, 2015 12:18 AM
To: CoCoList for Color Computer Enthusiasts
Subject: Re: [Coco] SCS - $FF40 behavior

On 10/16/2015 12:04 AM, Chad H wrote:
> What do you mean "only after banking in the ROMs"?  From what I observed on my CoCo 2, a simple PEEK or POKE triggered the SCS line low for a few microseconds then it was released.  Is the CoCo 3 doing something different here?
Sorry, I confused two things.

Yes, $ff40 will drop SCS

But, to read from the ROM space on the Coco, you have to bank it in:

 > On a Coco3 you must force ROM mode before attempting to read the external ROM. So, first POKE&HFFDE,0 and then try to read the Typing cart; ?PEEK(&HC000).


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