4-TOS SWITCHER
By Vezz!
----------------------
http://come.to/vezz.it
----------------------

Sometimes you need TOS 2.06. Sometimes you need MagiC. Sometimes that old
games works only with TOS 1.6... no more TOS loaders and dirty tricks with 
this easy hardware TOS switcher.

WHAT YOU NEED
-------------
- Two 4Mbit 512K*8 eproms (for example 27C4001 or 27C040)
- Two 2-position 1-way switches (a total of 3 pins each)
- Some wire
- A soldering iron

HOW IT WORKS
------------
All TOSes since v1.6 are 256K in size. Usually you can find two 1mbit eproms
in your Atari.
We program the 4bit eproms in pages of 1mbit, so we have 4 different
combinations of TOSes. In a very simple way with the switches the correct 
address is set, and that's it.
Let's give a look at eprom schematics.

        274001                           271001
     512kx8 EPROM.                     1Mbit EPROM      
                                                             
    +--------------+                +------\/------+         
VPP |1    +--+   32| VCC        VPP |1           32| VCC     
A16 |2           31| A18 *      A16 |2           31| /PGM      
A15 |3           30| A17 *      A15 |3           30| NC      
A12 |4           29| A14        A12 |4           29| A14     
 A7 |5           28| A13         A7 |5           28| A13     
 A6 |6           27| A8          A6 |6           27| A8      
 A5 |7           26| A9          A5 |7           26| A9      
 A4 |8   274001  25| A11         A4 |8   271001  25| A11     
 A3 |9           24| /OE         A3 |9           24| /OE     
 A2 |10          23| A10         A2 |10          23| A10     
 A1 |11          22| /CE         A1 |11          22| /CE     
 A0 |12          21| D7          A0 |12          21| D7      
 D0 |13          20| D6          D0 |13          20| D6      
 D1 |14          19| D5          D1 |14          19| D5      
 D2 |15          18| D4          D2 |15          18| D4      
GND |16          17| D3         GND |16          17| D3      
    +--------------+                +--------------+     
                          

A17 and A18 drives the eprom page. If we send '1' or '0' to these two pins,
we get four combinations... 00,01,10 and 11, corresponding to 4 different
1mbit pages.
That's it!


Programming the Eproms
----------------------
You need some practice with your eprom programming software for this.
Select 4mbit eprom type, then load 'LO' part of first tos at address 00000.
Then load 'LO' part of the second tos at address 20000. Third TOS will go
at 40000, fourth at 60000. Then program the 4mbit 'LO' eprom.
Do the same (keep same TOS order!!) for the 'HI' part.
If your eprommer won't get 4mbit eproms but only 1mbit eproms, you can make
it anyway.
Install switches as it is explained later, and program by pages of 1mbit 
the eproms, flipping switches appropriately. Remember to bend out of the
socket pin 30 and 31.


Installation
------------
Bend pins 31 and 30 and keep them out of the socket.
Take the switches... solder a wire for each pin.

        SW1                              SW2
                                        
     +5V o                            +5V o    
                                    
     A17 o                            A18 o           
                                    
     GND o                            GND o    
     
You can grab +5V from PIN 32 of the eprom, and GND from PIN 16.
If you like, drill two hole somewhere in your ST case and fit switches there.
Make note of the correspondence between position and TOS version.

How to switch TOS
-----------------
Simply turn off ST, flip the switches, turn on ST.
You can switch TOS even while you keep pushed the 'reset' button.
If you switch TOS while system is running, the system crashes, but there
are no hardware faults.

Why not TOS 1.0?
----------------
TOS 1.0 is 192K in size, and located at $FC00000, while TOS 1.6 and upper
are 256K in size and located at $E000000.
The size is not a problem, but the different location is a BIG problem.
The tos should be relocated, and currently I haven't found how to do it.

Copyrights blah blah
--------------------
This simply hardware modification is free.
No one can sell it.
Everyone can do it.
TOS is (C) By Atari, thus is not included in this package.
Do not mail me for TOS images, but look around in the Internet and you'll
find dozens of them.

Marcello Vezzelli - Italy
-------------------------
The Atari Hardware Hack Page
http://come.to/atarihacks
                                              
