Build this project at your own risk! This document is for reference information only!
Your Standard Atari OS is a 16k Rom Image. The 27256 E-Prom is 2 times as large as the rom you took out of your machine.
27128 Rom = $0000-$3FFF mapped into $C000-$FFFF
27265 Rom = $0000-$7FFF that can be broke into two banks mapped into $C000-$FFFF
Bank1 = $0000-$3FFF
Bank2 = $4000-$7FFF
How to Build your own Dual OS
Part need for this project:
1 High quality 28 pin IC sockets
Few feet of 22 gauge wire
1 27256 E-Prom
1 Single pole single throw switch
1 1 kohm 1/8watt resister
Desolder with a devack Soldering tool the OS chip in your Atari. With a 15 watt Soldering iron solder in your socket onto your mother board.
First thing you need to do is pick 2 Atari OS images. Then burn the OS images you want into the 2 16k banks. Bend out pins 27. (Make sure you always put the Standard OS into one of the banks for compatibility)
Solder one leg of one resister to pin 27. Solder 1 wire to pin 27 and solder the other end of the wire to one leg of your switch.
bend the leg of the other end of the resister to pin 28 of your 27256 E-Prom. Make sure to leave enough of pin 28 to be insert into your socket.
Now solder 1 wire to pin 14 (ground) and the other end to one of the unsolder legs of the switch. .
While there are many OS upgrades this is a very stable one. This upgrade will not work with a Ultra Speed Plus OS Image.
Here are the Operating Systems Documentation I use the Most and their documentation. Their is E-Prom Images in my download section of this web site
Other Applications for Dual E-Prom Configurations
You can use this same circuit to create a OSS Super Cart with all the OSS language Roms. Here is a picture of an OSS Super Cart that uses 1 27128 E-Prom. Not all of the OSS Super Cart use a single Chip. Fact is the Vast number of OSS Super Carts use 2 2764 Roms for their carts. But if you have a OSS Cart that is single chip here is a rom image to build an OSS Super Cart with Mac/65, Basic XL, Basic XE, and Action.
Just select two OSS cart images you want to use and burn them into the 32k by 8 Rom. Next solder your switch and resister and pack them all up and put your project into the case.
Once again these are modification I own and have. If you have a mod I do not have please e-mail me I would Like to here about it.