2,060 bytes added,
17:44, 15 January 2012 == Description ==
This modification will allow you to have two DOSs in a CPC6128 without the need of an addition PCB. By replacing the standard 16K EPROM with a 32K and switching the MSB (A14), you can choose the DOS that the CPC uses.
== Warning / Disclaimer ==
Although I have taken the utmost care preparing this documentation, I do not guarantee that it is error free and I accept no responsibility for damage to anyone’s hardware or other personal equipment or injury inflicted on you or others.
== Construction ==
First you need to remove the 40015 IC from the mainboard. Tips on how to do this can be found [[IC Repair|here]].
[[Image:IC_Repair9.png|600px|centre]]
Next you will need to prepare a 28pin socket. To make the mod exactly like I have done, you should use a low-profile socket where the pins protrude below the sockets plastic.
Pin 27 should be cut off completely below the plastic and a 10K resistor should be soldered between pin 27 and pin 1. This is a pull-up resistor to ensure that A14 of the EPROM is held at 5V when the OS switch is open.
[[Image:DualOS1.png|600px|centre]]
Next you need to solder a flylead to pin 1 and pin 27. These leads need to go to a standard normally open switch mounted on the CPCs casing.
Closing the switch will set A14 to 0 and opening it sets A14 to one. This allows you to enable the upper or lower 16K of the EPROM and thus select which OS the CPC sees.
[[Image:DualOS2.png|600px|centre]]
The following diagram gives a clearer picture of how the connections should be.
[[Image:OS Switch.png|600px|centre]]
The socket now needs to be soldered onto the mainboard, but pin 27 should not make contact with the PCB track.
[[Image:DualOS3.png|600px|centre]]
The EPROM should be burnt with whatever two OSs you wish to have, the first OS is saved from &0000 to &3FFF and the second OS should be located from &4000 to &7FFF.
The ROM image below is a single 32K file with AMSDOS and ParaDOS.
[[Image:DualOS4.png|600px|centre]]
[[File:DualOS.ROM]]
Bryce.
[[Category:DIY]]