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Here's how to add a switchless multi kernal mod to your flat 128, 
based on the C64 Kernel Switcher and Restore Key Reset/Selector By Adrian Black. 
This requires a 5v arduino, I'm using a pro mini I had laying around. 
The mod needed one more line defined to add the second rom chip for 128 mode.


C128 Flat Roms.

U32 Contains 64 Mode Basic and Kernal, so we can hold 4 total sets on a 27512 with no board mods.
U35 Contains 128 Mode Kernal, so It can contain 4, but I only chose 2 (Original And Jiffy Dos.)

For U32 make a bin file using the following command in a dos prompt.

copy /b basic.bin+kernal0.bin+basic.bin+kernal1.bin+basic.bin+kernal2.bin+basic.bin+kernal3.bin new-u32.bin

Basic.bin is just 64's basic rom each time, and kernal could be whatever ones you want. I left stock kernal as the first one.

For U35, for 2 Kernals you can use a 27256, or do what I did and just double up for a 27512 (I have a bunch of 27512 eproms..)

copy /b kernal128.bin+jiffy128.bin+kernal128.bin+jiffy128.bin+kernal128.bin+jiffy128.bin+kernal128.bin+jiffy128.bin new-u35.bin


Burn your new roms, then we need to modify them before you install.

For U35 bend up pin 27. All other pins go in the socket.

For U32 bend up pins 1 and 27. All other pins go in the socket.

For mounting my promicro, I used some double sided tape to the right of u55, right above C67.
The Right hand side of C67 is +5, and the left side in GND.. Verify with your meter.

Wiring..

I prefer to use 30 gauge kynar wire, I've been using it for decades, so I'm used to it. It also allows you to run the wires under
other components.

For U35, connect pin 27 to pin 9 of the pro mini.

For U32, connect pin 27 to pin 6 of the pro mini.
         connect pin 1 to pin 5 of the pro mini.
		 
		 
Now for the other signals.

Reset can be found on pins 2 or 3 on U63, which is the chip closest to the user port.
U63 (7406) Pin 2 to pin 7 of the pro mini.

Restore key..
This is available 2 places.
U16 pin 9, this is next to the 8502 cpu, or the right hand side of r9, located under the z80 cpu.

Connect to pin 8 of the pro mini.

EXROM

The exrom goes to the mmu and pla, so I'm tapping in to the via right next to pin 12 of the pla.
Connect to pin 3 of the pro mini.


LED.

I'm using a 220uf resistor inline with pin 4 of the pro mini.
This goes to either side of the 3 pin female header. 
There are 2 gnd pins available on the pro mini, or use any gnd from the motherboard.


Arduino Power.

Looking at c67, the right plane is +5v, and the left side is GND. Just attach to the Via's

For More info see 

https://imgur.com/gallery/tCfId77