Stackable CPU boards

[hus]

(continuing from the comments)
Sharing signals shouldn't be a problem - it's just a matter of defining which CPU controls what line (and when) and putting all others in some high-Z state. But it would be more useful just to share power and a communication channel like I2C, and connect all other pins just to one side, e.g. the upper connector. If space allows, connection to the lower connector could be through solder bridges.

[knickers]

What kind of projects might you use this for? It seems to me that it would be easier to just have a separate stack to handle a different task.

[tinybuilder]

Alternately, a "stackable CPU shield", just in the sense of it having a connector on the bottom so that it can be anywhere in the stack.

Considering the current design, if a "coin cell shield" is created with a connector on top and the holder on the bottom, coupling the new "stackable CPU shield" with the new "coin cell shield", the current configuration is simulated.

[calvinthedestroyer]

I would recommend having a shared static ram chip in the middle of the two CPU boards, that way each CPU can send signals to each other in the way of Status flags. IE: CPU1 sets register 1 to high, CPU2 reads register 1 and it knows that CPU1 is done with whatever the task was.

On the old machines I worked on:
CPU1 could write to the even registers, but could only read the odd registers.
CPU2 could write to the odd registers, but could only read the even registers.