QUAD
QUAD (at least the functionality) is modeled heavily after the PG Designs ram bank boards.
They both provide multiple banks main ram, which you can switch between using a special menu program that overlays the built in main rom menu. QUAD's "0quad" is modeled after PG Designs "0menu"
Differences are: PG Designs came in several sizes from a single extra 32k up to 7 additional 32k banks added to the built-in.
QUAD is one size, 4 banks.
PG Designs boards added banks to the built-in. The original built-in 8k-32k remains active and accessible as the main "Bank 0" when the PG Designs board is added.
QUAD displaces the built-in ram. So when you install a QUAD, the built-in ram becomes hidden and inaccessible, and the QUAD itself provides the ram for all 4 available banks. (This could be an advantage. If the built-in ram has bad addresses or bad chips, or if there is only 8, 16, or 24k built-in instead of the full 32k, the QUAD will give you full 32k in bank 0 as well as 1-3)
PG Designs boards use a button battery right on the ram board to maintain the memory. Quad uses the computer's internal memory battery. There are advantages and disadvantages to both approaches. The PG Designs board can be pulled out of the M100 and put into another, and the memory will still be preserved on it. The QUAD will lose it's memory if you unplug it. However, the button cell on the PG Designs must be replaced periodically. The QUAD has no consumable parts to maintain.