The CPC6128 second bank of 64K RAM is controlled by a PAL 16L8 chip. It has the Amstrad part number 40031.
On the CPC 6128 schematic, it is top centre: [http://www.cpcwiki.eu/imgs/4/4a/CPC6128_Schematic.png CPC6128 Schematic] however the X inputs aren't distinguished.
The following equations were worked out by Porchy (member on CPCWiki Forum). These can be used to program replacements: