What you describe sounds like a "Memory Base 128"
You can verify this by checking the picture at
http://pcenginefx.com/TS/system_-_pc_engine_memory_base.html
There is not, to my knowledge, a manual for this
I believe that it is used by only a select few games, and the instructions
vary by (and are specific to) the game in question, so any instructions
would be in the game manual
Different games use it for different purposes (mostly to store saved games)
- but the majority of games do not use it all
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