Programmable Flash Memory Expansion

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The PFM512 boot device modification replaces the standard 16KiB 28-pin boot EPROM with a 512KiB 32-pin Atmel 29c040 flash chip. The chip is visible in four 128KiB banks residing at memory pages 0xF0 - 0xFF. Each page is selected via two re-purposed (normally unused) TMS9901 outputs:

PFM512 Bank Selection
CRU 0x28 CRU 0x3A Bank Notes
0 0 Bank 0 Default Boot code and Operating System
0 1 Bank 2 Flashdisk bank #1
1 0 Bank 1 Flashdisk bank #2
1 1 Bank 3 Flashdisk bank #3