RAM expansions can even be at an adjacent address range. For example, the [[Y-MEM]] is a 512k RAM expansion that uses the &7Exx range instead of the usual &7Fxx.
Given this diversity of RAM expansions, [[SymbOS]] does the right thing by testing each 64k page individually to detect the available RAM.
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However, if the RAM expansion decodes the [[Standard_Memory_Expansions#Extended_1M-4M_Expansions_.28RAM7.2FYarek-style.29|A8 address line]], you can access the internal second RAM bank of the 6128 from another range (&7Exx). The [[X-MEM]] and [[CPC_iRAM#CPC_iRAM.2F640|iRAM/640]] expansions implement this and therefore can provide 640K of total RAM. [https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/extram-512kb-to-640kb-for-cpc6128-on-one-dip20-chip-slg46533v-dip/msg247763/#msg247763 Source]
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== 4MB RAM expansions ==
Beware about 4MB RAM expansions. The upper 2MB (Port 7Bxxh, 7Axxh, 79xxh, 78xxh) of a 4MB RAM expansion can conflict with other hardware as setting A10 to 0 is also used for many hardware expansions.
See the [[Default I/O Port Summary|I/O Port Allocation Rules]] to understand why.
The practical safe maximum on Amstrad CPC with [[Standard Memory Expansions]] is 2MB.
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