from BASIC. Obviously it shouldn't be lower than &170 or higher than HIMEM's initial value. (Sometimes I personally don't use this at all. If I compile something at &4000 and my BASIC program is small, then I just don't bother with MEMORY and HIMEM because usually nothing gets overwritten.)
[http://cpcrulez.fr/firmware.txt The CPCEMU Amstrad CPC Firmware Guide] has, among other things, detailed information about the RAM layout on 464 and 6128. Yet it's not good practice to just POKE there as it may be incompatible with other models (e.g. the Plus range) or make your program crash whenever the system is not exactly like yours (e.g. expansion boards, which are now a substantial fraction of the remaining CPC users).
=== CPC Hardware & Firmware ===