If I do a FUNCTION or SUB When is good to make it fastcall ?

[boriel]

Usually when the function body is in ASM, or when you pass 0 parameters to it.
Fastcall functions are slighlty faster, but the 1st parameter comes into register and ZX Basic currently does nothing with it.
Also the Stack must be rearranged by the user on return if more than 1 parameter is used (the compiler will issue a Warning)