Hi,
there is no globla memory approach. In order to be access any part of the flash you can use 16bit address only for spaces 4000~7FFF and C000~FFFF which are also addressable via PPAGE access via pages 3E and 3F together with window 8000~BFFF.
Moreover, the memory map can be affected by modification registers INITRG, INITRM and INITEE.
The memory map can be found in the attachment.
Moreover, I have attached a SW which can help you to create your own bootloader.
Best regards,
Ladislav