My understanding of JRCONFIG.SYS is that irregardless of which video page remap it chooses for the B8000-BFFFF area, it still tells DOS to start it's allocations at 10000h (+128K) to avoid DOS loading itself (MSDOS.SYS, IO.SYS, or COMMAND.COM) or applications in the slower shared motherboard RAM. Any idea how it does this?
I'm aware that DOS consults the BDA for the top usable limit where the transient portion of COMMAND.COM usually resides and obviously the heap limit. But I'm not sure how it determines the first available start address.