IIRC, in ISIS-II, programs were loaded somewhere above 3000H, depending on the buffer requirements of the program, but always above the ISIS resident, which always occupied the same space, regardless of memory size. On some early 8080 implementations, an 8259 wasn't used (expensive!) and a simpler circuit using a priority encoder and a latch to stuff 111iii11 onto the bus in response to an interrupt acknowledge. CP/M could get in the way of this scheme, if it was desired to use the RST0 vector for an interrupt. Otherwise, there was no problem, except for the RST vector (usually 7) used for DDT--but that's local to DDT and easily patchable (it's done for the Amstrad PCW, for example). After having used ISIS before CP/M, I was happy to see the "lean and mean" CP/M. ISIS was verbose, clumsy and slow (e.g., :F0: instead of A: for the first floppy drive; "DELETE" instead of "ERA"). Of course, had CP/M simply shifted the TPA up 256 bytes and used the area between 0100h and 01FFh for command-line storage, system request vectors and FCBs, that would have solved the problem, but for the DDT RST vector. Cheers, Chuck