For use EXOS calls and IRQ system needed the Page0 segment in the Z80 page 0. The machine code program loaded to this segment from 100h. (With carefuly programming you can page out this segment.)

In other words (what Zozo already said), when you want to call EXOS, or leaving EXOS interrupts working, you have to leave the Page0 (0xB0) segment alone. On that page there will be usually the 0xF8 segment (on a 128K machine), or I think 0xFC segment (on a 64K) machine. (Or any other working RAM segment detected by EXOS with the smallest segment number).

The other pages can be populated by wish. EXOS will save and restore those pages at an EXOS call. This will happen with almost all the registers, too.

"Parameters are passed to EXOS calls in registers A, BC and DE, and results are passed back in the same registers. Register A returns a status code which is zero if the call was successful and a non­zero error code otherwise. All other registers (HL, IX, IY, AF', BC', DE', HL') are preserved by all EXOS calls, and also the user's paging is not disturbed. EXOS calls can be made from any address in any Z80 page, and user's stack can be in any of the four pages."

http://gafz.enterpriseforever.com/Dokumentacio/Konyvek/EXOS_2.1_technikal_information/exos/kernel/Ch11.html

From 0000-002Fh and 005C-00FFh also can be used under EXOS. Many programs place SP to 100h (using stack under 0100h).

About 1 years ago I uploaded for you a sample.asm for the begining.

Yes, I was reading this last night. 

Thank-you everyone for the information, that's very helpful!