Zenith Z-100 Reference Manual page 130

Proceed in this manner to define each character in your set.
When you have all the characters defined and placed in memory, the first character
will have a hexadecimal code of 80 and the last a code of FF.
All that remains now is to reset the pointer at interrupt vector 1F to the start
of the memory section where your characters reside.
From now on, whenever you call a character code between 80 and FF in graphics
mode, the corresponding character from the set you created will be used in place
of the firmware character normally used.
Video Graphics Programming
Page 7.41