Int 21H, AH=65H, AL=00H

7.1 and higher

SET GENERAL INTERNATIONALIZATION INFO

CF clear if successful
AX = system code page
CX = number of bytes copied from supplied buffer
CF set on error
AX = error code

 nothing.  next call to AH=59h will return values from fields AX,BX,CX,
  DX,DI, and ES in corresponding registers

This function will not overwrite the country or code-page numbers, but can be used to modify all other data about the country and code page

 AH=65h"GET EXTENDED" - AH=70h
 

Text based on Ralf Brown Interrupt List Release 61