int 21 is from dos and you dont use it anymore in windows, dos is dead.
there isnt any way to switch to ring0 in xp, 9x is very easy.
only way to use ring0 is code your own ring0 driver aka .sys
but deroko already told you best way from ring3.
ds is data segment used in dos. now they all point into same segment.
in 64bit env they are dead due amd and intel used cs ds es fs gs segment bits for memory range. to not change gdt table item size, so we saved 3 bits and can page 8x more memory. dont waste time for old and useless things