I have a problem with my study

I read a program,but some of my program confuse me
what are these "push ds
sub ax,ax
push ax " and "move ah,2" do?



prognam segment
main proc far
assume cs rognam
start:
push ds
sub ax,ax
push ax
mov ch,4
rotate: mov cl,4
rol bx,cl
mov al,bl
and al,0fh
add al,30h
cmp al,3ah
jl printit
add al,7h
printit:
mov dl,al
mov ah,2
int 21h
dec ch
jnz rotate
ret
main endp
prognam ends

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