Need some comments here

[toodle]

can someone add some comments here so i can understand the process of the codes
im a beginner here at assembly language
by the way im using tasm for assembly

.model small
.stack
.data

msg1 db 13,10,"Enter a string with dollar symbol as a break :$"
msg2 db 13,10,"Modified string is:$"

buf db 80 DUP(0)
revbuf db 80 DUP(0)
strlen db ?

.code

main proc
mov ax,@data
mov ds,ax

lea dx,msg1
mov ah,9h
int 21h
lea si,buf

read: 
mov ah,1h
int 21h
mov [si],al
inc si
cmp al,24h
je check
jmp read

check:
lea si,buf
lea di,revbuf

start:
mov al,[si]
cmp al,'$'
je dis
cmp al,60h
jb lower
cmp al,7Ah
jb upper
jmp start

lower:
cmp al,40h
jb skip
cmp al,5Ah
jb up

up:
add al,20h
mov [di],al
inc di
inc si
jmp start

upper:
cmp al,60h
ja lo

lo:
sub al,20h
mov [di],al
inc di
inc si
jmp start

skip:
mov [di],al
inc si
inc di
jmp start

dis:
mov al,'$'
mov [di],al

lea dx,msg2
mov ah,9h
int 21h

lea dx,revbuf
mov ah,9h
int 21h

ou:
mov ax,4c00h
int 21h

main endp
end