d_seq segment
    filenamein db 'D:\file1.txt', 0
    buf dw 100 dup (?)
    blen dw 100
    handle dw 0
    filenameout  db  'D:\file2.txt',0
    descr   dw  ?
d_seq ends

s_seq segment
db 256 dup (?)
s_seq ends

c_seq segment
assume cs:c_seq, ss:s_seq, ds:d_seq
start:

    mov     ax, d_seq
    mov     ds, ax
    mov     ax, ss
    mov     ss, ax

    mov     AH, 3dh
    mov     AL, 0
    mov     DX, offset filenamein
    int     21h
    mov     handle, AX

    mov     AH, 3fh
    mov     BX, handle
    mov     DX, offset buf ;
    mov     CX, 100
    int     21h

    mov     DI, offset buf ;
    mov     BX, 100 ;
    mov     byte ptr [DI+BX], '$'

    mov     AH, 9 ;
    mov     DX, offset buf
    int     21h


    xor     dx, dx
    xor     ah, ah
    mov     ah, 2
    mov     dl, 0dh
    int     21h

    mov     DX, offset buf

    mov bx, dx
    xor ch, ch
    mov cl, [bx]
    mov dx, bx

@@next:
    mov al, [bx]

    cmp al, 'a'
    jb @@skip

    cmp al, 'A'
    jb @@skip

    cmp al, 'z'
    ja @@skip

    cmp al, 'Z'
    jbe @@save

@@save:
    xor byte ptr [bx], 20h
@@skip:
    inc bx
    loop @@next

    mov byte ptr [bx],'$'

    mov ah, 09h
    int 21h

;----------------------------------------------------------
    xor dx, dx

    mov   ah,3Ch  ;Create File
    xor   cx,cx
    lea   dx,filenameout
    int   21h
    mov   descr,ax

    mov   ah,3Dh  ;Open File
    mov   al,2
    lea   dx,filenameout
    int   21h
    mov   descr,ax

    xor   ax,ax
    mov   al,100
    cbw
    mov   bp,ax
    xor   ax,ax

    mov   bx,descr
    call  write

    mov   ah,3Eh  ;Close File
    mov   bx,descr
    int   21h

    mov ax,4C00h
    int 21h
write:
    xor cx,cx
    mov   ah,40h  ;Record File
    mov   cx,bp
    lea   dx,buf
    mov   cx, 65
    int   21h

    int 20h
c_seq ends
end start