unix2dos

So I tried co make unix to dos (dos to unix) end line converter and tried do use stdin/stdout. This posed a problem to recognize when was end of file reached. So starting from int 21h, ah=08h to read stdin and ah=02h to write stdout, I ended up reading with func. ah=3Fh, and writing with ah=02h.

So question is this (u2d-2):
1. is there a way to get EOF with ah=02h, or similar?

If I stick with ah=3Fh, and wanted to read more than 1 byte at a time, then:
2. how do I distinguish between reading less than requested, especially 0, and EOF?

If I use ah=40h to write (u2d-3) then:
3. does it make sense to check correctness of writing to stdout?
4. if I check then how do I know separate recoverable writing "error" from irrecoverable?

u2d-2:

Code:

;
; read from stdin and write to stdout replacing '\n' with '\r\n'
;
format MZ

main:
        push    cs
        pop     ds
L1:
        mov     dx, buf
        mov     ah, 3Fh
        mov     bx, STDIN
        mov     cx, 1
        int     21h     ; read char from file into buffer
        cmp     al, 0   ; number of bytes read
        je      EXIT

        mov     al, [buf]
        cmp     al, LF
        je      WR_CR
WR_BUF:
        mov     dl, al
        mov     ah, 02h
        int     21h     ; write char form DL to stdout
        jmp     L1
WR_CR:
        mov     dl, CR
        mov     ah, 02h
        int     21h     ; write CR
        mov     al, 10
        jmp     WR_BUF  ; write NL

EXIT:
        mov     ax, 4C00h
        int     21h; exit program

desc db 'unix2dos',0
buf db ?
CR = 13
LF = 10
STDIN = 0
STDOUT = 1

u2d-3:

Code:

;
; read from stdin and write to stdout replacing '\n' with '\r\n'
;
format MZ

main:
        push    cs
        pop     ds
L1:
        mov     dx, buf
        mov     ah, 3Fh
        mov     bx, STDIN
        mov     cx, 1
        int     21h     ; read char from file into buffer
        cmp     al, 0   ; number of bytes actually read
        je      EXIT    ; exit at EOF
                        ; check for LF
        mov     al, [buf]
        cmp     al, LF
        je      WR_CR
WR_BUF:
        mov     dx, buf
        mov     ah, 40h
        mov     bx, STDOUT
        mov     cx, 1
        int     21h     ; write char from buffer to file
;       cmp     al, 0   ; number of bytes actually written
;       je      EXIT    ; do ... on unsuccessful write/-
        jmp     L1
WR_CR:
        mov     dx, _cr ; write CR
        mov     ah, 40h
        mov     bx, STDOUT
        mov     cx, 1
        int     21h
;       cmp     al, 0
;       je      EXIT
        jmp     WR_BUF  ; back to writing buf

EXIT:
        mov     ax, 4C00h
        int     21h; exit program

desc db 'unix2dos',0
buf db 0,0
CR = 13
LF = 10
_cr db 13       ; CR ('\r')
_lf db 10       ; LF ('\n')

STDIN = 0
STDOUT = 1