Equivalent assembly code: 762 705 683 670 670 characters (Counting newline as 2 chars, CR LF)

org $100
t equ $61
xor ax,ax
mov bx,10
xor cx,cx
mov si,$81
mov di,$80
mov cl,[di]
add di,cx
a:inc si
mov cl,[si]
sub cl,'0'
mul bx
add al,cl
cmp si,di
jb a
mov bx,ax
mov cx,ax
mov ax,$1001
mov dx,-1
xor si,si
xor di,di
int t
push bx
mov ax,$1200
mov di,$80
mov cx,1
mov dx,-1
int t
mov ax,$1400
int t
xor ax,ax
mov bx,10
add di,6
lea cx,[di+4]
b:add al,[es:di]
adc ah,0
inc di
cmp di,cx
jb b
mov di,$84
mov cx,$80
mov bx,10
c:dec di
xor dx,dx
div bx
add dl,'0'
mov [di],dl
cmp di,cx
ja c
push ds
pop es
mov ax,$130e
pop bx
mov di,$80
mov cx,4
mov dx,-1
int t
mov ax,$1100
mov dx,1
int t
mov ax,$4c00
int $21

Equivalent assembly code:

org 0x100
tcp equ 0x61    ; NTCPDRV interrupt

    xor ax,ax
    mov bx,10
    xor cx,cx
    mov si,0x81     ; [ds:81]-[ds:FF] = command line args
    mov di,0x80     ; [ds:80] = strlen(args)
    mov cl,[di]
    add di,cx
    
@@: inc si
    mov cl,[si]     ; get character
    sub cl,'0'      ; convert char to int
    mul bx          ; ax *= 10
    add al,cl
    cmp si,di
    jb @b
    ; now ax = port number

    mov bx,ax       ; source port (leaving this 0 doesn't work?)
    mov cx,ax       ; dest port
    mov ax,0x1001   ; open TCP socket for listening
    mov dx,-1       ; infinite timeout
    xor si,si       ; any dest IP
    xor di,di
    int tcp
    ; ^ I think this call should block until a connection is established, but apparently it doesn't.

    push bx         ; bx = socket handle, save it for later

    mov ax,0x1200   ; read from socket
    mov di,0x80     ; es:di = buffer (just reuse argument area to save space)
    mov cx,1        ; one byte
    mov dx,-1
    int tcp         ; this will block until a client connects and sends one byte

    mov ax,0x1400   ; get TCP session status, bx=handle
    int tcp
    ; now es:di points to a struct containing the source/dest IP addresses and ports
    ; the docs say it's two dwords for each IP address, then two bytes for "ip_prot" and "active" (whatever that means)
    ; ...but actually each IP address is followed by the port number (one word)

    xor ax,ax
    mov bx,10
    add di,6        ; [es:di+6] = client IP
    lea cx,[di+4]
@@: add al,[es:di]  ; add all bytes together
    adc ah,0
    inc di
    cmp di,cx
    jb @b
    ; now ax contains the IP address sum

    mov di,0x84     ; recycle arguments area again
    mov cx,0x80
    mov bx,10
@@: dec di
    xor dx,dx
    div bx          ; dl = ax mod 10
    add dl,'0'      ; convert int to char
    mov [di],dl
    cmp di,cx
    ja @b
    ; now [ds:80]-[ds:83] contains an ascii representation of the IP address sum

    push ds
    pop es
    mov ax,0x130e   ; send data with newline, wait for ack
    pop bx          ; socket handle
    mov di,0x80     ; es:di = data
    mov cx,4        ; sizeof data
    mov dx,-1
    int tcp

    mov ax,0x1100   ; close TCP socket
    mov dx,1
    int tcp

    mov ax,0x4c00
    int 0x21

8086 assembly (16-bit DOS), 762 705 683 670 characters

(Counting newline as 2 chars, CR LF. Assembled size: 163 156 148 148 bytes)

org $100
t equ $61
mov ax,0
mov bx,10
mov cx,0
mov si,$81
mov di,$80
mov cl,[di]
add di,cx
a:
 inc si
mov cl,[si]
sub cl,'0'
mul bx
add al,cl
cmp si,di
jb a
mov bx,ax
mov ax,$1001
mov cx,bx
mov dx,-1
mov si,0
mov di,0
int t
push bx
mov ax,$1200
mov di,$80
mov cx,1
mov dx,-1
int t
mov ax,$1400
int t
mov ax,0
mov bx,10
add di,6
lea cx,[di+4]
b:
 add al,[es:di]
adc ah,0
inc di
cmp di,cx
jb b
mov di,$84
mov cx,$80
mov bx,10
c:
 dec di
mov dx,0
div bx
add dl,'0'
mov [di],dl
cmp di,cx
ja c
push ds
pop es
mov ax,$130e
pop bx
mov di,$80
mov cx,4
mov dx,-1
int t
mov ax,$1100
mov dx,1
int t
mov ax,$4c00
int $21

8086 machine code (16-bit DOS), 163 156 148 148 142 bytes

00000000  31 c0 bb 0a 00 31 c9 be  81 00 bf 80 00 8a 0d 01  |1....1..........|
00000010  cf 46 8a 0c 80 e9 30 f7  e3 00 c8 39 fe 72 f2 89  |.F....0....9.r..|
00000020  c3 89 c1 b8 01 10 ba ff  ff 31 f6 31 ff cd 61 53  |.........1.1..aS|
00000030  b8 00 12 bf 80 00 b9 01  00 ba ff ff cd 61 b8 00  |.............a..|
00000040  14 cd 61 31 c0 bb 0a 00  83 c7 06 8d 4d 04 26 02  |..a1........M.&.|
00000050  05 80 d4 00 47 39 cf 72  f5 bf 84 00 b9 80 00 bb  |....G9.r........|
00000060  0a 00 4f 31 d2 f7 f3 80  c2 30 88 15 39 cf 77 f2  |..O1.....0..9.w.|
00000070  1e 07 b8 0e 13 5b bf 80  00 b9 04 00 ba ff ff cd  |.....[..........|
00000080  61 b8 00 11 ba 01 00 cd  61 b8 00 4c cd 21        |a.......a..L.!|
0000008e

Equivalent assembly code: 762 705 683 670 670 characters (Counting newline as 2 chars, CR LF)

org $100
t equ $61
xor ax,ax
mov bx,10
xor cx,cx
mov si,$81
mov di,$80
mov cl,[di]
add di,cx
a:inc si
mov cl,[si]
sub cl,'0'
mul bx
add al,cl
cmp si,di
jb a
mov bx,ax
mov cx,ax
mov ax,$1001
mov dx,-1
xor si,si
xor di,di
int t
push bx
mov ax,$1200
mov di,$80
mov cx,1
mov dx,-1
int t
mov ax,$1400
int t
xor ax,ax
mov bx,10
add di,6
lea cx,[di+4]
b:add al,[es:di]
adc ah,0
inc di
cmp di,cx
jb b
mov di,$84
mov cx,$80
mov bx,10
c:dec di
xor dx,dx
div bx
add dl,'0'
mov [di],dl
cmp di,cx
ja c
push ds
pop es
mov ax,$130e
pop bx
mov di,$80
mov cx,4
mov dx,-1
int t
mov ax,$1100
mov dx,1
int t
mov ax,$4c00
int $21

8086 16-bit assembly (DOS), 762 705 683 characters

(163 156 148 bytes compiled)

org $100
t equ $61
mov ax,0
mov bx,10
mov cx,0
mov si,$80
mov di,si
mov cl,[si]
add di,cx
inc si
@@:
inc si
mov cl,[si]
sub cl,'0'
mul bx
add al,cl
cmp si,di
jb @b
mov bx,ax
mov ax,$1001
mov cx,bx
mov dx,-1
mov si,0
mov di,0
int t
push bx
mov ax,$1200
mov di,$80
mov cx,1
mov dx,-1
int t
mov ax,$1400
int t
mov ax,0
mov bx,10
add di,6
lea cx,[di+4]
@@:
add al,[es:di]
adc ah,0
inc di
cmp di,cx
jb @b
mov di,$84
mov cx,$80
mov bx,10
@@:
dec di
mov dx,0
div bx
add dl,'0'
mov [di],dl
cmp di,cx
ja @b
push ds
pop es
mov ax,$130e
pop bx
mov di,$80
mov cx,4
mov dx,-1
int t
mov ax,$1100
mov dx,1
int t
mov ax,$4c00
int $21

This assumes ntcpdrv is loaded at INT 0x61 (and any suitable packet driver at 0x60). Compile with fasm tcpserv.asm.

It has some issues though:

• It doesn't check if the argument is a valid port number, or if it's a number at all.
• The client must send at least one byte, since I can't seem to find any other way to tell if a client has connected.
• It only works once, and hangs on a second attempt. Works again after a reboot.
• The returned value is left-padded with zeroes.
• This is my very first code golf entry, and also my very first 8086 asm program. I'm sure there are ways to improve this further.

8086 assembly (16-bit DOS), 762 705 683 670 characters

(Counting newline as 2 chars, CR LF. Assembled size: 163 156 148 148 bytes)

org $100
t equ $61
mov ax,0
mov bx,10
mov cx,0
mov si,$81
mov di,$80
mov cl,[di]
add di,cx
a:
inc si
mov cl,[si]
sub cl,'0'
mul bx
add al,cl
cmp si,di
jb a
mov bx,ax
mov ax,$1001
mov cx,bx
mov dx,-1
mov si,0
mov di,0
int t
push bx
mov ax,$1200
mov di,$80
mov cx,1
mov dx,-1
int t
mov ax,$1400
int t
mov ax,0
mov bx,10
add di,6
lea cx,[di+4]
b:
add al,[es:di]
adc ah,0
inc di
cmp di,cx
jb b
mov di,$84
mov cx,$80
mov bx,10
c:
dec di
mov dx,0
div bx
add dl,'0'
mov [di],dl
cmp di,cx
ja c
push ds
pop es
mov ax,$130e
pop bx
mov di,$80
mov cx,4
mov dx,-1
int t
mov ax,$1100
mov dx,1
int t
mov ax,$4c00
int $21

This assumes ntcpdrv is loaded at INT 0x61 (and any suitable packet driver at 0x60). Compile with fasm tcpserv.asm.

It has some issues though:

• It doesn't check if the argument is a valid port number, or if it's even a number at all.
• The client must send at least one byte, since I can't seem to find any other way to tell if a client has connected.
• It only works once, and hangs on a second attempt. Works again after a reboot.
• The returned value is left-padded with zeroes.
• This is my very first code golf entry, and also my very first 8086 asm program. I'm sure there are ways to improve this further.