INT 61 - FTP Software PC/TCP - TCP/IP TSR System Call interface
        AH = system call number (see also entries below)
            01h "pkt_alloc" (v2.05; this is "net_config" in v2.1+)
            02h "pkt_free" (v2.05; this is "get_kernel_info" in v2.1+)
Return: CF clear if successful
        CF set on error
            AL = basic error (see below)
            AH = suberror number
Notes:  the installation check consists of testing for the signature "TCPTSR"
          three bytes beyond the start of the interrupt handler
        INT 61 is the default; PC/TCP v2.05 may be configured to use any
          interrupt from 20h through E0h
BUG:    the SLIP kernel for v2.05 bounds-checks the wrong register, so values
          greater than 54h in AH may crash the system.  Other kernels may have
          this bug as well.
SeeAlso: INT 61/AH=00h"PC/TCP",INT 61/AH=2Ah,INT 61/AH=54h
Index:  installation check;PC/TCP

Values for error code:
 00h "NET_NOERR" successful
 01h "NET_ERR_INUSE" protocol or socket already in use
 02h "NET_DOS_ERR" MS-DOS error (returned as suberror code in AH)
 03h "NET_ERR_NOMEM" out of memory
 04h "NET_ERR_NOTNETCONN" not a network descriptor
 05h "NET_ERR_ILLEGALOP" invalid operation on given kind of network descriptor
 06h "NET_ERR_BADPKT" illegal or corrupted packet
 07h "NET_ERR_NOHOST" no host bound to specified connection
 08h "NET_ERR_CANTOPEN" unable to open file
 09h "NET_ERR_NET_UNREACHABLE" network is unreachable
 0Ah "NET_ERR_HOST_UNREACHABLE" host is unreachable (see subcodes below)
 0Bh "NET_ERR_PROT_UNREACHABLE" protocol is unreachable
 0Ch "NET_ERR_PORT_UNREACHABLE" port is unreachable
 0Dh "NET_ERR_TIMEOUT" operation timed out
 0Eh "NET_ERR_HOSTUNKNOWN" unable to resolve host name
 0Fh "NET_ERR_NOSERVERS" no name servers configured
 10h "NET_ERR_SERVER_ERR" bad reply from name server
        Subcodes: 0= no error, 1 = Host unreachable
 11h "NET_ERR_BADFORMAT" bad format for IP address or field in IP address struc
                        is zero
 12h "NET_ERR_BADARG" invalid argument
 13h "NET_ERR_EOF" foreign host closed its end of connection
 14h "NET_ERR_RESET" connection has been reset
 15h "NET_ERR_WOULDBLOCK" recv() call was done on a non-blocking connection
                        with no data available
 16h "NET_ERR_UNBOUND" insufficient resources to do operation
 17h "NET_ERR_NODESC" could not allocate network descriptor
 18h "NET_ERR_BADSYSCALL" invalid/unsupported kernel call
 19h "NET_ERR_CANTBROADCAST" unable to broadcast
 1Ah "NET_ERR_NOTESTAB" operation illegal because connection not established
 1Bh kernel busy, try again later
 1Ch "NET_ERR_ICMPMESG" an ICMP message was received (not on streams)
        (see subcodes below)
---v2.1+---
 1Dh "NET_ERR_TERMINATING" internal kernel fatal error
 1Eh "NET_ERR_TAG_LOCKED" not allowed to set this tag (net_config)
 1Fh "NET_ERR_BAD_INTERFACE" non existent interface specified
 20h "NET_ERR_BADCONFIG" kernel cannot run - bad configuration
 21h "NET_ERR_EMM" expanded memory error
 22h "NET_ERR_CANT_SHUTDOWN" cant unload kernel (multitasker running)
 23h "NET_ERR_PARKED_IN" unable to unhook DOS interrupt
 24h "NET_ERR_NOQIOS" ran out of resources; try again later
 25h "NET_ERR_WOULD_TRUNCATE" datagram too large and "don't truncate" was set

Values for subcodes of error 0Ah "NET_ERR_HOST_UNREACHABLE":
 00h    no error
 01h    host unreachable
 02h    ARP failed
 03h    hardware failure
 04h    link failure
 05h    no route
 06h    gateway down

Values for subcodes of error 1Ch "NET_ERR_ICMPMESG":
 07h    unrecognised
 08h    can't fragment
 09h    srcr_fail
 0Ah    source quench
 0Bh    time exceeded
 0Ch    parameter problem
 0Dh    admin_prohib. see also code 0Ah

.