INT 21 - pcANYWHERE IV/LAN - INSTALLATION CHECK
        AX = 2B44h ('D')
        BX = 4D41h ('MA')
        CX = 7063h ('pc')
        DX = 4157h ('AW')
Return: AX = 4F4Bh ('OK') if large host resident
           = 6F6Bh ('ok') if small host resident
        CX:DX -> API entry point
SeeAlso: INT 16/AH=79h

Call API entry point with:
        AX = 0000h get pcANYWHERE IV version
            DS:SI -> BYTE buffer for host type code
            Return: AH = version number
                    AL = revision number
                    DS:DI buffer byte filled with
                        00h full-featured host
                        01h limited-feature LAN host
                        other API may not be supported
        AX = 0001h initialize operation
            DS:SI -> initialization request structure (see below)
            Return: AX = function status (see below)
        AX = 0002h get status
            Return: AH = current operating mode (see init req structure below)
                    AL = current connection status
                        bit 0: a physical connection is active
                        bit 1: remove screen updating is active
                        bit 2: connection checking is active
                        bit 3: hot key detection is active
                        bit 4: background file transfer is active
        AX = 0003h suspend remote screen updates
            Return: AX = function status (see below)
        AX = 0004h resume screen updates
            Return: AX = function status (see below)
        AX = 0005h end current remote access session
            DS:SI -> termination request structure (see below)
            Return: AX = function status (see below)
        AX = 0006h remove pcANYWHERE IV from memory
            Return: AX = status
                        0000h successful
                        FFD2h unable to release allocated memory
                        FFD1h unable to release interrupt vectors
        AX = 8000h read data from communications channel
            DS:BX -> buffer
            CX = buffer size
            Return: AX >= number of characters read/available
                    AX < 0 on error
        AX = 8001h write data to communications channel
            DS:BX -> buffer
            CX = buffer size
            Return: AX >= number of characters written
                    AX < 0 on error
        AX = 8002h get connection status
            Return: AX = status
                        > 0000h if connection active
                        = 0000h if connection lost
                        < 0000h on error

Format of initialization request structure:
Offset  Size    Description
 00h    BYTE    operating mode
                00h wait for a call
                01h hot key activates
                02h incoming call activates
                03h initiate a call
 01h  3 BYTEs   user ID to append to config file names
 04h    WORD    DS-relative pointer to path for config files
 06h    WORD    DS-relative pointer to path for program files

Format of termination request structure:
Offset  Size    Description
 00h    BYTE    operating mode after termination
                00h wait for a call
                01h hot key activates
                02h incoming call activates
                80h use current mode
                FFh remove from memory

Values for function status:
 0000h  function completed successfully
 FFF2h  unable to establish a connection when operating mode is
        "Initiate a call"
 FFF3h  modem configuration is invalid (corrupt config)
 FFF4h  modem initialization failed (no modem response)
 FFF5h  the communications device could not be initialized
 FFF6h  the host operator aborted the function
 FFF7h  the communications driver type specified in the configuration file is
        different than the one loaded when pcANYWHERE IV was initially started
 FFF9h  the configuration file is invalid
 FFFAh  the configuration file could not be found
 FFFBh  no session is active
 FFFCh  a remote access session is active
 FFFDh  the specified operating mode is invalid

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