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INT 7F - HLLAPI (IBM 3270 High-Level Language API)
        AX = 0104h (HLLAPI gate ID)
        BX = 0000h
        DS:SI -> parameter control block (see below)
Return: parameter control block updated

Format of parameter control block:
Offset  Size    Description
 00h  3 BYTEs   signature = 'PCB'
 03h    BYTE    function number (see below)
 04h    WORD    segment of control string
 06h    WORD    offset of control string
 08h    WORD    length of control string, unless explicit end-of-str char set
 0Ah    BYTE    unused
 0Bh    WORD    return code
 0Dh    WORD    maximum length of control string

Values for function number:
 00h    Query system (Attachmate implementation only)
 01h    Connect presentation space
 02h    Disconnect presentation space
 03h    Send string of keystrokes as if typed from keyboard
 04h    Wait ~60s, returns status of presentation space
 05h    Copy current presentation space into a user-defined buffer
 06h    Search presentation space for first occurrence of a specified string
 07h    Query cursor location in current presentation space
 08h    Copy part or all of current presentation space into user buffer
 09h    Set session parameters; parameters vary by vendor
 0Ah    Get info on sessions currently connected
 0Bh    Lock current presentation space
 0Ch    Unlock previously locked presentation space
 0Dh    Return copy of operator info area (OIA) of current presentation space
 0Eh    get attribute byte for given position in the current presentation space
 0Fh    copy string of characters to the current presentation space
 10h    workstation control functions
 11h    storage manager functions, intended primarily for BASIC applications
 12h    set delay period in half-second intervals
 14h    get info on level of workstation support used
 15h    reset session parameters to default values
 16h    get detailed info on the current session
 17h    start host notification to application on presentation sp or OIA update
 18h    check host update when host notification enabled
 19h    stop host notification
 1Eh    search field within current presentation space for string
 1Fh    get first positionof a selected field in the current presentation space
 20h    get length of specified field
 21h    copy string into a specified field
 22h    copy specified field into a user-defined buffer
 23h    create alternate presentation space (IBM only), don't use with BASIC
 24h    switch to alternate presentation space (IBM only), not with BASIC
 25h    display cursor in specified area (IBM only), don't use with BASIC
 26h    display alternate presentation space (IBM only), don't use with BASIC
 27h    delete alternate presentation space (IBM only), don't use with BASIC
 32h    start intercepting keystrokes to allow filtering
 33h    get keystrokes after turning on interception
 34h    notify operator when keystroke rejected by filter subroutine
 35h    stop intercepting keystrokes
 5Ah    send file
 5Bh    receive file
 5Ch    run a program
 5Dh    execute DOS command
 63h    change presentation space position to PC display row/col or vice versa
 FFh    Get info on DCA implementation

Session Parameters for function 09h:
 ATTRIB         return attributes in hex
 NOATTRIB       return attributes as blanks
 CONPHYS        make physical connection
 CONLOG         only make logical connection
 EAB            copy extended attribute bytes along with data
 NOEAB          copy data only
 ESC=n          set escape character to "n" (default '@')
 EOT=n          set end of string character (default 00h)
 FPAUSE         full-duration pause
 IPAUSE         interruptible pause
 STRLEN         use explicit string lengths
 STREOT         use terminated strings
 SRCHALL        search entire presentation space
 SRCHFROM       search from specified offset
 SRCHFRWD       search forward from position 1
 SRCHBKWD       search backward from last position in presentation space
 TWAIT          wait specified time for keyboard ready
 LWAIT          wait until keyboard ready
 NWAIT          no wait
 TRON           enable tracing
 TROFF          disable tracing
 AUTORESET      send reset before sending keys with function 03h
 NORESET        don't send reset
 QUIET          don't display messages sent with INT 21/AH=9
 NOQUIET        allow messages to be displayed
 TIMEOUT=n      set timeout in 30-second intervals, 0 = wait until reak
 XLATE          translate extended attribute bytes
 NOXLATE        don't translate
 NEWRET         use HLLAPI v3.0 return code conventions
 OLDRET         use HLLAPI v2.0 return code conventions