INT 60 u - HP 95LX System Manager - OPEN FILE
        DI = 0500h
        STACK:  2 WORDs unused dummies (for calls from high level languages)
                DWORD   pointer to file state record (see below)
                DWORD   pointer to filename
                WORD    length of filename
                WORD    ???
                WORD    suppress buffering if nonzero
Return: AX = status
        STACK unchanged
SeeAlso: INT 60/DI=0501h,INT 60/DI=0502h,INT 60/DI=0508h

Format of file state record:
Offset  Size    Description
 00h    WORD    DOS file handle
 02h    WORD    flags
                bit 0: buffer contents valid
                bit 1: buffer is dirty and must be written
                bit 2: unbuffered I/O
                bit 3: file is a character device
 04h    DWORD   current DOS physical file offset (FFFFFFFFh if unknown)
 08h    DWORD   DOS file offset of start of buffer
 0Ch    DWORD   effective file offset as seen by caller
 10h    WORD    number of bytes in file buffer
---buffered I/O only---
 12h 512 BYTEs  file buffer

.