AH = 3Dh
        AL = access code
            0 = Read Only
            1 = Write Only
            2 = Read/Write
        AL bits 7-3 = file-sharing modes (DOS 3.x)
            bit 7    = inheritance flag, set for no inheritance
            bits 4-6 = sharing mode
                      000 compatibility mode
                      001 exclusive (deny all)
                      010 write access denied (deny write)
                      011 read access denied (deny read)
                      100 full access permitted (deny none)
            bit 3    = reserved, should be zero
        DS:DX = address of ASCIZ filename
        DX = OFFSET ADDRESS OF ASCIIZ
Return: CF = 1 if error
            AX = Error Code
        CF = 0 successful
            AX = file handle
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