INT 45 - Acorn BBC Master 512 - "OSFILE" - READ/WRITE FILE OR DIRECTORY INFO
        AL = function
            00h save block of memory as file
            01h update directory entry for existing file
            02h set load address for existing file
            03h set execution address for existing file
            04h set attributes for existing file
            05h read directory
            06h delete file
            FFh load file
        DS:BX -> control block (see below)
Return: FLAGS destroyed
        AL = file type
            00h not found
            01h file found
            02h directory found
            FFh protected file
SeeAlso: INT 40"Acorn",INT 41"Acorn",INT 44"Acorn",INT 46"Acorn"

Format of control block:
Offset  Size    Description
 00h    WORD    address of CR-terminated filename
 02h    DWORD   load address of file
 06h    DWORD   execution address of file
 0Ah    DWORD   start address of data to save
 0Eh    DWORD   end address of data to save, or file attributes
                file attributes in low byte (see below)
                other three bytes are filing-system specific file attributes

Bitfields for file attributes:
 bit 0  no owner read access
 bit 1  no owner write access
 bit 2  not executable by owner
 bit 3  not deletable by owner
 bit 4  no public read access
 bit 5  no public write access
 bit 6  not executable with public access
 bit 7  not deletable with public access

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