Version notes

    All the Int 21h functions listed are supported by DOS 2.0 and
    later DOS versions, unless noted, e.g. DOS 3.3+ means supported
    by DOS version 3.30 and later versions. However, the descriptions
    in this guide are not necessarily 100% accurate for DOS versions
    prior to version 5.0.

    New programs should not use superseded functions, since support
    for these functions "may be removed at any time".

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    The following functions under interrupt 21h are obsolete (i.e. not
    supported by DOS 5.0+) according to MS-DOS Programmer's Reference
    version 5.0:
    18h, 1dh, 1eh, 20h, 61h, and 63h

    The following functions are reserved in DOS 5.0:
    37h, 52h, 53h, 55h, and 60h

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    DOS 1.0 documented support for the following Int 21h functions:
    00h..17h, 19h..1ah, and 21h..2eh,
    as well as for the following interrupts:
    20h, 21h, 22h, 23h, 24h, 25h, 26h, and 27h