Category: <b>DOS kernel</b>
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<pre><b>INT 21 - DOS 1+ - NULL FUNCTION FOR CP/M COMPATIBILITY</b>

	AH = 20h
Return: AL = 00h
Note:	corresponds to the CP/M BDOS function "get/set default user
	  (sublibrary) number", which is meaningless under MS-DOS
SeeAlso: AH=18h,AH=1Dh,AH=1Eh,AX=4459h
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