Path: utzoo!utgpu!water!watmath!mks!alex
From: alex@mks.UUCP (Alex White)
Newsgroups: comp.sys.ibm.pc
Subject: Bug in MSC5.0 routine _dos_read() in small model
Keywords: Bug MSC5.0 _dos_read
Message-ID: <410@mks.UUCP>
Date: 24 Feb 88 14:08:42 GMT
Organization: Mortice Kern Systems, Waterloo, Ont.
Lines: 13

The routine _dos_read takes a far pointer as the buffer address.
This pointer is loaded into DS:DX, and the int call is issued.
The problem is that the number of bytes actually read is stored thru a
pointer in the argument list - but this is done BEFORE the value of DS
is restored.  From the debugger, the code in _dos_read reads:
	...
	int	21
	mov	bx,[bp+0c]	# Fetch address of returned byte count
	mov	[bx], ax	# Store it
	pop	ds		# Restore ds as it was before we loaded the
				# far pointer buffer address
Thus the byte count isn't returned - but perhaps worse is an arbitrary word
in memory [possibly in the data you read in] has been destroyed.