INT 21 U - PTS-DOS 6.51 & S/DOS 1.0 - DIRECT DISK READ                      
	AH = 90h
	CX = FFFFh (for partitions > 32 Mb)
	    DS:BX -> request block (see #04094)
	CX = sector count (for partitions < 32 Mb)
	    DX = starting sector number
	    DS:BX -> buffer
Return: CF set on error
	    AX error code
	CF clear on success
Notes:	This function is supported by Paragon Technology Systems S/DOS 1.0
	  (and most probably by PTS-DOS 6.51, maybe even earlier).
	To access big partitions (> 32 Mb), the CX != FFFFh is not allowed and
	  the buffer must always be used. Otherwise, the system will return
	  error code 0207h.
	It is currently unknown if this function is also supported by
	  PhysTechSoft PTS-DOS 6.60, 6.70, or 2000.
SeeAlso: INT 21/AH=91h"PTS",INT 25h,INT 26h


Format of PTS-DOS & S/DOS disk read/write request block:
Offset	Size	Description	(Table 04094)
 00h	DWORDDoubleword; four bytes.	 Commonly used to hold a 32-bit segment:offset or selector:offset address.	sector number
04h	WORD	counter
06h	DWORDDoubleword; four bytes.	 Commonly used to hold a 32-bit segment:offset or selector:offset address.	pointer to buffer