HP OpenVMS systems documentation

The existing 32-bit Buffered I/O (BUFIO) packet format will continue to be supported. In addition, a new 64-bit BUFIO packet format will be supported. These BUFIO packets are "self identifying". That is, it is possible to distinguish a 32-bit from a 64-bit format BUFIO packet from information in the packet.

Although the structure type code DYN$C_BUFIO is defined and there is an expected layout for the header of buffered I/O packet, there currently is no formal definition of a structure. Existing code in drivers and IOCIOPOST.MAR uses numeric constants as offsets.

The existing 32-bit BUFIO packet will be formally defined along with a new 64-bit BUFIO packet format. The 64-bit BUFIO structure format will also be used for 64-bit diagnostic buffer packets (see Table A-3).

Table A-3 BUFIO Packet

Field	Type	Comments
bufio$ps_pktdata	void *	Pointer to the buffered data within the packet.
bufio$ps_uva32	void *	32-bit pointer to user's address space. On a read function, data is transfered from that user virtual address to the buffer packet during FDT processing. On a write function, data is transfered to that user virtual address from the buffer packet during I/O Postprocessing. If this cell contains the value BUFIO$K_64 (-1), then the pointer to the user buffer is in bufio$pq_uva64 .
bufio$w_size	unsigned short	Size of the BUFIO packet in bytes.
bufio$b_type	unsigned char	Nonpaged pool packet type code, DYN$C_BUFIO
BUFIO$K_HDRLEN32	constant	Size in bytes of the minimal buffered I/O packet header with a 32-bit user virtual address (12).
bufio$pq_uva64	VOID_PQ	64-bit pointer to user's address space. On a read function, data is transfered from that user virtual address to the buffer packet during FDT processing. On a write function, data is transfered to that user virtual address from the buffer packet during I/O Postprocessing. This cell contains a valid address only if the bufio$ps_uva32 cell contains the value BUFIO$K_64 (-1).
BUFIO$K_HDRLEN64	constant	Size in bytes of the minimal buffered I/O packet header with a 64-bit user virtual address (24).

A.4 Complex Chained Buffer (CXB)

The CXB structure defines the format of entries that are linked together to build a complex chained buffered I/O packet.

The CXB structure will be enhanced such that it can be used by existing code with no source changes to support a 32-bit caller's buffer address. However, the same enhanced CXB structure can be used to support a 64-bit caller's buffer address as well (see Table A-4).

Table A-4 CXB Structure Changes

Field	Type	Comments
cxb$ps_pktdata	void *	Pointer to the buffered data within the packet. This cell will be overlaid on the existing cxb$l_fl cell to reflect its current alternate use.
cxb$ps_uva32	void *	32-bit pointer to user's address space. If this cell contains the value BUFIO$K_64 (-1) then the pointer to the user buffer is in cxb$pq_uva64 . This cell will be overlaid on the existing cxb$l_bl cell to reflect its current alternate use.
cxb$pq_uva64	VOID_PQ	64-bit pointer to user's address space. This cell contains a valid address only if the cxb$ps_uva32 cell contains the value BUFIO$K_64 (-1). This cell will be inserted as the last aligned quadword just before the end of the standard CXB header which is CXB$K_LENGTH bytes long.

A.5 Data Chain Block (DCBE)

The DCBE structure is the Data Chain Block that is used by the OpenVMS LAN driver VMS Communications Interface (VCI). A DCBE is used to connect to a VCRP all or part of the data to be transmitted. A chain of DCBEs is used when the data is contained in more than one discontiguous buffer in virtual memory.¹

There are two mutually exclusive methods that a DCBE can use to identify the start of the buffer:

1. When the dcbe$l_buffer_address cell contains a zero, the buffer address is specified by the dcbe$l_svapte and dcbe$l_boff cells. A fixed-size primary DIOBM structure will be added to the DCBE. This embedded DIOBM structure is available for use by an upper-level VCM if it needs to derive a 32-bit SVAPTE from a 64-bit VA_PTE for the PTEs that map the buffer. The lower-level VCM will not alter this embedded DIOBM or make any assumptions about it.
2. When the dcbe$l_buffer_address cell contains the a non-zero value, this value is the system virtual address of the buffer. This method remains unchanged.

Because a VCRP can also be used as a DCBE, the named DCBE cells must be at the same offsets as their VCRP counterparts. Therefore, DCBE changes are reflected in the VCRP and changes to the common portion of the VCRP are reflected in the DCBE.

In addition, SYS$PEDRIVER overlays a DCBE with the vcrp$t_internal_stack area within the VCRP. Therefore, an increase in the size of the DCBE must be reflected by a corresponding increase in the size of the internal stack area within the VCRP (see Table A-5).

Table A-5 DCBE Structure Changes

Field	Type	Comments
dcbe$l_reserved	int32[13]	This existing vector of 6 filler longwords has been increased to 13 fill longwords to reflect the increased size of the common portion of the VCRP. The common portion of the VCRP has been increased to accommodate either an ACB64 or ACB structure.
dcbe$pq_buffer_addr64	VOID_PQ	64-bit buffer address. This cell is available for use by upper-level VCMs only. Note that this cell does not replace the dcbe$l_buffer_address cell which continues to be used by lower-level VCMs. The dcbe$pq_buffer_addr64 cell has been added after the dcbe$l_bcnt cell.
dcbe$r_diobm	DIOBM	Embedded fixed-size primary "direct I/O buffer map" structure. This DIOBM structure is available for use by upper-level VCMs that need to lock down a buffer and provide a value for the dcbe$l_svapte cell. This structure has been added just before the end of the DCBE header.