HP OpenVMS systems documentation

• The caller's virtual address for the buffer is used only in FDT context.
• Most of the driver identifies buffer start by IRP$L_SVAPTE and IRP$L_BOFF.
• Driver "layering" in start I/O or fork environments.
• Most drivers use either OpenVMS-supplied upper-level FDT routines or FDT support routines.
• The moving of the page tables has a significant impact:
  1. Only the current process's PPTEs are available at any given time.
     This is called the "cross-process PTE access" problem.
  2. A 64-bit address is required to access page table entries in page table space: process, global, and system PTEs.
  3. Because "SVAPTE, BOFF, BCNT" are used in many device drivers, the impact of 1 and 2 is not insignificant.

4.5.3 Direct I/O Buffer Map (DIOBM)

Figure 4-3 Direct I/O Buffer Map Data Structure

• Use PTE vector in DIOBM for buffers up to 64 Kb
• Use "secondary" DIOBM for buffers up to 5.2 Mb
• Use PTE window method with DIOBM for larger buffer
• DIOBM embedded in IRP, IRPE, VCRP, DCBE
• MMG_STD$IOLOCK_BUF replaces MMG_STD$IOLOCK
• New DIOBM routines; for example IOC_STD$FILL_DIOBM
• Also of interest to LAN VCI clients

4.5.4 64-Bit AST

Figure 4-4 64-Bit AST

• Both ACB and ACB64X formats are supported.
• ACB packets are self-identifying.
• An ACB64 is an ACB with an immediate ACB64X.

4.5.5 64-Bit ACB Within the IRP

Figure 4-5 Embedded ACB64

An IRP created by the $QIO system service uses the ACB64 layout unconditionally.

IRP$B_RMOD	New ACB$V_FLAGS_VALID bit always set
IRP$L_ACB_FLAGS	New ACB$V_64BITS bit always set
IRP$L_ACB64X_OFFSET	Contains hex 28

4.5.6 I/O Function Definitions

I/O functions are defined as follows:

1. Direct I/O, raw data transfer
   Functions in this category are implemented as direct I/O operations and transfer raw data from the caller's buffer to the device without significant alteration or interpretation of the data.
2. Direct I/O, control data transfer
   Functions in this category are implemented as direct I/O operations and transfer control information from the caller's buffer to the device driver. The device driver usually interprets the data or uses it to control the operation of the device.
   Usually these functions do not support 64-bit addresses. In contrast to the raw data transfers, control data transfers tend to be smaller and are invoked less frequently. Thus, there is less need to be able to store such data in a 64-bit addressable region. The only area impacted in the driver are the corresponding FDT routines. However, control data often introduces the problem of embedded 32-bit pointers.
3. Buffered I/O, raw data transfer
   Functions in this category are implemented as buffered I/O operations by the device driver but are otherwise the same type of raw data transfer from the caller's buffer as the first category.
4. Buffered I/O, control data transfer
   Functions in this category are implemented as buffered I/O operations by the device driver but are otherwise the same type of control data transfer from the caller's buffer as the second category.
5. Control operation, no data transfer, with parameters
   Functions in this category control the device and do not transfer any data between a caller's buffer and the device. Since there is no caller's buffer it does not matter whether the function is designated as a buffered or direct I/O function. The control operation has parameters that are specified in p1 through p6 however these parameters do not point to a buffer.
6. Control operation, no data transfer, with no parameters
   Functions in this category control the device and do not transfer any data between a caller's buffer and the device. Since there is no caller's buffer it does not matter whether the function is designated as a buffered or direct I/O function. In addition, there are no parameters for these functions.

Table 4-3 Guidelines for 64-Bit Support by I/O Function

Function Type	64-Bits	Area of Impact
Direct I/O, raw data transfer	Yes	FDT only
Direct I/O, control data transfer	No	FDT only
Buffered I/O, raw data transfer	No/yes	Entire driver, new BUFIO packet
Buffered I/O, control data transfer	No	Entire driver, new BUFIO packet
Control, no data transfer, param	No	Entire path but usually simple
Control, no data transfer, no params	Moot	None

4.6 64-Bit Support in Example Driver

This section summarizes changes made to the example device driver (LRDRIVER.C) to support 64-bit buffer addresses on all I/O functions.

This sample driver is available in the SYS$EXAMPLES directory.

1. All functions are declared as capable of supporting a 64-bit P1 parameter.
2. The 64-bit buffered I/O packet header defined by bufiodef.h is used instead of a privately defined structure that corresponds to the 32-bit buffered I/O packet header.
3. The pointer to the caller's set mode buffer is defined as a 64-bit pointer.
4. IRP$Q_QIO_P1 is used instead of IRP$L_QIO_P1.
5. The EXE_STD$ALLOC_BUF_64 routine is used instead of EXE_STD$DEBIT_BYTCNT_ALO to allocate the buffered I/O packet.

No infrastructure changes were necessary to this driver. The original version could have been simply recompiled and relinked and it would have worked correctly with 32-bit buffer addresses.