Record Selection and Data Transfer

Record Pointers

The File System uses record pointers to find specific records for use. Record pointers block and deblock records and indicate specific records in a file. A file opened with the inhibit buffering option parameter set to BUF (the default) is accessed by using a record pointer. (A file opened with the inhibit buffering option parameter set to NOBUF is accessed via a block pointer.)

	NOTE: Buffering does not apply to ordinary disk files.

Pointer Initialization

When you open a file, the HPFOPEN or FOPEN intrinsic sets the record pointer to record 0 (the first record in the file) for all operations. If you have opened the file with Append access (using the access type option parameter), MPE/iX moves the record pointer to the end of the file prior to a write operation. This ensures that any data you write to the file is added to the end of the file instead of written over existing data. (Append and other access types are described later in this manual.) Following initialization, the record pointer can remain in position at the head of your file, or it can be moved by the intrinsics used in record selection (for example, FREAD, FWRITE, and FPOINT).

Record Selection

Various File System intrinsics are designed to transfer records to and from files. The record pointer indicates the specific location where a file can be accessed. Records can be transferred to or from this location, or the pointer can be moved to another place in the file you wish to access.

There are five methods of record selection for accessing a file:

For detailed information on record selection methods and the intrinsics used for data transfer, refer to Accessing Files Programmer's Guide (32650-90017).

Mapped Access to Disc Files

MPE/iX employs a "mapped files" technique for performing disc file access. File access efficiency is improved when code and data portions of files required for processing reside in memory. Accessing memory is faster than performing physical disc I/O operations. The mapped files technique eliminates file system buffering and optimizes global system memory management.

File mapping is based on MPE/iX demand paged virtual memory, which uses to advantage the large amount of virtual memory on the system. When a file is opened, it is logically mapped into virtual memory. An open file and its contents are referenced by virtual addresses. Each byte of each opened file has a unique virtual address.

File mapping improves I/O performance without imposing additional CPU overhead or sacrificing data integrity and protection. Traditional disc caching schemes for increasing I/O performance impose a CPU overhead penalty. The 900 Series hardware and system architecture allow MPE/iX to perform file mapping without incurring this penalty. System hardware performs the virtual to physical address translations for locating portions of the mapped files, thus eliminating CPU overhead for this function.

If the required pages are not in memory, the MPE/iX Memory Manager fetches them directly from disc and places them in memory. This eliminates File System buffering. Pages are "prefetched" to reduce the amount of physical disc I/O. Prefetching means that the page specified for fetching and the group of pages surrounding it are a fetched all at once. This improves efficiency because the processor is likely to require pages that are located near each other. Two benefits of this are:

MPE/iX File System access intrinsics are built on the mapped file technique. Programs using file access methods supported by MPE file types and intrinsics obtain the benefits of file mapping without requiring changes.

You can directly access mapped files when programming in Native Mode languages with pointers by using the HPFOPEN intrinsic. For example, you can obtain the advantage of File System naming and data protection for accessing array type structures and developing specialized access methods.

You can write programs that address files through virtual memory, instead of calling File System intrinsics for disc reading and writing. The file interface provides opening and closing of user mapped files with normal naming and security, but with improved LOAD and STORE speed on file references.

Multiple Record Transfers

In most applications, programs conduct input and output in normal recording mode, where each read or write request transfers one logical record to or from the stack. In specialized applications, however, you may want a program to read or write, in a single operation, data that exceeds the logical record length defined for the input or output file. For example, multirecord transfer may be preferable, as a good programming practice, for applications that run only on MPE/iX.

You can bypass the normal record-by-record input and output, instead receiving large data transfers by specifying multirecord mode (MR) by using the multirecord option parameter in the HPFOPEN or FOPEN call, or by using the :FILE command. For detailed information and examples on multiple record transfers, refer to Accessing Files Programmer's Guide (32650-90017).

Data Transfer Control Operations

To move a record pointer to a particular place without necessarily transferring data, the following intrinsics perform three types of record selection:

For detailed information and examples on spacing, pointing, and rewinding, refer to Accessing Files Programmer's Guide (32650-90017). For detail information on FSPACE, FPOINT, and FCONTROL, refer to MPE/iX Intrinsics Reference Manual (32650-90028).

Reading from a File

There are several ways to move data to a program from a disc file or device file. The File System intrinsics used for reading data from a file are:

If the standard input device ($STDIN) and the standard list device ($STDLIST) are opened with an HPFOPEN or FOPEN intrinsic call, the FREAD and FWRITE intrinsics can be used with them. For example, you can use the FREAD intrinsic to transfer information entered from a terminal to a buffer in the stack, and you can use the FWRITE intrinsic to directly transfer information from the stack to the standard list device.

You can use the FREADDIR intrinsic to read a record from a file opened by the HPFOPEN or FOPEN intrinsic and transfer the record to an array in the stack. When FREADDIR reaches the end of a file, the end-of-file condition code CCG is returned. If FREADDIR does not successfully read information, a CCL condition code is returned.

When a labeled tape file has been opened, you can use the FREAD intrinsic to read data from the opened file. The system uses the block size, record size, and file format on the tape label to determine the amount of data to read. You can call FGETINFO or FFILEINFO to obtain these values.

You can use the FREADLABEL intrinsic to read a user-defined label on a labeled magnetic tape file. To read a user-defined header, a program must call FREADLABEL before issuing the first FREAD for the file. Execution of the first FREAD causes MPE/iX to skip past any unread user-defined header labels.

Other specialized read intrinsics are READX and FREADLABEL.

For detailed information on reading a file, refer to Accessing Files Programmer's Guide (32650-90017).

Writing to a File

You can move data from a program to a disc file or to a device file in several ways. The File System intrinsics used for writing the data are as follows:

If the standard input device ($STDIN) and the standard list device ($STDLIST) are opened with an HPFOPEN or FOPEN intrinsic call, the FREAD and FWRITE intrinsics can be used with these devices. For example, the FWRITE intrinsic can be used to transfer information from a buffer in your process stack directly to the standard list device.

A program can use the FWRITEDIR intrinsic to read records from one file and write them, in reverse order, into a second file. You can use the FGETINFO intrinsic to locate EOF in the file to be read. This information is returned to the program in a variable.

You can write to either a labeled or an unlabeled magnetic tape file. Writing to a labeled tape file differs from writing to an unlabeled tape file when the user program attempts to write over or beyond the physical end-of-tape (EOT) marker.

For detailed information on writing to a file, refer to Accessing Files Programmer's Guide (32650-90017).

Updating a File

The FUPDATE intrinsic updates a logical record of a disc file. It affects the last logical record (or block for NOBUF files) accessed by any intrinsic call for the file named and writes information from a buffer in the stack into this record. Following the update operation, the record pointer is set to indicate the next record position.

As a physical data storage device, magnetic tape is not designed to enable the replacement of a single record in an existing file. An attempt to perform this type of operation causes problems in maintaining the integrity of records on the tape. Magnetic tape files, therefore, should not be maintained (updated) on an individual record basis, but should be updated during copy operations from one file to another. For detailed information on updating files, refer to Accessing Files Programmer's Guide (32650-90017).

Using Mapped Access to a File

The MPE/iX File System employs access to mapped files, accessible through the use of pointers in the HPFOPEN intrinsic. A mapped file is a file that can be accessed directly through machine loads and stores, bypassing File System intrinsics normally used for disc files.

Mapped access is not available through the standard Input/Output statements of most programming languages. Thus, the programmatic use of HPFOPEN adds capability to programs. You can achieve mapped access to a file by declaring a pointer in a program and passing it to the HPFOPEN intrinsic. The pointer is initialized to point to the beginning of the data area of the opened file.

After HPFOPEN returns the address of the file, simply reference the pointer as an array or any type you want. The machine architecture translates the address to a file page and ensures integrity and protection of the file.

Standard disc files with fixed-length or undefined-length record formats are allowed any type of access (Read, Write, Read/Write, and so on) when opened for mapped access. The following file types are allowed Read Only access when opened for mapped access:

The following file types cannot be opened for mapped access:

The File System provides protection by keeping a list of file rights. When a page fault or protection fault occurs, it checks the list. If you pass the security check, your protection ID is placed in a range of pages allowing access to the file.

All File System and data transfer intrinsics applicable to the file can be used with a mapped file. When mixing FREAD and FWRITE calls with mapped access, you must consider the file's data type (ASCII or binary), record format, and record size to ensure that data written to the file using mapped access makes sense when read by FREAD.

When you open a file using mapped access and write data to it, you must use the FPOINT and FCONTROL intrinsics to reset end-of-file (EOF) before closing the file. Otherwise, all data written to the file is lost when you close it. In the case of a newly created file, the EOF initially points to record zero. Mapped accessing of a file bypasses File System services that, otherwise, automatically set various File System pointers, including EOF and the logical record pointer. You are responsible for resetting EOF prior to closing the file.

For detailed information on the advantages of mapped access to files, refer to Accessing Files Programmer's Guide (32650-90017).

Device File Buffers

A buffer is an area in memory used by the File System to hold one block (one physical record) of a file. When a program reads a record from a buffered file, a block is brought into the buffer. For detailed information, refer to Accessing Files Programmer's Guide (32650-90017).