Document revision date: 30 March 2001

A skip file or skip record operation that uses the standard space-by-records algorithm is terminated when two consecutive tape marks are encountered when the tape moves in the forward direction. After the operation terminates, the tape remains positioned between the two tape marks that were detected. The I/O status block (IOSB) returns the status SS$_ENDOFVOLUME and the actual number of files (or records) skipped during the operation prior to the detection of the second tape mark. The skip count is returned in the high-order word of the first longword of the IOSB.

An optimized skip file that uses the IO$M_ALLOWFAST modifier is terminated when the end-of-data position is encountered. If two consecutive tape marks immediately precede the end-of-data position on the tape, the tape is positioned between these two tape marks. The SS$_ENDOFVOLUME status and the skip count are returned in the IOSB.

Subsequent skip record (or skip file) requests terminate immediately when the tape is positioned between the two tape marks, producing no net tape movement and returning the SS$_ENDOFVOLUME status with a skip count of zero.

To move the tape beyond the second tape mark, you must employ another I/O function. For example, the IO$_READLBLK function, if issued after receipt of the SS$_ENDOFVOLUME status return, terminates with an SS$_ENDOFFILE status and with the tape positioned just past the second tape mark. From this new position, other skip functions could be issued to produce forward tape motion (assuming there is additional data on the tape).

If three consecutive tape marks are encountered during a skip file function, you must issue two IO$_READLBLK functions, the first to get the SS$_ENDOFFILE return and the second to position the tape past the third tape mark.

3.4.6 Write End-of-File

The write end-of-file (EOF) function writes an extended interrecord gap (of approximately 3 inches for non-return-to-zero-inverted (NRZI) recording and 1.5 inches for phase-encoded (PE) recording) followed by a tape mark. No device- or function-dependent arguments are used with IO$_WRITEOF.

An end-of-tape (EOT) status is returned in the I/O status block if either of the following conditions is present and no other error conditions occur:

• A write EOF function is executed while the tape is positioned past the EOT marker.
• A write EOF function causes the tape position to enter the EOT region.

The rewind offline function rewinds and unloads the tape on the selected drive.

The I/O operation is completed as soon as the tape movement is initiated. The actual finish of the mechanical rewind or unload operation may occur long after the I/O operation completes.

If the IO$M_RETENSION function modifier is specified and the device supports the retension operation, the rewind offline function positions the tape to the physical end-of-tape (EOT) marker and rewinds the tape to the beginning-of-tape (BOT) marker. If the tape does not support the IO$M_RETENSION modifer, a SS$_ILLIOFUNC error is returned.

No device- or function-dependent arguments are used with IO$_REWINDOFF.

3.4.8 Unload

The unload function rewinds and unloads the tape on the selected drive. The unload function is functionally the same as the rewind offline function. If the IO$M_NOWAIT function modifier is specified, the I/O operation is completed as soon as the rewind operation is initiated. No device- or function-dependent arguments are used with IO$_UNLOAD.

3.4.9 Sense Tape Mode

The sense tape mode function senses the current device-dependent and extended device characteristics (see Tables 3-2 and 3-3).

The operating system provides the following function codes:

• IO$_SENSEMODE---Sense mode
• IO$_SENSECHAR---Sense characteristics

Sense mode requires logical I/O privilege. Sense characteristics requires physical I/O privilege. For TMSCP drives, the sense mode function returns magnetic tape information in a user-supplied buffer, which is specified by the following function-dependent arguments:

• P1---Optional. Address of a user-supplied buffer.
• P2---Optional. Length of a user-supplied buffer.

If P1 is not zero, the sense mode buffer returns the tape characteristics. (If P2=8, the second longword of the buffer contains the device-dependent characteristics. If P2=12, the second longword contains the device-dependent characteristics and the third longword contains the tape densities that the drive supports and the extended tape characteristics.) The extended characteristics are identical to the information returned by DVI$_DEVDEPEND2 (see Table 3-3). Figure 3-3 shows the contents of the P1 buffer.

Regardless of whether the P1 buffer is specified, the I/O status block returns the device-dependent characteristics in the second longword (see Figure 3-6). These characteristics are identical to the information returned by DVI$_DEVDEPEND (see Table 3-2 in Section 3.3).

Figure 3-3 Sense Mode P1 Buffer

Set mode operations affect the operation and characteristics of the associated magnetic tape device. The operating system defines two types of set mode functions: set mode and set characteristics.

Set mode requires logical I/O privilege. Set characteristics requires physical I/O privilege. The following function codes are provided:

• IO$_SETMODE---Set mode
• IO$_SETCHAR---Set characteristics

These functions take the following device- or function-dependent arguments (other arguments are ignored):

• P1---The address of a characteristics buffer.
• P2---Optional. The length of the characteristics buffer. The default is 8 bytes. If a length of 12 bytes is specified, the third longword (which is for TMSCP drives only) specifies the extended tape characteristics.

Figure 3-4 shows the P1 characteristics buffer for IO$_SETMODE. Figure 3-5 shows the same buffer for IO$_SETCHAR.

Figure 3-4 Set Mode Characteristics Buffer for IO$_SETMODE

Figure 3-5 Set Mode Characteristics Buffer for IO$_SETCHAR

The first longword of the P1 buffer for the set characteristics function contains information on device class and type, and the buffer size. The device class for tapes is DC$_TAPE.

The $DCDEF macro defines the device type and class names. The buffer size is the default to be used for tape transfers (this default is normally 2048 bytes).

The second longword of the P1 buffer for both the set mode and set characteristics functions contains the tape characteristics. Table 3-5 lists the tape characteristics and their meanings. The $MTDEF macro defines the symbols listed. If P2=12, the third longword contains the extended tape characteristics for TMSCP drives, which are listed in Table 3-6. The extended tape characteristics are defined by the $MT2DEF macro and are identical to the information returned by DVI$_DEVDEPEND2.

Table 3-5 Set Mode and Set Characteristics Magnetic Tape Characteristics

Characteristic1	Meaning
MT$M_PARITY	If set, all data transfers are performed with even parity. If clear (normal case), all data transfers are performed with odd parity. Even parity can be selected only for non-return-to-zero-inverted recording at 800 bits/inch. Even parity cannot be selected for phase-encoded recording (tape density is MT$K_PE_1600) or group-coded recording (tape density is MT$K_GCR_6250) and is ignored.
MT$V_DENSITY MT$S_DENSITY	Specifies the density at which all data transfers are performed. Tape density can be set only when the selected drive's tape position is at the BOT marker. Possible density values are as follows: MT$K_DEFAULT Default system density. MT$K_GCR_6250 Group-coded recording, 6250 bits/inch. MT$K_PE_1600 Phase-encoded recording, 1600 bits/inch. MT$K_NRZI_800 Non-return-to-zero-inverted recording, 800 bits/inch. MT$K_BLK_833 Cartridge block mode recording 2.
MT$V_FORMAT MT$S_FORMAT	Specifies the format in which all data transfers are performed. Possible format values are as follows: MT$K_DEFAULT Default system format. MT$K_NORMAL11 Normal PDP-11 format. Data bytes are recorded sequentially on tape with each byte occupying exactly one frame.

Table 3-6 Extended Device Characteristics for Tape Devices

Characteristic1	Meaning
MT2$V_WBC_ENABLE	Enable write-back caching on a per-unit basis.
MT2$V_RDC_DISABLE	Disable read caching on a per-unit basis.

Application programs that change specific magnetic tape characteristics should perform the following steps, as shown in Example 3-2 in Section 3.6:

1. Use the IO$_SENSEMODE function to read the current characteristics.
2. Modify the characteristics.
3. Use the set mode function to write back the results.

Failure to follow this sequence will result in clearing any previously set characteristic.

3.4.11 Multiple Tape Density Support (Alpha Only)

As of Version 7.2, OpenVMS Alpha permits the selection of any density and any compression supported by a tape drive. You can write to tapes using any density and any compression algorithm supported by the tape drive. Exchanging tapes among tape drives with different default settings for density or compression is much easier with this enhancement.

Mutiple tape density support is provided by changes in the QIO interface. These changes are guided by device/density tables in system libraries and the corresponding class drivers. This enhancement functions with tape drives that support multiple tape density switching via the standard MODE_SENSE and MODE_SELECT mechanisms. All density and compression options available for a given drive will be accessible by the system. The QIO interface uses MT3DEF to identify the drives, and to match them with their density and compression code options. Some newer drives may not be included in the module.

These enhancements allow IO$_SETMODE and IO$_SENSEMODE to function with most density values and a wider variety of drives. The system management utilities BACKUP and MOUNT take advantage of this added functionality. For more information about multiple tape density support with these utilities, refer to the OpenVMS System Management Utilities Reference Manual. For more information about enchancements in INITIALIZE, refer to the OpenVMS DCL Dictionary.

3.4.12 Data Security Erase

The data security erase function erases all data from the current position of the volume to 10 feet beyond the EOT reflective strip, and then rewinds the tape to the BOT marker. It is a physical I/O function and requires the access privilege necessary to perform physical I/O functions. The following function code is provided:

IO$_DSE

If the function is issued when a tape is positioned at the BOT marker, all data on the tape will be erased.

IO$_DSE takes no device- or function-dependent arguments.

3.4.13 Modify

Specifying the ATR$C_USERLABEL or ATR$C_ENDLBLAST attributes with IO$_MODIFY results in a bad attribute error. If any other attributes are specified, the IO$_MODIFY function is treated as a no-operation; that is, the function returns success, but no action is performed.

3.4.14 Pack Acknowledge

The pack acknowledge function sets the volume valid bit for all magnetic tape devices. It is a physical I/O function and requires the access privilege to perform physical I/O. The following function code is provided:

IO$_PACKACK

This function code takes no function-dependent arguments.

IO$_PACKACK must be the first function issued when a volume is placed in a magnetic tape drive. IO$_PACKACK is issued automatically when the DCL commands INITIALIZE or MOUNT are issued.

3.4.15 Available

The available function clears the volume valid bit for all magnetic tape drives, that is, it reverses the function performed by the pack acknowledge function (see Section 3.4.14). A rewind of the tape is performed (applicable to all tape drives). No unload function is issued to the drive. The following function code is provided:

IO$_AVAILABLE

This function takes no function-dependent arguments.

3.4.16 Flush

The flush function is used to ensure that all previously issued cached commands have fully completed. Normally, hosts use this function to establish or maintain synchronization with write-back cached commands issued to the specified tape unit. The I/O request does not complete until all cached data is written successfully to the media in the exact order that the user specified.

IO$_FLUSH

This function code takes no function-dependent arguments.

3.5 I/O Status Block

The I/O status block (IOSB) for QIO functions on magnetic tape devices is shown in Figure 3-6. Appendix A lists the status returns for these functions. (The OpenVMS system messages documentation provides explanations and suggested user actions for these returns.) Table 3-2 (in Section 3.3) lists the device-dependent data returned in the second longword. The IO$_SENSEMODE function can be used to return that data.

Figure 3-6 IOSB Contents

The byte count is the actual number of bytes transferred to or from the process buffer or the number of files or blocks skipped. (If a IO$_SKIPRECORD function is terminated by the detection of a tape mark, the count returned in the IOSB is an unsigned number reflecting the number of blocks skipped, plus 1.

3.6 Magnetic Tape Driver Programming Examples

This section presents magnetic tape driver VAX MACRO programming examples.

Example 3-2 shows the recommended sequence for changing a device characteristic. It retrieves the current characteristics using an IO$_SENSEMODE request, sets the new characteristics bits, and then uses IO$_SETMODE to set the new characteristics.

Example 3-3 shows ways of specifying sense mode and set mode, both with and without a user buffer specified, and with user buffers of different lengths.

In addition, Example 3-4 shows how data is written to and read from magnetic tape through the magnetic tape ACP.

Example 3-2 Device Characteristic Program Example

$QIOW_S - ; Get current characteristics. FUNC = #IO$_SENSEMODE,- ; - Sensemode CHAN = CHANNEL,- ; - Channel IOSB = IO_STATUS,- ; - IOSB P1 = BUFFER,- ; - User buffer supplied P2 = #12 ; - Buffer length = 12 . . . (Check for errors) . . . (Set desired characteristics bits) . . . $QIOW_S - ; Set new characteristics. FUNC = #IO$_SETMODE,- ; - Set Mode CHAN = CHANNEL,- ; - Channel IOSB = IO_STATUS,- ; - IOSB P1 = BUFFER,- ; - User buffer address P2 = #12 ; - Buffer length = 12 . . . (Check for errors) . . .

Example 3-3 Set Mode and Sense Mode Program Example

.PSECT IMPURE, NOEXE, NOSHR $IODEF DEVICE_NAME: ; Name of device .ASCID /MUA0/ ; CHANNEL: ; Channel to device .WORD 0 ; BUFFER: .BLKL 3 ; Set/Sense characteristics ; buffer IO_STATUS: ; Final I/O status .QUAD 0 ; .PSECT CODE, RD, NOWRT, EXE .ENTRY MAIN,^M<> $ASSIGN_S - ; Assign a channel to device DEVNAM = DEVICE_NAME,- ; CHAN = CHANNEL ; BSBW ERR_CHECK2 ; Check for errors $QIOW_S - ; Get current characteristics FUNC = #IO$_SENSEMODE,-; No user buffer supplied CHAN = CHANNEL,- ; IOSB = IO_STATUS ; BSBW ERR_CHECK ; Check for errors $QIOW_S - ; Get current characteristics FUNC = #IO$_SENSEMODE,-; User buffer supplied, length CHAN = CHANNEL,- ; defaulted IOSB = IO_STATUS,- ; P1 = BUFFER ; BSBW ERR_CHECK ; Check for errors $QIOW_S - ; Get current characteristics FUNC = #IO$_SENSEMODE,-; User buffer supplied, length CHAN = CHANNEL,- ; = 8 IOSB = IO_STATUS,- ; P1 = BUFFER,- ; P2 = #8 ; BSBW ERR_CHECK ; Check for errors $QIOW_S - ; Get extended characteristics FUNC = #IO$_SENSEMODE,-; User buffer supplied, length CHAN = CHANNEL,- ; = 12 IOSB = IO_STATUS,- ; P1 = BUFFER,- ; P2 = #12 ; BSBW ERR_CHECK ; Check for errors $QIOW_S - ; Set new characteristics FUNC = #IO$_SETMODE,- ; Length defaulted CHAN = CHANNEL,- ; IOSB = IO_STATUS,- ; P1 = BUFFER ; BSBW ERR_CHECK ; Check for errors $QIOW_S - ; Set new characteristics FUNC = #IO$_SETMODE,- ; Length = 8 CHAN = CHANNEL,- ; IOSB = IO_STATUS,- ; P1 = BUFFER,- ; P2 = #8 ; BSBW ERR_CHECK ; Check for errors $QIOW_S - ; Set extended characteristics FUNC = #IO$_SETMODE,- ; Length = 12 CHAN = CHANNEL,- ; IOSB = IO_STATUS,- ; P1 = BUFFER,- ; P2 = #12 ; BSBW ERR_CHECK ; Check for errors RET .ENABLE LSB ERR_CHECK: BLBS IO_STATUS,ERR_CHECK2 ; Continue if good IOSB MOVZWL IO_STATUS,-(SP) ; Otherwise, set up for stop BRB 10$ ; Branch to common code ERR_CHECK2: BLBS R0,20$ ; Continue if good status PUSHL R0 ; Otherwise, set up for stop 10$: CALLS #1,G^LIB$STOP ; Stop execution 20$: RSB .DISABLE LSB .END MAIN

Example 3-4 MAGNETIC_TAPE.MAR Device Characteristic Program Example

; ********************************************************************* ; .TITLE MAGTAPE PROGRAMMING EXAMPLE .IDENT /01/ ; ; Define necessary symbols. ; $FIBDEF ;Define file information block ;symbols $IODEF ;Define I/O function codes ; ; Allocate storage for the necessary data structures. ; ; ; Allocate magtape device name string and descriptor. ; TAPENAME: ; .LONG 20$-10$ ;Length of name string .LONG 10$ ;Address of name string 10$: .ASCII /TAPE/ ;Name string 20$: ;Reference label ; ; Allocate space to store assigned channel number. ; TAPECHAN: ; .BLKW 1 ;Tape channel number ; ; Allocate space for the I/O status quadword. ; IOSTATUS: ; .BLKQ 1 ;I/O status quadword ; ; Allocate storage for the input/output buffer. ; BUFFER: ; .REPT 256 ;Initialize buffer to .ASCII /A/ ;contain 'A' .ENDR ; ; ; Now define the file information block (FIB), which the ACP uses ; in accessing and deaccessing the file. Both the user and the ACP ; supply the information required in the FIB to perform these ; functions. ; FIB_DESCR: ;Start of FIB .LONG ENDFIB-FIB ;Length of FIB .LONG FIB ;Address of FIB FIB: .LONG FIB$M_WRITE!FIB$M_NOWRITE ;Read/write access allowed .WORD 0,0,0 ;File ID .WORD 0,0,0 ;Directory ID .LONG 0 ;Context .WORD 0 ;Name flags .WORD 0 ;Extend control ENDFIB: ;Reference label ; ; Now define the file name string and descriptor. ; NAME_DESCR: ; .LONG END_NAME-NAME ;File name descriptor .LONG NAME ;Address of name string NAME: .ASCII "MYDATA.DAT;1" ;File name string END_NAME: ;Reference label ; ; ********************************************************************* ; ; Start Program ; ; ********************************************************************* ; ; The program first assigns a channel to the magnetic tape unit and ; then performs an access function to create and access a file called ; MYDATA.DAT. Next, the program writes 26 blocks of data (the letters ; of the alphabet) to the tape. The first block contains all A's, the ; next, all B's, and so forth. The program starts by writing a block of ; 256 bytes, that is, the block of A's. Each subsequent block is reduced ; in size by two bytes so that by the time the block of Z's is written, ; the size is only 206 bytes. The magtape ACP does not allow the reading ; of a file that has been written until one of three events occurs: ; 1. The file is deaccessed. ; 2. The file is rewound. ; 3. The file is backspaced. ; In this example the file is backspaced zero blocks and then read in ; reverse (incrementing the block size every block); the data is ; checked against the data that is supposed to be there. If no data ; errors are detected, the file is deaccessed and the program exits. ; .ENTRY MAGTAPE_EXAMPLE,^M<R3,R4,R5,R6,R7,R8> ; ; First, assign a channel to the tape unit. ; $ASSIGN_S TAPENAME,TAPECHAN ;Assign tape unit CMPW #SS$_NORMAL,R0 ;Success? BSBW ERRCHECK ;Find out ; ; Now create and access the file MYDATA.DAT. ; $QIOW_S CHAN=TAPECHAN,- ;Channel is magtape FUNC=#IO$_CREATE!IO$M_ACCESS!IO$M_CREATE,-;Function - ;is create IOSB=IOSTATUS,- ;Address of I/O status - ;word P1=FIB_DESCR,- ;FIB descriptor P2=#NAME_DESCR ;Name descriptor CMPW #SS$_NORMAL,R0 ;Success? BSBW ERRCHECK ;Find out ; ; LOOP1 consists of writing the alphabet to the tape (see previous ; description). ; MOVL #26,R5 ;Set up loop count MOVL #256,R3 ;Set up initial byte count ;in R3 LOOP1: ;Start of loop $QIOW_S CHAN=TAPECHAN,- ;Perform QIOW to tape channel FUNC=#IO$_WRITEVBLK,- ;Function is write virtual - ;block P1=BUFFER,- ;Buffer address P2=R3 ;Byte count CMPW #SS$_NORMAL,R0 ;Success? BSBW ERRCHECK ;Find out ; ; Now decrement the byte count in preparation for the next write ; operation and set up a loop count for updating the character ; written; LOOP2 performs the update. SUBL2 #2,R3 ;Decrement byte count for ;next write MOVL R3,R8 ;Copy byte count to R8 for ;LOOP2 count MOVAL BUFFER,R7 ;Get buffer address in R7 LOOP2: INCB (R7)+ ;Increment character SOBGTR R8,LOOP2 ;Until finished SOBGTR R5,LOOP1 ;Repeat LOOP1 until alphabet ;complete ; ; The alphabet is now complete. Fall through LOOP1 and update the ; byte count so that it reflects the actual size of the last block ; written to tape. ; ADDL2 #2,R3 ;Update byte count ; ; The tape is now read, but first the program must perform one of ; the three functions described previously before the ACP allows ; read access. The program performs an ACP control function, ; specifying skip zero blocks. This is a special case of skip reverse ; and causes the ACP to allow read access. ; CLRL FIB+FIB$L_CNTRLVAL ;Set up to space zero blocks MOVW #FIB$C_SPACE,FIB+FIB$W_CNTRLFUNC ;Set up for space ;function $QIOW_S CHAN=TAPECHAN,- ;Perform QIOW to tape channel FUNC=#IO$_ACPCONTROL,- ;Perform an ACP control - ;function P1=FIB_DESCR ;Define the FIB CMPW #SS$_NORMAL,R0 ;Success? BSBW ERRCHECK ;Find out ; ; Read the file in reverse. ; MOVL #26,R5 ;Set up loop count MOVB #^A/Z/,R6 ;Get first character in R6 LOOP3: ; MOVAL BUFFER,R7 ;And buffer address to R7 $QIOW_S CHAN=TAPECHAN,- ;Channel is magtape FUNC=#IO$_READVBLK!IO$M_REVERSE,- ;Function is read - ;reverse IOSB=IOSTATUS,- ;Define I/O status quadword P1=BUFFER,- ;And buffer address P2=R3 ;R3 bytes CMPW #SS$_NORMAL,R0 ;Success? BSBW ERRCHECK ;Find out ; ; Check the data read to verify that it matches the data written. ; MOVL R3,R4 ;Copy R3 to R4 for loop count CHECKDATA: ; CMPB (R7)+,R6 ;Check each character BNEQ MISMATCH ;If error, print message SOBGTR R4,CHECKDATA ;Continue until finished DECB R6 ;Go through alphabet in reverse ADDL2 #2,R3 ;Update byte count by 2 for ;next block SOBGTR R5,LOOP3 ;Read next block ; ; Now deaccess the file. ; $QIOW_S CHAN=TAPECHAN,- ;Channel is magtape FUNC=#IO$_DEACCESS,- ;Deaccess function P1=FIB_DESCR,- ;File information block (required) IOSB=IOSTATUS ;I/O status ; ; Deassign the channel and exit. ; EXIT: $DASSGN_S CHAN=TAPECHAN ;Deassign channel RET ;Exit ; ; If an error had been detected, a program would normally ; generate an error message here. But for this example the ; program simply exits. ; MISMATCH: ; BRB EXIT ;Exit ERRCHECK: ; BNEQ EXIT ;If not success, exit RSB ;Otherwise, return .END MAGTAPE_EXAMPLE

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