Chapter IV Magnetic Tape
4.1 TAPE FORMAT
The control writes lateral characters, ie it writes transverse lines on tape with nine or seven bits of infor-mation per line, one bit in each track. The density of the inforinfor-mation written is determined by a switch at the transport. Every character is in either a data record or a file mark. A data record contains both data characters and error-checking characters; every data character consists of a data byte and a parity bit, which the control generates so that the number of Is in the line is odd or even as specified by the program. The data bytes in a record taken together correspond to a block of words sent from memory to the control. To separate adjacent records the control automatically erases a segment of tape, a record gap, between them. The control always stops tape in a gap.
Transfers between memory and control are of full words even though the tape chadlcters may contain 8-bit or 6-bit data bytes. The minimum length of a record is two words (four data characters), the maximum length is 4096 words. To write, the control divides the words into data bytes, and when reading, it reassembles them.
There are two ways in which this is done. For 9-track format the control writes each word as two characters, each containing an 8-bit data byte. After the control writes the last data line for a record, it writes three blank 4-1
o 2
FIRST CHARACTER 3 I 4
DATA
FORWARD _ _ _
5 6
SECOND CHARACTER
7 8 9 10 11 12 I 13
3 BLANK L1NES\ LC:--EOR GAP (3 LINES) LPCC .... ~
.6" DATA
RECORD GAP ONE RECORD
9-TRACK FORMAT
14 15
-lines, a cyclic redundancy character (CRC), three more blank -lines, and a longitudinal parity check character (LPCC). The first three zero characters constitute the end-of-record gap (EOR)" so called because the control uSes it to detect the end of record; this is so even in writing, as the tape encounters the write head first, and the control detects everything shortly after writing it. The control generates the CRC as described in §6 of USAS X3.22-1967, USA Standard, Recorded Magnetic Tape for Information Interchange. Taking the CRC bits as numbered in that document, CRC bit 1 corresponds to the parity track, bits 2-9 correspond to the tracks that receive the bits from left to right in each data byte. The LPCC (which may be zero) produces even longitudinal parity in each of the tracks along the length of the record. The minimum record gap is .6 inch. For compatibility with IBM format, a record must be written in high density and odd parity.
Whenever the control reads or writes a data record, it checks that the lateral parity of every data line agrees with the parity specified by the program and checks that every track has even longitudinal parity.
For 7-track format the control writes bits 2-7 and bits 10-15 of each word in two characters, ignoring bits 0, 1, 8 and 9 altogether. After writing the last data line, the control writes an EOR gap and an LPCC. The
FIRST CHARACTER SECOND CHARACTER
o 2 3 I 4 5 6 7 8 9 10 II 12 I 13 14 IS
LPCC ~ EOR GAP (3 LINES)
'
-DATA .75" DATA
FORWARD _ _ _ RECORD GAP ONE RECORD
7-TRACK FORMAT
minimum record gap is .75 inch. When reading 7-track tape, the control assembles pairs of bytes into words in the positions shown above, with Os in the unused bits.
When writing in even parity, the program must take care not to supply a word containing a zero data byte in the recording format selected, as this would result in a missing character (a blank line), and no words beyond 4-2
that point would reassemble correctly. The control does not check for missing characters when reading, but two or more contiguous missing characters would be interpreted as an EaR gap, so the command would terminate with the Bad Tape flag set.
To facilitate tape processing the program can group sets of data records into files. The end of a file is indicated by a 3-inch gap followed by a file mark, which is a special record containing a single, special data
character and its (equivalent) LPCC. A space command automatically terminates when a file mark is encountered.
Every tape has two physical markers to indicate its extremities. These markers are reflective strips that are sensed by photoelectric cells in the transport. At least ten feet in from the beginning of the reel is the loadpoint marker, which is the logical beginning of tape (BOT). Reverse commands stop automatically at this marker. A loadpoint gap of at least three inches precedes the first record on the tape. The end-of-tape marker (EaT) is at least fourteen feet from the physical end of the tape; the final ten feet of tape should be left for trailer, ie the program should not record more than a few feet beyond the EaT (this is more than enough for a record of maximum length at low density). A status bit indicates when the tape is beyond the EaT, but this condition stops the tape automatically only when it is spacing forward.
An annular groove is molded into the back of every reel. The control cannot write on the tape unless the supply reel has a plastic (write enable) ring in this groove. By leaving the ring out, the operator can protect the data on the tape from accidental destruction (overwriting or erasure) .
While the control is actually processing the data part of a record, the data transfer rate is fixed. But in a lengthy tape run the effective (average) transfer rate depends on record length, which determines the percentage of tape taken up by gaps (at the higher density each record gap could hold an additional 240 words). The effective transfer rate is therefore a function of record length as well as tape speed and density.
4.2 INSTRUCTIONS
The tape control has two 16-bit buffer registers to provide double buffering of data between tape and data channel; hence the channel has almost three character times in which to respond to requests by the tape control.
To run the tape, the program must select a transport and a command; most of the latter also require speci-fication of parity, an initial address (to the IS-bit address counter) for data channel access, and the (twos complement) negative of a word count. Space commands use the 12-bit word counter for counting records.
The tape system uses five of the 10 transfer instructions. Busy and Done are controlled or sensed by bits 8 and 9 in all 10 instructions with device code 22, mnemonic MT A. Interrupt Disable is controlled by interrupt priority mask bit 10. A second tape system connected to the bus would have device code 62. The Clear functivn (F
=
10) clears Busy and Done and also clears the command register and the status flags in the control. Start (F = 01) clears Done, sets Busy, clears many of the flags, and places the control and the selected transport in operation.DOA -,MTA Data Out A, Magnetic Tape
o
ACo o
Fo o o o
o 2 3 4 5 6 7 8 9 10 11 12 \J 15
Load the contents of AC bits 9--15 into the tape command register as shown, and perform the function specified byF.
4-3
o I 9 10-12
13-15
2 3 4 5 6 7
o
selects odd parity, 1 selects even.These bits select the command as follows.
0 Read
1 Rewind 2
3 Space Forward 4 Space Reverse 5 Write
6 Write End of File 7 Erase
Numbers 0-7 address transports 0-7.
DOB -,MTA Data Out B, Magnetic Tape
I
0 AC I 1o o
o 2 3 4 5 6 7
8
PAR-I COMMAND
ITY . UNIT
9 10 11 12 13 14 15
F
o o o o
I
8 9 10 11 12 13 14 15
Load the contents of AC bits 1-15 into the address counter (AC bit 0 should be 0), and perform the function specified by F.
Note: If this instruction is given with a 1 in AC bit 0 and if the control then executes a Read command in which the word counter does not overflow, the control reads the CRC at the end of the record and sends it to the next memory location specified by the address counter. This is primarily for maintenance, for the program to check whether the CRC is being generated properly.
DOC -,MTA Data Out C, Magnetic Tape
I
0 AC I1 1
o
F Io
1o 2 3 4 5 6 7 8 9 10 11
Load the contents of AC bits 4-15 into the word counter, and perform the function specified by F.
DtA -,MTA Data In A, Magnetic Tape
o
1 AC Io o
1 F Io o o
o 2 3 4 5 6 7 8 9 10 11 12 13
Read the status of the tape system into AC as shown, and perform the function specified by F.
4-4
o
14 15
o
14 15
DATA RE- ILLE- HIGH PAR- END END LOAD 9 BAD SENJ?K FIRST WRITE ODD UNIT ERROB LATE WIND- GAL DEN- ITY OF OF POINT TRACK TAPE CLOC CHAR LOCK CHAR READY
ING SITY ERROR TAPE FILE
o 2 3 4 5 6 7 8 9 10 II 12 13 14 15
Bits 11, 12 are for maintenance only and are not discussed further here_ Start clears Error, Data Late, Parity Error, End of File, and Bad Tape; Clear clears these plus Illegal; the remaining flags are supplied by the addressed transport (which is automatically unit 0 after Clear is given).
O B i t 1, 3, 5, 6, 7, 8, 10 or 14 is 1.
1 The data channel has failed to respond in time to a request for access (eg because of a long indirect addressing chain or preemption of the channel by faster devices).
2 The addressed transport is now rewinding.
3 This bit sets if the program gives Start when any of the following conditions holds:
• The command is Write, Erase or Write End of File, and Write Lock (bit 13) is 1.
• The command is Space Reverse and Loadpoint (bit 8) is I_
• Busy is 0 but Unit Ready (bit 15) is also O.
The setting of Illegal prevents the tape control from going into operation and sets Done, requesting an interrupt if Interrupt Disable is clear. The program must give Clear before proceeding (Start does not clear Illegal).
4 The addressed transport is set to high density (0 indicates low density).
5 In Read or Write the control has encountered a data character whose lateral parity differs from that specified with the command or has discovered a track with odd parity the length of a record. Incorrect parity in a CRC or LPCC does not set this bit, but specifying the wrong parity when reading a file mark does.
6 The addressed tape is beyond the EOT marker. (Reverse motion clears this bit.)
7 The control has written a file mark or has encountered one in reading or spacing. If there is an error in a file mark it is not recognized as such, ie the control interprets it as a very short data record.
8 The addressed tape is at loadpoint.
9 The addressed transport handles 9-track tape (0 indicates 7-track).
10 The control has encountered either data in a record gap or a false end of record (two or more contiguous blank characters). Spacing reverse over an unrecognized file mark also sets Bad Tape.
13 The write enable ring is not in the supply reel on the addressed transport.
14 An odd number of characters were detected while reading or writing.
15 The addressed transport is ready for operation by the program.
DlB -, MTA Data In B, Magnetic Tape
I
0 ACo
F !o o o o
o 2 3 4 6 7 8 9 10 II 12 13 14 IS
Read the present contents of the address counter into AC bits 1-15, and perform the function specified by F.
Clear AC bit O.
4-5