FP6000 PROJECT MEMORANDUM #23

October 3, 1963
Revised - Sept 22 1964

The following specification has been drawn up by Product Planning in conjunction
with Engineering. It completely replaces the issue of December 19, 1963.
Revisions have been underlined.

W. R. Whittall
Product Planning
Computer Systems

DISTRIBUTION LIST:

1. General

The magnetic tape control system consists of one unit control for each magnetic
tape unit (Burroughs BC422) together with one or more channel controls. Each unit
control has its own special register address for communication with executive and an
associated control word in the store for use by hesitation control. The special register
address is determined by the wiring of a decoder card in the unit control (the unit number
switch on the BC422 is not used). All tape operations except rewind require exclusive
use of a channel control. Provision is made for connecting each unit control to three
channel controls. At least twelve unit controls can be connected to the same channel control.
The unit control automatically selects a free channel control when the operation is initiated
by executive and releases it at the end of the operation. A special register bit will be set
in the unit control if all the channel controls connected to it are busy.

2. Tape Format

The FP6000 Magnetic Tape System will read and write IBM compatible 1/2”
7-track tapes at 120" per second, with variable length blocks of up to 131,072 6-bit
characters separated by gaps of not less than 5/8" on reading, or 3/4", +5/32", -l/l6" on
writing. Each 6-bit character will bo written across the tape, one bit per track, with
an odd or even parity bit in the seventh track. At the end of each block there is an even
longitudinal parity bit for each of the 7 tracks.

3. Density

The recording density can be either 200 or 555.6 characters per inch as selected
by the High/Low switch on each magnetic tape unit. This corresponds to a transfer rate
of 24 Kc/s or 66.7 Kc/s at 120" per second.
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4. Coding

Odd or even character parity will be selected by executive. The four
most significant bits of each character will be inverted before being written on tape
and inverted again when read. Computer character octal 74 cannot be written on the
tape with even parity and is therefore used as a stop character in even parity mode.
Binary data must therefore be written with odd parity and for this reason the modes
are called "Binary” and "BCD".

5. Checking

During a write operation the parity bits are generated by the channel control
and written on the tape with the data. When the information passes the read heads it
is checked for correct parity. During an erase operation the read heads are used to check
for complete erasure. During a read operation (forward or backward) the information is
checked for correct parity.

6. Tape Operations

The following tape operations can be initiated by executive by writing to the
unit control special register

In the case of Read, Write, Backward Read and Erase, N can have any value up to 32,767
and the 9 least significant bits of N should be placed in the control word with the starting
address for the transfer before writing to the special register. N = 0 signifies 32,768

7. Read (forward or backward

A block containing any number of characters from 1 to 131,072 can be read
by choosing a large enough value of N. Stop characters (octal 74) will be used to fill up
the last word transferred if necessary. The number of data characters in the last word
transferred (modulo 4) will be available to executive by reading the special register.
In BCD mode, if the number of words (including any partword) in a block on tape is less
than N. an extra word of stop characters is transferred to store and a special "Stop Word"
bit is set in the special register, if the block on tape is longer than N words the first
N words only are transferred, the rest of the block is read and checked, and a special
"Long Block" bit is set in the special register,

8. Write and Erase

These operations can be carried out in the forward direction only. Erase is
identical to Write except that the tape is erased and the read check detects any unerased
1 bit and sets parity fail if any are found. The length of tape erased depends on N and
density selected on the magnetic tape unit mid is only approximate, due to tape speed
fluctuations.

An operator attention interrupt will occur if an attempt is made to write or
erase on a reel without a Write Permit ring. About 2 1/2 inches of tape will be automatically
erased before the first write operation at beginning of tape. When writing or erasing
in BCD mode the first stop character (Octal 74) will cause the operation to be terminated
and the "Long Block" and "Number of data characters in last word" bits to be set in the
special register.

9. Backward Read

This is similar to the Read operation except that the tape is moved backwards
and the characters are placed in the store in decreasing address sequence starting with
character 3 of the word addressed by the control word. An attempt to perform a Backward
Read operation at Beginning of Tape will result in an Operator Attention interrupt without
moving tape.

10. End of File Blocks

The End of File block consists of an IBM tape-mark (octal 17) character
followed by its longitudinal check bits. When read into the computer it will be converted
into octal 63. A special End of File bit will be set in the special register whenever an
End of File block passes the read heads in either direction except during a rewind. (Revised)

11. Rewind

This causes the tape to rewind at high speed (2,400 feet in 90 seconds) until
the beginning of tape marker is sensed when an interrupt will be generated. The Rewind
operation does not use a channel control.

12. Simultaneous Transfers

The channel control will be double buffered so that up to three (6 us machine)
or seven (2 us machine) READ, WRITE. BACKWARD READ or ERASE operations can take
place simultaneously if there is a free channel control for each transfer and if there are no
other fast peripherals in use.

13. Accept Facility

If the "Remote” (On-Line) button on the magnetic tape unit is pressed while
the unit is in "Local”, at the beginning of tape, and otherwise ready, an interrupt will be
generated with the "Accept" bit set in the special register.

14. Information Available to Executive

Notes

1. End is set at the end of any operation (including rewind)

2. Operator Attention is set:-

(a) If any operation is initiated when the unit is off-line.

(b) If the unit goes off-line while busy.

(c) If an attempt is made to write on a reel without a write permit ring.

(d) If an attempt is made to Backward Read or File Skip Backward at
beginning of tape.

(e) If Beginning of tape is reached during a Backward Read or File Skip
Backward.

3. Beginning of Tape is set when the tape reaches the Beginning of Tape marker
moving backwards and remains set until the tape is moved forwards

4. End of Tape is set when the tape reaches the End of Tape marker moving
forwards and remains set until the tape is moved backwards. Approximately
8 feet of tape can be used beyond the End of Tape marker in its normal position.

5. The unit can be set On-Line (Remote) or Off-Line (Local) by a switch on the unit.

6. TREL is set during any operation, other than REWIND, except for
about 5ms, after the end of any previous operation. TWRL is set only
if TREL is set and the tape supply reel mounted on the unit is fitted with
a write permit ring. TREL will only be used to define when TWRL is
dependent only on the presence or absence of the write permit ring.