LINCTAPE TYPE TC12

The basic LINCtape system consists of either two DECtape Transports Type TU55 or one DECtape Transport Type TU56 controlled by a fully buffered subprocessor. A single ten-channel tape head serves for both reading and writing. Information is redundantly recorded; one line of tape contains five bits, each recorded in two non-adjacent channels, as shown in Figure 3-16. Three bits are actual data; the other two provide control information for the tape processor. The Timing track determines the position of each recorded line. Four lines are required to accommodate a full 12-bit word; the corresponding Mark Track code identifies the nature of the data word. The recording technique and tape layout are described in detail in the PDP-12 Maintenance Manual.

3.6.1 Organization of Data

On a standard-format LINCtape, information is recorded in blocks of 256 12-bit words each, with identifying data at each end of the block. One reel of Standard format LINCtape has a capacity of 512 blocks, for a total of 131,0721 0 words of data. (In other formats, this capacity may be extended to 225,0001 0 words.)

The organization of a tape is schematically presented in Figure 3-16. At each end of the tape is a long End Zone which allows the transport to reverse direction or come to rest without pulling the tape off the reel. Between the end zones and the terminal blocks, and between blocks, are Interblock Zones which ca_n be sensed by the LINC instruction IBZ. An interblock zone is 5 words long.

Figure 3-l 6B represents a typical block of 256 data words preceded and followed by control and identifying information. The serial bit sequence on the Mark Track, is decoded so that the control can determine whether the adjacent data bits form true data, checksum words, guard words, etc. The symbols BM, CM, GM, etc. (defined below), refer to these Mark Track bit patterns.

A. FULL

-Figure 3-16. Standard LINCtape Format

END

A block consists of 256 data words preceded and followed by control and identifying information, as shown in the second drawing in Figure 3-16.

Block Number (BM) Guard Word

(GM)

Data Words (DM,FM)

Checksum (CM)

Check Words (CM)

Reverse Block Number (RBM) Su bprocessor

This identifies the block. On a standard LINCtape, block numbers are sequential, from 0000 through 0777 8 •

This protects the Block Number from transients when the read/write current is turned on and off, and allows time for the tape processor to switch from Search to Read or Write modes.

This is the information recorded on tape from core memory. The final Data Word is specially identified, to signal the end of the block. When writing a tape, this signal conditions the tape processor to write the checksum in the next word position.

This is the two's complement of the 12-bit sum of all the data words in the block plus the constant 7777 _{8 •} The result of adding the data sum to the checksum should be 0000; this provides a check (hence the name) on the accuracy of the transfer.

These are dummy words whose Mark Track code is the same as that for the Checksum. They are provided to insure that the Write current will be turned off before the Reverse Block Number is encountered. The Guard and Check words are not of general interest to the programmer except as they affect timing.

This is the Block Number that identifies the block when tape is being searched in the reverse direction.

The LINCtape processor controls all information transfers between memory and tape. It is fully buffered; once an operation has been initiated by a LINC mode instruction, it is carried to completion by the tape processor. The central processor may either wait until the tape transfer is complete, or proceed immediately after the tape instruction has been initiated, testing at some later time for completion of the operation.

Transfers are effected in either Standard or Extended modes. In Standard mode, transfers are made to and from fixed memory locations. Extended Operations provide for a flexible addressing facility, program interrupt, and additional tape units.

-As can be seen in Figure 3-17, the tape subprocessor contains seven registers which provide the transmission path for data and for control information.

Data Path

Read/Write Buffer (RWB), 12-Bits - When reading, the four lines of a data word are assembled in this register, in the bit positions shown in the third drawing of Figure 3-16. When writing', the contents of the RWB are disassembled and written on four consecutive lines of tape. Essentially, the RWB is a three-section shift register, with the three bits of a tape line entering (or leaving) the register at four-bit intervals, as indicated by the arrows in Figure 3-17.

SEARCH ONLY

TBN 12 bits

TMA 12 bits

ADDER TMA SETUP 12 bits

XOB 12 bits

LINCtape PROCESSOR

AC

<

_ _ _ _..,. CONTROL INFORMATION ( 12 BITS )

WRITE

CENTRAL PROCESSOR

T-B

TAC

12 bits SHIFT REGISTER

READ/SEARCH

TB+TAC 12 bits

MB

Figure 3-17. LINCtape Processor Information Paths

READ/

WRITE

12-0105

Tape Buffer (TB), 12 Bits - When reading, the assembled word is transferred from the RWB to the TB, and from there sent to the MB in the central processor. When writing, the direction is reversed; information from the MB enters the TB, and from there is placed in the RWB for disassembly onto the tape.

Tape Accumulator (TAC), 12 Bits - As each data word is read or written, the data sum is accumulated in the TAC.

When reading, this sum is added to the Checksum read from tape, to determine whether the transfer was completed accurately. When writing, the data sum is complemented when the final data word signal is received, and the resulting Checksum is written in the word position following the final data word. The contents of the TAC can be brought into the central processor AC, using the Linc mode TAC instruction. In searching operations. the TAC also holds the sum of the desired block number and the last block number read from tape.

Control Registers

In Standard mode operations, these registers are automatically set up; in Extended Operations, the program must set the XOB and TMA Setup Register.

Tape Block Number (TBN), 12 Bits - This contains the number of the block to be accessed in a data transfer. As the tape is searched, the Block Number read from tape is compared with that in the TBN; when the numbers match, the tape is positioned so that the transfer can begin. During group operations, the TBN contains the number.

of the first block to be accessed.

Tape Memory Address (TMA), 12 Bits - This contains the address of the memory location to or from which the data is being transferred. In extended address mode, TMA is loaded from the TMA Setup Register at the beginning of a tape instruction; in Standard mode, the MBLK and TBLK information in the second tape instruction word are used to determine the initial contents of TMA. The TMA is incremented by 1 for each data word transferred.

TMA Setup Register, 12 Bits - In Extended Address mode, the register retains the first memory address of the data to be transferred. If the transfer is not successful, the contents of TMA Setup are placed in the TMA, and the operation is repeated. The TMA Setup Register is loaded from the AC, using the TMA instruction.

Extended Operations Buffer (XOB) 12 Bits - The contents of this register determine which of the various extended tape operations are in effect. These include extended memory addressing, tape interrupt, the no-pause condition, hold motion, and extended units.

3.6.2 Programming

The tape transfer operations are the same for both Standard and Extended Operation modes. Data may be read or written in single blocks or groups of contiguous blocks (in the extended address mode, only single blocks are transferred), with or without error-checking. Step-by-step searches can be performed, and block numbers can be identified without reading or writing data.

All LINCtape instructions require two words. The first word specifies the operation to be performed, one of two tape units, and the motion of tape at the end of the operation. The second word gives the tape block number and in Standard mode also gives the memory block number. The structure of the two words is shown in Figure 3-18.