![[ GIF image of Figure 3-17 ]](doco/images/Fig3-17.gif)
Figure 3-17. LINCtape Processor Information Paths
The basic LINCtape system consists of two TU55 transports 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.
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
LINCtape reel has a capacity of 512 standard blocks, a total of 131,072 words
of data.
INTERBLOCK
ZONE /-------- 512 BLOCKS ------\
\ \/ \
+------------+--+---------+---+-- --+---------+---+------------+
FULL | | | | I | | | I | |
TAPE | END ZONE | | 1 BLOCK | B | ... | 1 BLOCK | B | END ZONE |
| | | | Z | | | Z | |
+------------+--+---------+---+-- --+---------+---+------------+
\ /
About 5 feet
++-----+----+---+----------------------+----+---+---+-----+-----+
ONE ||INTER| | | | | | | | |
BLOCK ||BLOCK| BN | G | DATA | CS | C | C | RBN | IBZ |
||ZONE | | | | | | | | |
++-----+----+---+----------------------+----+---+---+-----+-----+
Words -> 5 1 1 256 1 1 1 1 5
---+---+---+---+---+-----
ONE Data Channel 1 -> | 0 | 1 | 2 | 3 |
DATA Data Channel 2 -> | 4 | 5 | 6 | 7 |
WORD Data Channel 3 -> | 8 | 9 |10 |11 |
---+---+---+---+---+-----
|
+-- Bit Location
<------- Tape Direction Forward --------------
Figure 3-16. LINCtape Format
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 can be sensed by the LINC instruction IBZ. An interblock zone is 5 words long.
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.
Subprocessor
The LINCtape subprocessor controls all information transfer 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 begun, 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
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.
Figure 3-17. LINCtape Processor Information Paths
The tape transfer operations are the same for both Standard and Extended Operation modes. Data can be read or written in single blocks or groups of contiguous blocks, 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 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.
+---+---+---+---+---+---+---+---+---+---+----+----+
First Word | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
+---+---+---+---+---+---+---+---+---+---+----+----+
\ / ^ ^ \ /
\--- Instruction Code ---/ | | \ Operation /
| |
Motion --+ +-- Unit
/----- Block Number (Extended Addressing) ------\
/ \
+---+---+---+---+---+---+---+---+---+---+----+----+
Second Word | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
+---+---+---+---+---+---+---+---+---+---+----+----+
\ /\ /
\ Memory / \---- Tape Block Number (TBLK) ----/
Block Number
(MBLK)
Figure 3-18. LINCtape Instruction Format
Memory LINC LINC Block Number Memory Field Field Addresses (MBLK) (if IF=0 & DF=1) ---------------------------------------------------------------- 0 Instruction Field 0000-0377 1 Instruction Field 0400-0777 2 Instruction Field 1000-1377 3 Instruction Field 1400-1777 4 Data Field 2000-2377 5 Data Field 2400-2777 6 Data Field 3000-3377 7 Data Field 3400-1777
In Standard Mode addressing, the contents of a tape block and a memory block correspond exactly. All single-block transfers are effected between the tape block and the memory block specified by the second word of the tape instruction. In group transfers, where several contiguous blocks are transferred, the second word is interpreted in a slightly different way. (See description of RCG and WCG instructions.) The two non-transfer instructions, MTB and CHK, use the second word in still different ways and are described below.
Although tape can be read or written only in the forward direction, it may be searched either forward or backward. Bit 7 of the first word of a LINCtape instruction determines the motion of the tape when a LINCtape operation has been completed.
If bit 7 (the I-bit) is set to 1, the tape is left moving in the direction it was going at the completion of the operation. Except for searches backward, this will usually be the forward direction.
If bit 7 is set to 0, the tape processor enters the Turnaround state at the completion of the operation. Regardless of its former motion, the tape is set moving backwards. When a block mark is encountered, the tape stops, leaving the Turnaround state and entering the idle state. If a new tape instruction occurs while the tape processor is in the Turnaround state, searching begins with the tape moving backwards. (A tape instruction is considered to be complete when the tape processor enters either the Turnaround or the idle state.)
If the tape has stopped, a new tape instruction will always cause it to begin moving in the forward direction. It will not search backwards until one block number has been encountered and compared with the desired block number.
NOTE
The foregoing discussion applies only to tape motion at the completion of an instruction. It should not be confused with the NO PAUSE Extended Operation, which affects central processor action after a tape operation has begun.
In the subsequent instruction descriptions, the following terms are used:
RDE - Read Tape
Form: RDE I U
Octal code: 0702+ 20I+ 10U
Execution time: 3.2 µsec
Operation: Block TBLK is read from tape into memory block MBLK. The transfer
check is left in the TAC and AC. The contents of tape are unchanged.
RDC - Read and Check
Form: RDC I U
Octal code: 0700 + 20I + 10U
Execution time: 3.2 µsec
Operation: Block TBLK is read from tape into memory block MBLK. If the block
is transferred correctly, the transfer check is left in the TAC and AC;
otherwise, the operation is repeated. The information on tape is unchanged.
RCG - Read and Check Group
Form: RCG I U
Octal code: 0701 + 20I + 10U
Execution time: 3.2 µsec
Operation: MBLK is the number of additional consecutive blocks to be
transferred. Block TBLK is read from tape into the memory block designated
by the three low-order bits of MBLK, e.g., tape block 773 is read into memory
block 3, tape block 027 into memory block 7, etc. The next consecutive TBLK
blocks are read into successive memory blocks. Tape block 000 follows tape
block 777, and memory block 0 follows memory block 7.
Example: Transfer blocks 202-205 from unit 1 to memory, leaving the unit in motion at the end.
...
RCG I l 0731
3202 /MBLK=3, THE NUMBER OF ADDITIONAL BLOCKS
/TBLK=202.
Data is transferred from tape block 202 into memory block 2, then from 203 to memory block 3, 204 to memory block 4, and 205 to memory block 5.
Each block transfer is checked; if the transfer is successful, the transfer check (7777) is left in the TAC otherwise, that block is repeated. If the entire group is transferred successfully, 7777 is left in the TAC and AC at the end of the operation.
WRC - Write and Check
Form: WRC I U
Octal Code: 0704 + 20I + 10U
Execution Time: 3.2 µsec
Operation: The contents of memory block MBLK are copied into block TBLK. If
the transfer is successful, the transfer check (7777) is left in the TAC;
otherwise, the operation is repeated.
WRI - Write Tape
Form: WRI I U
Octal code: 0706 + 20I + 10U
Execution time: 3.2 µsec
Operation: The contents of memory block MBLK are copied into block TBLK; the
transfer check is left in the TAC. The contents of memory are unchanged.
WCG - Write and Check Group
Form: WCG I U
Octal code: 0705 + 20I + 10U
Execution time: 3.2 µsec
Operation: MBLK is the number of additional consecutive memory blocks
whose contents are written on tape. The low-order digit of TBLK specifies the
first memory block to be accessed. The contents of this block are copied into
block TBLK of tape. The contents of the remaining MBLK memory blocks are
copied into the next successive tape blocks. Memory block 0 follows memory
block 7, and tape block 000 follows tape block 777. The scheme is identical
with that for RCG.
The following two instructions do not transfer any data.
MTB - Move Toward Block
Form: MTB I U
Octal code: 0703 + 20I + 10U
Execution time: 3.2 µsec
Operation: Subtract the next tape block number (or reverse block number, if
the tape is moving backwards) encountered from TBLK, leaving the difference
in the TAC and AC. If I = 0, the tape stops. If I = 1, the tape is left moving
forward if the difference is positive or 0, and backward if negative. If the
tape is at rest when this instruction is given, it starts moving forward;
otherwise it continues in the direction it had been going. The MBLK bits of
the second word are ignored.
CHK - Check Tape Block
Form: CHK I U
Octal code: 0707 + 20I + 10U
Execution time: 3.2 µsec
Operation: Tape block TBLK is found, and its contents read into the tape
control registers only, to form the data sum (no transfer takes place). The
checksum is read, and the transfer check is left in the TAC and AC. The
information on tape is unchanged, and the MBLK bits of the second word
are ignored.
The contents of the Tape Accumulator can be examined by using the following instruction.
TAC - Tape Accumulator to AC
Octal code: 0003
Execution time: 1.6 µsec
Operation: The contents of the Tape Accumulator are placed in the central
processor AC. The previous C(AC) are lost; C(TAC) are unchanged.
LINCtape Extended Operations give the programmer a more flexible addressing scheme for information transfers, additional control functions, and a tape processor maintenance facility. These operations are controlled by the contents of the Extended Operations Buffer, defined as shown in Figure 3-19. The XOB can be loaded from the AC and vice versa.
AXO - AC to XOB
Octal code: 0001
Execution time: 1.6 µsec
Operation: The contents of the AC are placed in the Extended Operations
Buffer. The previous C(XOB) are lost: C(AC) are unchanged.
XOA - XOB to AC
Octal code: 0021
Execution time: 1.6 µsec
Operation: The contents of the Extended Operations Buffer are placed in the
AC. The previous C(AC) are lost; C(XOB) are unchanged.
EXTENDED OPERATIONS BUFFER
+---+---+---+---+---+---+---+---+---+---+----+----+
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
+---+---+---+---+---+---+---+---+---+---+----+----+
\ / ^ ^ ^ ^ ^ ^ \ /
Extended | | | | | | Extended
Memory | | | | | | Units
Address | | | | | |
| | | | | | Hold
Mark ----------+ | | | | +-- Unit
Condition | | | | Motion
| | | |
Enable | | | | Do Not
Tape --------------+ | | +------ Pause During
Interrupt | | Execution
| |
Maintenance ------------------+ +---------- Enable Extended
Mode Address Mode
Figure 3-19. Extended Operations Buffer Bit Assignments
This facility releases the programmer from the limitation of block to block transfers, as described previously. Instead, a block transfer may begin in any register of any memory bank, regardless of the settings of the Memory Field Registers.
When the Extended Address Mode is enabled (by setting bit 7 of the XOB to 1), all subsequent tape transfers are executed as follows:
The starting address of the area in memory is placed in the TMA Setup Register by the program, using the instruction TMA.
TMA - Load TMA Setup Register
Octal code: 0023
Execution time: 1.6 µsec
Operation: The contents of the AC are placed in the Tape Memory Address
Setup Register. The previous contents of TMA Setup are lost; C(AC) are
unchanged.
At the occurrence of a tape transfer instruction, the contents of the TMA Setup Register are placed in the TMA. The second word of the instruction is taken as a full 12-bit block number, and placed in the TBN. The transfer is effected between tape and the designated area of the 4096-word memory bank specified by bits 0-2 of the XOB. The transfer is thus independent of the LINC Memory Field assignments.
As in all extended memory operations, whether with tape or not, the transfer will not cross memory bank boundaries; register 7777 of a given bank is followed by register 0000.
NOTE
The group transfer instructions RCG and WCG cannot be used in extended address mode.
The non-transfer instructions MTB and CHK are not affected.
Example: Read the contents of block 365 of unit 0 into memory bank 1 (second 4K memory bank) starting in register (10)540.
Octal
Instruction Code Action
LDA I 0 1020 /LOAD AC WITH STARTING ADDRESS
0540 0540 /STARTING ADDRESS OF TARGET AREA
TMA 0023 /PLACE STARTING ADDRESS IN TMA SETUP
LDA I 0 1020 /LOAD AC WITH EXTENDED OPERATIONS BITS
1020 /BANK I:ENTER EXTENDED ADDRESS MODE.
AXO 0001 /LOAD XOB FROM AC.
...
...
RDC 0 0700 /READ AND CHECK FROM UNIT 0
0365 0365 /BLOCK 365
...
The data will be read into registers 540-1137 of memory bank 1.
The two Extended Units bits (XOB[10,11]) may be thought of as an extension of the unit bit of a LINCtape instruction (bit 8). Taken together, the three bits can select one of up to eight TU55 transports which may be attached to the TC12 tape control. The logical unit numbers are assigned by rotating the dials on the transports; they correspond to the unit select bits as follows:
Extended unit Instruction Transport Bits (XOB) Unit Bit Selected 10 11 8 ---------------------------------------------------- 0 0 0 8 0 0 1 1 0 1 0 2 0 1 1 3 1 0 0 4 1 0 1 5 1 1 0 6 1 1 1 7Tape Interrupt Enable
When this bit (XOB[5]) is set, a program interrupt will occur whenever a tape operation is not in progress. As with other LINC interrupts, control is transferred to register 0041 of memory field 0; the contents of the PC are stored in register 0040. (If the central processor is in PDP-8 mode, the interrupt uses registers 0001 and 0000.)
Normally, the central processor waits until a tape operation is finished before proceeding. Such delays are eliminated by setting the No Pause condition bit (XOB[8]). When this condition is enabled, the processor continues with the program as soon as the LINCtape instruction has heen interpreted and the operation initiated.
Subsequently, the program can monitor the Tape Done (STD) flag to determine when the operation has finished, before starting a new one. When NO PAUSE is set, the transfer of the Transfer Check to the accumulator at the end of tape instruction inhibited.
STD - Skip if Tape Done
Form: STD I
Octal code: 0416 + 20I
Execution time: 1.6 µsec
Operation: If I = 0, skip the next instruction if the tape operation is
completed; otherwise execute the next instruction. If I = 1, skip if the
operation is still in progress. This instruction is identical to SXL I 16.
Used in conjunction with the tape flag and tape interrupt, the No Pause condition can save considerable amounts of time, and gives the programmer added flexibility in the processing of data before a transfer is completed.
Normally, a tape transport stops as soon as another unit has been selected. When XOB[9] is set, however, the transport will continue in the direction it has been moving when the unit is deselected. This is a useful feature for certain operations involving several units, and must be used with care. Note that it is not the same as the motion bit of a LINCtape instruction, which determines the motion state of a unit at the completion of an instruction only.
This bit (XOB[4]) is used in conjunction with the MARK switch on the operator's console, to allow the MARK12 program (see Chapter 6, Section III on Program Library) to record Timing and Mark tracks on a new tape. The interaction between the switch and the XOB is designed to minimize the possibility of accidentally destroying a tape by enabling the MARK flip-flop. The flip-flop can be set only when the MARK switch is held down while an AXO instruction is being executed with AC[4] set to 1.
When bit 6 of the XOB is set to 1, all timing signals and data are prevented from entering the tape control registers from the reader- writers. Instead, signals generated by PDP-8 IOT instructions are used as input to the tape control, in order to simulate the functions of the tape head and the tape processor. The Maintenance Mode is designed for diagnostic purposes only and is not intended for general use.
Whenever the TAPE TRAP and INSTRUCTION TRAP Special Functions are enabled (ESF with AC[2,3] set to 1s), LINCtape instructions are not executed. When one is encountered, a program trap to register 0140 of memory field 0 occurs. The Tape Trap is intended primarily for use with LINC-8 programs and the I/O Handler (PROGOFOP simulator) to ensure compatibility.