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PDP-11/60

INTRODUCTION

The PDP-11/60 computer was introduced in 1977 and was a follow-on to the PDP-11/40. This model was delayed into production by 12 to 18 months due to design and debugging issues. Not many of these models were sold as it did not have any more significant advantages over the 11/34, despite it being a more complex implementation of the PDP-11 instruction set. One notable feature was the writable control store (WCS) that allowed a user to implement additional instructions in micro-code. However, despite this feature, 11/60s were not commercially successful. By 1981 DEC shipments of 11/60s had ceased.

ARCHITECTURE

The PDP-11/60 was based on the KD11-K processor, providing the full FP11 floating point instruction in microcode. The hardware FP11-E Floating Point Processor, which provided significantly better floating point performance, was available as an option. A single slot in the CPU's backplane could hold either 1KWord of WCS (M7870, KU-116), an extended Control Store (ECS, M7871-YA) with 1.5KWord ROM microcode or a Diagnostic Control Store (DCS, M7871). It featured 18-bit memory management and standard main memory was either the MM11-WP core meory or the MF11S-KF MOS Memory option.

PDP-11/60 Handbook

The system was available in four different configurations. Three configurations were housed in a Low Boy Corporate cabinet. The main difference between these three was the installed mass storage option in the top left and right side. The choice available was either two RK06 removable pack disk drives, or a combination of a RK05f fixed and RK05j removable pack disk drive, or no mass storage peripherals at all, just two blank panels. The fourth configuration is housed in a High Boy Corporate cabinet and does not have any peripherals in the cabinet. An article by Chilton-computing.ork.uk provides amore detailed description of the 11/60 as documented below.

The PDP-11/60 was in the 11/04-11/34 line. It utilised the UNIBUS alone and runs it at 2Mbps. As opposed to some PDP-11 models using a seperate path to memory as in the FASTBUS model. The 11/60 fits between the 11/34 and 11/70, It used a mix of technology which gave it high computational power at a fairly competitive price. Since its introduction in the spring of 1977, sales in the U.K. alone topped the 40 mark and worldwide the total was more than 600.

The machine was oriented for high-performance, real-time applications and medium-performance, multi-user, multi-task, time-shared applications. The PDP-11/60 was offered in a variety of configurations. Every 11/60 used the same basic CPU but models differ in the cabinet shapes and sizes, the range of peripherals supplied, the amount and type of memory included in the basic packages. A bootstrap loader and a display console were provided to facilitate operator control. Also standard is a 2KB cache memory facility, hardware multiply and divide, and 32 and 64-bit single and double-precision floating point arithmetic. A faster floating point (FP11-E) is provided as an extra-cost option.

INSTRUCTIONS

The 11/60 had more than 400 machine instructions. There was the basic PDP-11 instruction set with some additions:

  • Four memory management instructions provided for moves in logical address space.
  • The FP11-C floating point set provided 46 instructions which include the ability to switch between fixed and floating point. These reference six dedicated 48-bit registers - other instructions utilise the eight 16-bit general-purpose registers. Calculations can be single-precision (32 bits) or double (64).
  • The FP11-E is an optional floating point accelerator, a microprocessor which implemented the same FP instructions at about twice the speed.
  • The EIS Extended Instruction Set is standard. It provided two powerful multiple-shift instructions and a hardware multiply and divide for fixed-point operations. A double-precision 32-bit word can be handled. The EIS is basically the same as that on the 11/34, 11/35 and 11/40. There was a group of five maintenance instructions which allow the user to examine registers and test the result of floating point operations. FP11-E also incorporates similar functions.

MICROPROGRAMMING

With the 11/60, Digital Equipment stepped boldly into user microprogramming. The 11/60 was not pioneering in this, several other mid-to-large systems could offer this method of implementing computer control. Microprogramming is considerably more detailed and much closer to the hardware than assembler programming. Individual steps of data movement and manipulation can be controlled by the programmer. This allows increased flexibility, the ability to run faster and more efficient programs, and the opportunity to implement custom-tailored instruction sets. Microprograms relate closely to hardware operations, so decoding and executing them is very quick and they are held in fast-access memory, so getting them into execution mode is also speedy.

Three microprogramming options were offered to the PDP-11/60 user; note that only one of the options may be installed in the computer at the same time.

Writable Control Store (WCS):

DEC referred to this feature as User Control Store, but the industry lingo is really Writable Control Store. It had all the software tools for program development. It provided 1,024 48-bit microwords of random-access memory. Each microword could be used either as one 48-bit-micro-instruction or as three 16-bit data words. By storing frequently-used data within the CPU in this fashion, the information can be retrieved at a cycle time of 170ns, the internal speed of the microcode processor. The appeal of WCS was ideal if the end-product makes the investment in microprogramming worthwhile. User-written microcode can be used to speed frequently-executed routines, so a commercial system might put data entry formats or mathematical calculations into WCS. A user could also give the machine new macro-instructions.

Extended Control Store (ECS):

The ECS is hardware only. It was comprised of 1,536 48-bit words of read-only memory. It afforded a means of permanent non-destructive storage for microprograms developed with the WCS option. ROM made for very fast access. To emphasise how much capacity ECS provided, Digital Equipment was fond of pointing out that it could hold the entire PDP-11 instruction set.

Diagnostic Control Store (DCS):

This option contained 2K 48-bit words of ROM and represents a specific hardware-supported application of ECS. DCS was a fault isolator which facilitates fault identification in the central processor to the module level, usually an easily-replaced chip. Experience has shown that the DCS will locate something like 92 percent of all processor faults.

Minimal Down Time and System Reliability

Microprogramming was available for the more sophisticated user who is willing to master the internal structure of the PDP-11/60 and write microcode for specific or time critival applications. The design and packaging of the PDP-11/60 placed emphasis on the Reliability and Maintainability Program (RAMP). Digital Equipment put much effort into minimising down-time. This involved trying to cut back on Mean Time Between Failures (MTBF). The cabinet has built-in cable runs and facilities. More impressive is the attention given to MTTR; there is continuous error-monitoring, several levels of diagnostics, fault isolation, in most cases to the board level and easy module replacement.

Other features providing system reliability and maintanibility are indicated below:

  • The cabinet had swing-out card cages for easier access by engineers; better cooling, which seems to mean better than for other PDP-11 cabinets; cable routing within the cabinet, which has been designed to reduce noise and cross-talk; and partitioned logic, an approach which gets related logic on to a single board.
  • The bootstrap leader checked the processor, cache and main memory, and the basic instruction set at start-up. This is conventional enough, but thorough.
  • The Diagnostic Control Store option provided instant identification of many processor faults. It was run from buttons on the console and isolates faults to the module level.
  • For error-logging, the 11/60 allows software to access particular storage locations where data is retained on failures. Using RSX-11M, the users can obtain a complete error history - the device identification, date and time, contents of the status register, and the number of the jobs running when the error occurred. Many so-called error logs trapped only the address of the fault.
  • Memory is operated below its theoretical maximum performance' core, for instance, is rated at a cycle time of only one microsecond. That allows greater reliability and the provision of cache means that a perfectly respectable throughput overall could be attained.

SYSTEM CONFIGURATION

Physically the processor consists of six printed circuit boards. The optional floating point processor uses four more slots in the backplane, the microcode extension option occupies one, and a serial line interface for the console always takes another. A variety of configurations wre available; all include this basic processor cage. Also on a separate backplane weredisc and peripheral controllers. Memory was on a third, backplane (the 11/60 uses 64KB cards) and a minimum of 64KB (one card) is always configured. The maximum of 248KB can be contained with the basic chassis but it is a fairly bulky unit by comparison with the 11/34. The basic cabinet stands 125cm high by 120cm wide and 79cm deep. The 11/66 is sold usually in three versions:

  1. PDP-11X60 - a processor with 64KB memory (core or MOS); no built-in discs and no peripherals are included in the price.
  2. PDP-11T60 - a 64KB 11/60, a DECwriter, one fixed 5MB RL01 disc drive, one removable 2.5MB RLO1 cartridge. The two drives provided are built into the top of the cabinet.
  3. PDP-11S60 - a 128KB processor, a DECwriter, and two integral 28MB RK07 disc drives. The controller could run up to eight disc drives.
The following is a typical 11/60 system configuration (DD11-KF memory slot excluded from list) as documented on the pdp-11.nl site.
  1. M7870 - KU116 WCS
  2. M7872 - KD11-K micro word module
  3. M7873 - KD11-K decode module
  4. M7874 - KD11-K data paths module
  5. M7875 - KD11-K cache memory and MMU
  6. M7876 - KD11-K timing module
  7. M7877 - KD11-K status module
  8. M7878 - FP11-E FNUA (Next micro Address)
  9. M7879 - FP11-E FLTEXP (Exponent)
  10. M7880 - FP11-E MULNET (Multiplying Network)
  11. M7881 - FP11-E FALU (Arithmetic Logic Unit)
  12. M7856 - DL11-W serial I/O port
  13. M7856 - DL11-W serial I/O port
  14. M7856 - DL11-W serial I/O port, system console

MEMORY

The memory for the 11/60 consisted of a 2,048-byte bipolar cache located within the processor, and either core or MOS memory in any combination of 64KB increments up to 256KB. Parity was standard core; ECC (error correcting code) is optional on the semi-conductor memory. The ECC would detect and correct all single-bit errors before passing-on data accessed to the computer. As with most similar schemes, it could not correct errors involving two or more bits; it did however, detect and log multiple-bit errors. ECC is superior to the parity checking, which does no more than detect single-bit errors. MOS memory with ECC was approximately 60 percent more expensive on the 11/60 than core with parity checking.

Cache

The cache memory acted as a buffer between the central processor registers and main memory. Whenever a request was made to fetch data from memory, a check was made to see if the data is already in cache. If it was, no main memory read was required; the UNIBUS was not accessed and the instruction proceeded at the fastest rate. If the data is not in cache, two bytes are transferred from main memory for execution. Benchmark tests by Digital Equipment were done for what the supplier describes as an average program; information needed by the CPU is found in the cache memory 77 percent of the time. By comparison, other cache systems can have a higher hit rate; the 11/70 cache has the needed instractions approximately 90 percent of the time.

As well as an increase in instruction execution speed, cache memory provided a lower UNIBUS utilisation by the processor; as a consequence, the UNIBUS is made available more often for transfers between I/O devices and memory. That overcame the need for a dedicated, high-speed data channel and allowed the 11/60 to perform well with the single bus. Battery back-up was standard on the MOS memory. The rechargeable unit lasts 30 minutes if keeping power on to a full 256KB, two hours for 64KB.

Memory Management

Memory management on the PDP-11/60 was compatible with the memory management option on the 11/34, the older 11/35, and 11/40. This facility provided for memory extension, relocation and protection.

  • It extends memory space from the 56KB which can be addressed directly to 248KB; 18-bit direct address bytes are used for this. The uppermost 8KB of address space, which takes the directly-addressable maximum to 64KB, was always reserved for UNIBUS I/O device registers.
  • It allowed segmentation of memory for multi-user environments.
  • It provided automatic protection of memory segments for multiple users. Memory areas could be designated as read-only or no access. In that way, access is controlled and users cannot accidentally or deliberately corrupt other memory segments.

OPERATING SYSTEMS

The 11/60 supported most of the main PDP-11 operating systems:

  • RT-11: A single-user, disc-based system for program development and/or on-line applications.
  • RSX-11M: Multi-user, real-time system.
  • RSX-11S: A subset of RSX-11M which provides a dedicated, execute-only environment for monitoring and controlling real-time processes.
  • IAS: General-purpose operating system, derived from RSX-11, allowing simultaneous interactive, batch and real-time program development and execution.
  • RSTS/E: Time-sharing system allowing multiple users to process data on-line.
  • MUMPS-11: Interactive, multi-user, database management system.

RSX-11M was the most commonly-used operating system on the 11/60s sold. It was developed independently of RSX-11D and IAS, though each of these developments adopted the same user interface. RSX-11D was developed for the midrange PDP-11 processors (11/35 upwards), while the M version was aimed at a wider range (11/04 upwards). D was not suitable for the smaller machines because of its high memory requirement, which meant that memory management was needed and because of its high processor requirement the smaller PDP-11s were too slow in operation.

RSX-11

RSX-11M was launched in 1974 and in the intervening years had two major upgrades, Version 3.0 being available since early 1977. An update to 3.1 was available in early 1978. RSX-11M can be configured to run on any PDP11 processor, with the exception of the LSI-11. It has a minimum requirement of 32KB and supports a maximum of 56KB if a memory management option is not available. The limit is 248KB on the 11/60. It provides a resource-sharing environment, including the following features:

  • disc-based operation, thus allowing task check-pointing, dynamic memory allocation and the use of overlaying within a task.
  • Multiprogramming.
  • Priority scheduling of tasks.
  • Contingency exits, or system traps, to allow tasks to control their action in the event of some unanticipated condition. For example, execution of an illegal instruction.
  • Power fail shutdown and auto restart.
  • Facilities available to the user included an easy-to-use operator interface (MCR) which incorporated the ability to process predefined sets of MCR commands contained in files.
RSX-11M acted as the host system for program development and system generation of RSX-11M systems. RSX-11S is a subset of M and is a memory-based system used for dedicated application environments. To operate it required a minimum memory of 16KB. It was fully compatible internally with RSX-11M. Being memory-based, RSX11S did not support a file system, task check-pointing or dynamic memory allocation.

FCS

The File Control System (FCS) supported block-structured or record-structured files with full file protection and automatic space allocation. Useful utilities included a text editor, the familiar PIP file manipulation facility, checks for file consistency, source file maintenance, and a crash dump analysis utility. RSX-11M could be configured to be 2 single or multi-user system. In its multi-user form it required a minimum of 64KB, could handle up to 24 terminals, and incorporated a multi-user protection scheme.

PROGRAMMING LANGUAGES

Languages supported for the 11/60 were the MACRO-II assembler and Fortran II as standard. Fortran IV, Fortran IV-Plus and Basic were options. Other software available includee the DECnet networking facilities. RMS-11, a multi-key ISAM record management facility, could be used; the full DBMS-11 database manager was not supported.

PDP-11/60 GUIDES
Document Name Order Part No. Publication Date Domain
PDP-11/60 Cabinet and Power Supply Manual EK-11060-SV-01 February 1978 HW
PDP-11/60 Installation and Operation Guide EK-11060-OP-003 February 1978 HW
PDP-11/60 Field Maintenance Print Set MP00166 May 1977 HW
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