Introduction to the DEC "PDP 11/20 Handbook", 1969
copy at


PDP-11 is Digitalís answer to the demand for a modular system for real-time data acquisition, analysis and control. PDP-11 systems can handle a wide variety of real-time control applications-each system being individually tailored from a comprehensive array of modular building blocks. Digital is unique among manufacturers of small-scale computers-in its ability to pro- vide not only fast and efficient processing units, but also a large family of its own compatible I/O devices including A/D and D/A converters, magnetic tape, disk storage, paper tape, and displays, as well as a wide range of general-purpose modules. This capability offers the user a hew, more efficient approach to real-time systems.


The PDP-11 is available in two versions designated as PDP-ll/10 and PDP-11/20. The PDP-ll/ 10 contains a KA11 processor, 1,024 words of 16-bitread-only memory, and 128 16-bit words of read-write memory. The basic PDP-ll/PO contains a KA11 processor and 4096 words of 16-bit read-write core memory, a programmerís console, and an ASR-33 Teletype. Both ver- sions can be similarly expanded with either read-write or read-only memory and peripheral devices.


Unibus is the name given to the single bus structure of the PDP-11. The processor, memory and all peripheral devices share the same high-speed bus. The Unibus enables the processor to view peripheral devices-as active memory locations which perform special functions. Peripherals can thus be addressed as memory. In other words, memory reference insfructions can operate directly onícontrol, status, or data registers in peripheral devices. Data transfers from input to output devices can bypass the processor com- pletely.


The PDP-11 has adopted a modular approach to allow custom configuring Of systems, easy expansion, and easy servicing. Systems are composed of basic building blocks, called System Units, which are completely independent sub- systems connected only by pluggable Unibus and power connections. There is no fixed wiring between them. An example of this type of subsystem is a 4,096-word memory module.

System Units can be mounted in many combinations within the PDP-11 hardware, since there are no fixed positions for memory or l/O device con- trollers. Additional units can be mounted easily and connected to the system in the field. In case maintenance is required, defective System Units can be replaced with spares and operation resumed within a few minutes.


A complete package of user-oriented software includes:
* Absolute assembler providing object and source listings
* String-oriented editor
* Debugging routines capable of operating in a priority interrupt environment
* Input/output handlers for standard peripherals
* Relocatable integer and floating point math library