A brief overview of the characteristics of the DEC (Digital Equipment Corporation) PDP-12 (Programmed Data Processor-12) computer is provided in this section. A more complete description of the PDP-12 may be obtained in the PDP-12 System Reference Manual (Ref 2).
The PDP-12 is a versatile digital computer which contains two distinct operating modes within its central processor, each with its own instruction set. The central processor logic is fully parallel, using a basic word length of 12 bits. The processor cycle time is 1.6 microseconds +20U. Most instructions require from 1 to 3 cycles for execution. The FDP-12 operates in one mode as a LINC (Laboratory Instrument Computer) and in the other mode as a PDP-8 computer. The computer may be stopped and started in either mode. Both operating modes have equal priority and programs may be switched from one to the other at will. Computations in one mode are immediately available to programs operating in the other mode because only one set of processing registers is involved.
The principal unit of core memory is a module of 4096 (4K) 12-bit words. Up to seven additional modules may be added, providing a total of 32,768 words. The logical organization within each module depends on the operating mode. In LINC mode, each 4K module is divided into four 1024-word segments. Only two of these segments are active at any given time« 1) the Instruction Field, which contains the executable program and directly addressed data, and 2) the Data field, which contains only indirectly accessed data. In 8 mode, each 4K module (memory field) is divided into thirty-two 128-word pages. Data may be directly addressed to the current page or to page 0 only. Indirect addressing, through page 0, must be used to address data between pages.- Special instructions must be used to change Instruction Field or Data Field segments in LINC mode and to change memory fields in 8 mode.
Many of the peripheral devices available with the PDP-12 are controllable only in LINC mode. Of these, the A-D Converter and the C.7 Display are used in the recognition program listed in Appendix A. The CRT Display, however, is not essential to the program and its use will not be described in this report. The other peripheral devices used in the program» the Programmable Real-Time Clock and the Teletype device, may be controlled in either mode. A brief description of these devices and their use in the recognition program is given in the following paragraphs.
The A-D Converter consists of eight external input channels and eight internal input channels. The external input channels have an acceptable input voltage rcnge of +lv, corresponding to a sample value range of +777». The internal input channels (control knobs) also have a sample value range of +777«. One external input channel is used in the recognition program to sample the voltage level of the Morse code signal. Three internal input channels are used to set the input signal threshold level, the number of samples to be averaged, and the CHARACTERWORD boundary adjustment value. The Programmable Real-Time Clock consists of a 400 kHz crystal clock, a 12-bit counter register, and an overflow bit. The clock may be used to synchronize the central processor to external events, count external events, measure intervals of time between events, or provide program interrupts at intervals from 2.5 microseconds to over 40 seconds. The 400 kHz crystal clock may be used to provide pulses to the counter register at 100 Hz, 1 kHz, 10 kHz, 100 kHz, or 400 kHz rates; or an external source may be used to drive the counter. The clock is used in the recognition program to measure the time durations of pulses and spaces. An external source is used to permit variable counter rates in the 1 kHz to 10 kHz range.
The Teletype device is used to type in or print out information at a rate of “n to ten characters per second. Similar devices, such as the B€CKRITER, operate at much faster rates. A DECWRITER was used during this project to print the test messages shown in appendix D.
Certain distinguishing features of the two operating modes must be considered when changing modes within a program. One of these is the addressing scheme. In LINC mode, Instruction Fields and Data Fields consist of 1024-word segments. The addresses within each field range from to 1777g, regardless of the physical location in core memory. Thus, location 0100g in Data Field 3 corresponds to physical location 6100g in core memory. In 8 mode, address locations correspond exactly to physical locations.
Another distinguishing feature between the two modes concerns arithmetic operations. LINC mode uses l’s complement addition for most operations, whereas 2’s complement addition is used in 8 mode. As an example, 7777g is interpreted in LINC mode as -0 and in 8 mode as -1. Also, 7777g + 0001g yields 0001g in LINC mode and 0000g in 8 mode.