4. Unit overloaded.
5. Unit under manual control.
6. Power off.
A selected but unavailable unit holds up the TC if a motion order is given for the unit. The TC will be held up until the unit is ready.
_Flag Positions_
_IO Bit_ _Flag_
0 EOR - End of record 1 RWF - Read-Write 2 EOT - End of Tape 3 Parity
_Connection with High Speed Channel_
The high speed channel directs the tape control, and word transfer, just as a program would. A unit is first started reading or writing. The high speed channel is given the memory location of the information, and the number of registers the words read or written will occupy. The channel effects the information transfer. Thus, a high speed channel connected to a tape control handles the programming for the unit word transfers.
Completion of the block transfer is signified by either setting a program flag, or entering the sequence break.
_Connection with Sequence Break System_
When the TC is connected to the Sequence Break System, the program is automatically interrupted each time an MPS command needs to be given.
Programming is unaffected during reading and a record may be read with no flags set. The TC initiates breaks so that an MPS may be given in time.
Similarly, the break is initiated during writing each time an MPS needs to be given.
_Motion Command Summary_
_Time before _Time between _Time after End of _Flags that first MPS_ MPS"s_ Record to deselect_ may be set_
MWA 3 ms. 400 us. 10 ms. RWF (if unit MWB (longer time is deselected causes deselection) and MPS given, or unit becomes unavailable), Parity, EOT.
MRA 7 ms. < 400="" us.="" 5="" ms.="" rwf,="" (if="" mrb="" (longer="" time="" information="" misses="" information,="" is="" missed,="" or="" and="" unit="" becomes="" rwc="" set)="" unavailable),="" eot,="" eor,="">
MBA - - 10 ms. RWF (if unit MBB becomes unavailable), EOR, EOT.
CATHODE-RAY-TUBE DISPLAY
The PDP-3 Cathode Ray Tube Display is useful for presentation of graphical or tabular information to the operator. It uses a 16 inch round tube with magnetic deflection. For each In-Out transfer order, one point is displayed at the position indicated by the In-Out Register.
Bits 0-9 of the IO indicate the X coordinate of the position, and bits 18-27 indicate the Y coordinate. The display takes 60 microseconds.
An additional display option is a Light Pen. By use of this device the computer is signaled that the operator is interested in the last point displayed. Thus the program can take appropriate action such as changing the display or shifting operation to another program.
A smaller display is available. This display uses a five inch, high resolution cathode ray tube. The tube is equipped with a mounting bezel to accept a camera or photomultiplier device. The operation of this display is similar to that of the 16 inch, except that 12 bits are decoded for each axis.
REAL TIME CLOCK
A special input register may be connected to operate as a Real Time Clock. This is a counting register operated by a crystal controlled oscillator. The clock can be reset to zero by manual operation. A toggle switch interlock prevents an accidental reset. The state of this counter may be read at any time by the appropriate In-Out Transfer instruction.
LINE PRINTER
A 72 column Anelex printer and control are available as an option for PDP-3. The control contains a one line buffer. This buffer is cleared by the completion of an order to s.p.a.ce the paper one position (psp). The buffer is filled from the In-Out Register by a succession of 12 load buffer orders (plb). The first plb will put the six characters represented by C(IO) in the leading (left-hand) column positions of the buffer. After the buffer is loaded, the order, print (pnt), is given.
UTILITY PROGRAMS
FRAP-3 - The a.s.sembly Program
An a.s.sembler or compiler prepares a machine language tape suitable for direct interpretation by the computer from a program tape in operator language. Generally speaking, one statement accepted by FRAP produces one instruction for the machine. A single statement written for the PDP-3 compiler, DECAL-3, may cause several instructions to be written.
Thus, FRAP causes a 1 for 1 mapping of instructions for statements while DECAL may produce many instructions from one statement.
In addition to allowing program tapes to be prepared with off line equipment, an a.s.sembly program has other functions. Normally, the machine would require 36 bits or 12 octal digits to be written for each instruction used in the machine. FRAP allows mnemonic symbols to be used for the instructions. These mnemonic symbols aid the programmer by representing the instruction in an easily remembered form.
In addition to allowing mnemonic symbols to represent the instructions, variable length sequences of alphanumeric characters may be used to represent memory addresses in symbolic form. The a.s.sembly program does the address bookkeeping for the programmer. A short example of a FRAP program is on Page 29.
Since few characters limit or control the format of instructions written in FRAP-3 language, it is possible to write instructions in almost any format or style.
FRAP-3 may also be used to prepare tapes for interpretive programming, since arbitrary definitions for operation code symbols are permitted.
A feature useful both for ease of programming and for machine simulation is the ability to call for a series of instructions (macro-instruction) to be written. Frequently used instruction sequences thus need only to be defined once.
DECAL - The Compiler Program
DECAL-3 (Digital Equipment Compiler, a.s.sembler, and Linking loader for PDP-3) is an integrated programming system for PDP-3. It incorporates in one system all of the essential features of advanced a.s.semblers, compilers, and loaders.
DECAL is both an a.s.sembler and compiler. It combines the one-to-one translation facilities of an a.s.sembler, and the one-to-many translation facilities of a formula translation compiler. Problem oriented language statements may be freely intermixed with symbolic machine language instructions. A flexible loader is available to allow the specification of program location at load time. The programmer may specify that certain variables and constants are "systems" variables and constants.
The symbols so defined are universally used in a system of many routines. Thus, communications between parts of a major program is facilitated even though these parts may be compiled separately. Storage requirements for a large program are lessened by this technique.
DECAL is an open-ended programming system and can be modified without a detailed understanding of the internal operation. This is achieved by means of a recursive definition facility based on a skeleton compiler with a small set of logical capabilities. The skeleton compiler acts as a bootstrap for introducing more sophisticated facilities.
The compiler will be delivered with a fully defined subset of formula translation operators. Additional subsets may be defined by the user to best fit his source language.
FLOATING POINT SUBROUTINES
A set of subroutines are provided with the PDP-3 to perform floating point arithmetic. In these, the PDP-3 36 bit word is divided to form a 27 bit mantissa, a, and 9 bit exponent, b. Numbers, thus, appear in the form: k = ax2^b where, a, is considered to be in fractional form in the range 1/2 <= a="">< 1,="" and="" b="" is="" an="" integer,="" 0=""><= b="">< 29.="" this="" gives="" number,="" k,="" the="" range="" 10^{-76}="">< k=""><>
The subroutines are called with one operand in the acc.u.mulator. After the subroutine has been executed, the acc.u.mulator contains the answer.
Thus floating point numbers are essentially handled as regular logical works. The format of the number allows magnitude comparisons to be made by conventional arithmetic as bit 0 is the sign of the number, bits 1 to 9 the exponent, and the remaining 26 bits, together with the sign bit, the mantissa in ones complement arithmetic. The arithmetic subroutines are: add, subtract, multiply, divide, convert a floating point number to binary, convert a binary number to a floating number. Additional routines form: [square root of x], e^x, ln x, sine(~pi~/2)x, cos(~pi~/2)x, tan^{-1}x. There are also programs to convert between floating decimal numbers and PDP-3 floating numbers.
Routines which require two operands, e.g., add, subtract, multiply and divide, require an index register to specify the address of the second operand. An index register also specifies parameters in data conversions, e.g., the position of the binary point when converting a binary number to a standard floating number.
Using the floating point subroutines, additional routines may be written which handle complex floating numbers and vector and matrix algebra.