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 space 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 Assembly Program

An assembler 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 assembly 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
f