Single Cycle Switch — When the Single Cycle Switch is on, the computer will halt at the completion of each memory cycle. This switch is particularly useful in debugging programs. Repeated operation of the Continue Switch button will step the program one cycle at a time. The programmer is thus able to examine the machine states at each step.
Test Switch — When the Test Switch is on, the computer will perform the instruction indicated in the Test Address location. It will repeat this instruction either at the normal speed rate or at a single cycle rate if the Single Cycle Switch is up. This switch is primarily useful for maintenance purposes.
Sense Switches — There are six switches on the Console which are present for manual intervention.
STORAGE
The internal Memory System for PDP-3 consists of modules of 4096 words of coincident current magnetic core storage. Each word has 36 bits. The memory modules operate with a read-rewrite cycle time of 5 microseconds. The driving currents of the memory are automatically adjusted to compensate for normal room temperature variations.
Each core memory module consists of the memory stack, the required X and Y switches, the X and Y current sources and sense amplifiers for that stack.
The Memory Address Register, the Memory Buffer Register, and the Memory Timing Controls are considered to be part of the Central Processor. The standard PDP-3 Memory Address Register configuration is built to allow up to 8 modules of core memory (32,768 words). There is a space in the addressing section of the machine to allow expansion of the addressing by a factor of eight for a total addressing capacity of 262,144 memory registers.
The Core Memory may be supplemented by Magnetic Tape Storage. This is described under Input-Output.