512 THE PDP-l0 FAMILY
II. Good Human Engineering Throughout Systems
a. An executive command language interpreter which provides direct access to a large variety of small, commonly used system functions, and access to and control over all other subsystems and user programs. Command language forms should be extremely versatile, adapting to the skill and experience of the user.
b. Terminal interface design should facilitate intimate interaction between program and user, provide extensive interrupt capability, and full ASCII character set.
c. Virtual machine functions should provide all necessary options, with reasonable default values simplifying common cases, and require no system-created objects to be placed in the user address space.
d. The system should encourage and facilitate cooperation among users as well as provide protection against undesired interaction.
III. The System must be Implementable, Maintainable, and Modifiable
a. Software must be modular with well defined interfaces and with provision for adding or changing modules clearly considered.
b. Software must be debuggable and reliable, allowing use of available debugging aids and including internal redundancy checks.
c. System should run efficiently, allow dynamic manual adjustment of service if desired, and allow extensive reconfiguration without reassembly.
d. System should contain instrumentation to clearly indicate performance.
Dan Murphy (one of Tenex's designers/ implementers) came to DEC and led the architecture and development effort that produced TOPS-20. The effort at DEC has been to increase the performance of TOPS-20 to be competitive with the highly tuned Monitor while not losing its generality. The TOPS-20 structure does provide increased reliability and modifiability.
HARDWARE IMPLEMENTATION
While logic and memory technology are often considered the prime determinant of the performance and cost of a computer system, fabrication and packaging technology are equally important. This section surveys logic, manufacturing, and packaging technology as it affected the various DECsystem-10 models. Table 7 summarizes those various logic and packaging technologies.
Logic
The PDP-6 used a set of logic modules that evolved from the earlier PDP-1, which in turn were derived from the Lincoln Laboratory circuits developed for the TX-0 [Mitchell, Olsen, 1956] and TX-2 [Clark, 1957] (Chapter 4) computers as part of the air defense program. These circuits were direct-coupled transistor logic and included both series and parallel transistor circuits to give great flexibility in designs. The PDP-1 circuits operated at a 5 MHz clock, and new transistors enabled the PDP-6 circuits to operate at 10 MHz. The computer's clock was based on a delay line which carried pulses generated by a pulse amplifier using pulse transformers (this too came from Lincoln Laboratory via the early work at MIT on radar and pulse transformers) The pulses were used for register transfer operations (i.e., moving data among the registers) and some logic gating.