Understanding the Addressing Capabilities of PDP-11 Systems

When discussing the architecture and capabilities of the PDP-11, one often encounters questions about its address space and the types of interfaces it supported. This article aims to clarify common misconceptions and provide a comprehensive overview of the addressing capabilities of these systems, particularly focusing on their physical address space and input/output (I/O) interfaces.

Introduction to the PDP-11

The PDP-11 is a series of 16-bit minicomputers developed by Digital Equipment Corporation (DEC) in the early 1970s. Renowned for its innovative features and reliability, the PDP-11 played a significant role in the development of modern computer architectures. Despite its legacy, questions often arise regarding the maximum physical address it can handle and the types of I/O interfaces it supports.

Addressing in PDP-11 Systems

The addressing capabilities of the PDP-11 were carefully designed to support both 16-bit and 22-bit address spaces, which is a critical aspect of understanding their functionality. Prior to the PDP-11/70 model, earlier versions of the PDP-11 used a 16-bit address space, providing 64 KB of addressable memory. This was considered adequate for many applications of the time, especially given the limited hardware constraints and software requirements.

16-Bit Address Space

The 16-bit address space, which was commonly used in early PDP-11 models, allowed for addressing 65,536 (2^16) memory locations. This limitation was practical for many small and medium-sized applications but posed challenges for more complex systems requiring larger address spaces.

22-Bit Address Space in the PDP-11/70

A significant improvement over earlier models was introduced with the PDP-11/70, which supports a 22-bit address space. This enhancement greatly expanded the physical addressable memory from 64 KB to 4 MB, significantly increasing the system's capabilities.

Input/Output Interfaces

Regarding input/output (I/O) interfaces, the PDP-11 was equipped with various hardware options that allowed for flexibility in connecting different peripheral devices. However, it is worth noting that the D9 serial port, commonly found in earlier models, was not designed to support Ethernet connectivity. For connecting to Ethernet networks, additional I/O expansion boards or external network interfaces were required.

Serial Ports

The D9 serial port, which was a popular feature on PDP-11 systems, provided serial communication capabilities. However, it was limited to basic serial communication standards and did not support Ethernet or other high-speed networking protocols. The inclusion of the D9 port in a given PDP-11 system depended on the specific model and configuration.

Ethernet Interfaces

For systems requiring Ethernet connectivity, DEC introduced various I/O expansion boards and network interface cards (NICs) to support Ethernet communications. The PDP-11/70 and later models, in particular, featured expandable I/O capabilities, allowing for the addition of Ethernet interfaces through appropriate hardware upgrades.

Conclusion

In summary, the PDP-11 series of computers offered a range of addressing capabilities, from the initial 16-bit design used in earlier models to the more advanced 22-bit address space supported by the PDP-11/70. The input/output interfaces were designed to be flexible, with options for serial communication and later support for Ethernet networking. Understanding these capabilities is essential for anyone studying the architecture and history of early minicomputers.

Keywords: PDP-11, Addressing, Computer Architecture