PDP-11/03

Introduction

The PDP-11/03 was introduced in 1975 and heralded a new generation of PDP-11 systems based on the QBus. The 11/03 was first known as the LSI-11 (Large Scale Integration) and was DEC's first microprocessor based PDP-11 architecture, despite it being implemented over four 40 pin chips. A fifth 40 pin DIP slot was left blank so users could add chips that implemented application specific instruction sets. The LSI-11's chips were manufactured by Western Digital.

The CPU microcode includes a debugger: firmware with a direct serial interface (RS-232 or current loop) to a terminal. This lets the operator perform debug functions by typing commands and reading octal numbers, rather than operating switches and reading lights, the typical debugging method at the time. The operator can examine and modify the computer's registers, memory, and input/output devices, diagnosing and correcting failures in software and peripherals (unless a failure disables the microcode itself). The operator can also specify which device to boot from.

The quad width LSI-11 board featured 4K of on-board memory, although the KD-11 processors could support up to 32KW of memory. A dual width card was also available and appropriately named the LSI-11/2; half the width of the LSI-11. The processor has a 16-bit address bus and a 16-bit data bus. It can address up to 64 KB (32 KW) of memory and reserves the top 8 KB (4 KW) of the address space for I/O devices. This leaves up to 56 KB (28 KW) of memory for programs and data. Some configurations allow the I/O area to be reduce to 4 KB, allowing up to 60 KB (30 KW) of memory for programs.

QBus

The QBus differs from the Unibus primarily in that addresses and data are multiplexed onto a shared set of lines rather than having separate sets of lines. It also differs slightly in how it addresses I/O devices and it eventually allowed a 22-bit physical address (whereas the Unibus only allows an 18-bit physical address) and block-mode operations for significantly improved bandwidth (which the Unibus does not support). Whereas the Unibus backplane was Hex width, the QBus backplanes were only Quad width. Therefore all QBus modules were either double or quad width.

These innovations increased reliability and decreased the cost of LSI-11 systems. Later Q-Bus based systems such as the LSI-11/23, /73, and /83 used chip sets designed in house by Digital Equipment Corporation. Later PDP-11 Unibus systems were designed to use similar Q-Bus processor cards, using a Unibus adapter to support existing Unibus peripherals, sometimes with a special memory bus for improved speed.

LSI-11 (KD11) Models

There were several models of the Quad and Dual width KD-11 processors. Often the difference in models was the addition of the extended instruction sets, otherwise most models were physically similar. There w

Model	Module ID	On-Board Memory	Date Introduced	Module Width
KD11-F	M7264	4KW	1975	Quad
KD11-L	M7264	4KW	1975	Quad
KD11-P	M7264-BB	4KW	1977	Quad
KD11-H	M7264-YA	none	1976	Quad
KD11-N	M7264-YA	none	1976	Quad
KD11-Q	M7264-YB	none	1977	Quad
KD11-H	M7264-YC	none	1980	Quad
KD11-N	M7264-YC	none	1980	Quad
KD11-HA	M7270	none	1977	Dual
KD11-NA	M7270	none	1977	Dual