April 16th, 2020 ~ by admin

DEC M7260 KD11-A CPU: The PDP-11 Goes MSI

PDP-11/05 Front Panel (pic from vintage_electron)

Back in 1972 DEC released the ‘budget’ PDP-11/05 16-bit computer.  The original PDP-11/20 had been released 3 years before and its CPU (the KA11) was based on simple TTL, its ALU could perform adds and that was all, which meant its designers had to get creative in implementing the instruction set.  By 1972 however things had changed, there still was no 16-bit processors available but there was now single chip 4-bit ALU’s.  The ALU was the famous 74181 and formed the heart of the KD11-A, DEC’s 4th processor design (the ‘third’ was the KB11-A which was similar but based on the faster 74S181 and used in the PDP-11/45 and released at the same time) .

The KD11-A consisted of a pair of boards, the M7260 Data Path Module and the M7261 Control Logic and Microprogram Module.  All the processor functional components are contained on these modules. The M7260 Data Path Module contains: data path logic, processor status word logic, auxiliary arithmetic logic unit control, instruction register and decoding logic, and serial communications line interface. The M7261 Control Logic and Microprogram Module contains: internal address detecting logic, stack control logic, Unibus control logic, priority arbitration logic, Unibus drivers and receivers, microbranch logic, microprogram counter, control store logic, power fail logic, line clock, and processor clock.   The M7260 was he brain, and the M7261 told it what to do, containing the microcode to implement the PDP-11 instruction set.  This was the first version (with the 11/45) of the PDP-11 that was microcoded.

Fairchild 934159 74181 MSI 4-bit ALU made on a Bipolar – This example from very early 1971

The KD11-A ran off a single 150ns clock resulting in a raw clock speed of 6.67MHz, however performance was limited by memory access speed. The PDP-11/05 supported up to 32K Words (64KB) of core memory and this memory could only run at a 980ns cycle time.  This limited the 11/05 performance to around 1MHz.  This was still quite good for 1972!.

The 74181 was capable of running at 42MHz (and 90MHz for the 74S181 Schottky TTL versions) but in a set of 4 this drops to about 27MHz (with the carry generator taking some time).   Speed, however, is usually limited by other things rather then the ALU itself.   The 74181 ALU contains the equivalent of 62 logic gates (170 transistors) and can perform 16 different arithmetic and logic functions on a pair of 4-bit inputs.  Ken Shirriff did an excellent die level analysis of a ‘181 thats worth reading.  It includes pretty pictures even.

DEC M7260 – Data Path for the KD11-B CPU – Dated July 1972

This particular KD11-A board is one of the very first made.  It is dated July 20th 1972, a month after the initial release of the 11/05.  The big white chip is a General Instruments AY-5-1012 UART.  To its right you can see thr 4 74181 ALUs.  Each is 4-bit and together they form a complete 16-bit ALU for the CPU. A 74150 Multiplexer helps determine what data goes where.  The 74182 is the Look ahead carry generator for the ‘181’s.  Most of the rest of the chips on the board are ROMs and supporting logic.  There is also 4 Intel C3101A 35ns SRAM chips, these are 16×4 SRAMs used as scratch pad memories and only were used in the very first version of the CPU (later versions replaced them with cheaper 7489 TTL versions).  The Scratch Pad Memory is what forms the registers for the CPU.  There are 16 16-bit registers with the the first 6, R0-R5 being general purpose registers and the rest special purpose such as the Program Counter, Interrupt Vector, etc.

M7261 Control module – Contains the microcode for the CPU (pic from xlat.livejournal.com)

Another interesting point on this board is the very large amount of green wires running on the board.  These are called ECO wires, which are ‘Engineering Change Order’ wires, and are placed, by hand, after the board is made to correct faults in the board layout.  The goal is to not have these as they are expensive and delicate and can result in failures down the road, so further revisions of the board would have these fixed/implemented in the PCB.  You do not see these much at all any more as modern design/testing tools virtually eliminate the possibility of a faulty PCB layout making it into production.

When it was released the ~1MHz 11/05 cost $25,000, which in 2020 US Dollars is around $154,000.  THe PDP-11 series ended up being one of the most popular minicomputers, selling over 600,000 units over the years.  Later versions like the LSI-11 series moved the entire CPU to a single LSI chip, adding Extended Instructions, Floating Point Instructions, faster memories and other performance enhancements well into the 1980’s.   It was also widely comied, and enhanced in the Soviet Union and Russia.  It was on a Soviet PDP-11 clone that Tetris was developed, a game we are all rather familiar with.

Its amazing to see where computers have come in the span of but a few decades. but these important parts of history continue to be used.  Perhaps not the 11/05, but there are many PDP-11 systems still working away, typically inindustrial environments, ironically helping produce things likely far more advanced then themselves.

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CPU of the Day

November 22nd, 2017 ~ by admin

CPU of the Day: DEC LSI-11 Chipset

LSI-11 Chipset with EIS/FIS Chip – 1976-1977

Back in 2014 we discussed the Western Digital WD/9000 Pascal Microcomputer system.  Today we’ll look at the LSI-11 chip set, the basis of the Pascal.  Back in 1974 DEC (Digital Equipment Corporation) contracted Western Digital to design and build a 16-bit chipset to emulate the Bipolar PDP-11/05 Minicomputer.  Western Digital was paid $6.3 million for the work, and would be allowed to market and sell the resulting chipset themselves, as well as grant license to it to others (including DEC).

The LSI-11 was to be a 16-bit chipset, but was based around a 8-bit Data chips (the 1611).  The 1611 has an 8-bit ALU , 26 8-bit registers and a microinstruction register.  This is controlled by the 1621 control chip, which interprets macroinstructions from handles all the timing, as well as interrupts.  The 1621 control chip is what allows the 8-bit 1611 to be used as a 16-bit processor.  The chips are connected by an 18-bit  microinstruction bus, and a 16-bit address/data bus handles access to the rest of the system (memory/I/O).  Each microm is a 512 Word by 22-bit ROM, which can hold 80 instructions.  It is these MICROMs that allow the WD MCP1600 to function as a PDP-11/05.  The instructions in the the MICROMs (2 are required for the LSI-11) emulate the PDP-11 instructions.

DEC M7264 LSI-11 KD11-L Board from PDP- 11/03

First production of the LSI-11 chipset began in March of 1975 with shipments commencing that year.  The PDP-11/03 based on this chipset was released later that year.  The KD-11 M7264 board formed the hear of the 11/03 (as well as other DEC systems).  In typical DEC fashion it came in many flavors with different amounts of memory, as well as different instruction support.  This was completely due to the design of the LSI-11 chipset and its MICROMs .  The basic LSI-11 needs 2 MICROMs to handle the basic PDP-11 instructions, the chipset however supported 4.  This means that more instructions could be added.  One of the most common and useful additions was the EIS/FIS (Extended Instruction Set/Floating Point Instruction Set) microm.  This added 8 more instructions including MUL, DIV, FADD, FSUB, FMUL, FDIV and 2 register shifts (ASH, ASHC).  Adding the EIS/FIS chip to a standard KD-11-F board turned it into a KD-11-L (like the one pictured).

Western Digital 1611 Die –
Pauli Rautakorpi

There were other MICROMs available as well.  This included a set of 2 for support of DIBOL (Digital Business Oriented Language), a DEC language similar to COBOL.  Since the DIBOL chipset needed 2 chips a system could support DIBOL, OR EIS/FIS but not both.  MICROMs were revised as bugs were found, or faster ways of handing an instruction were made.  MICROMs revisions could also be made to support different PCB revisions.  In some ways they played the part of firmware to the PCB, as well as the instruction set for the processor.  In this way many MICROMs are specific to PCB etch revisions and other revisions of the system outside of the processor itself.  Matching the correct MICROMs, as well as Control and Data chips to the correct board is a bt of a task, and take several dozen pages of the LSI-11 maintenance manual.

Here are a few part #s to help sort things out

Data Chip
DEC 1611
Control Chip
DEC 2007C
MICROM 1 3010D/A
MICROM 2 3007D
21-11549-01 23-008B5-00 STD INST 1
21-15579-00 (1611H) 23-003C4-00 23-007B5-00 STD INST 2
21-16890-00 (1611H) 23-002C4-00 23-003B5-00 EIS/FIS
23-001C3 CP1621B14 23-009B5-00 EIS/FIS
23-001C2-01 CP1621B451 23-001B5-00 CP1631B103 STD INST 1
23-002B5 CP1631B073 STD INST 2
 23-091A5-01 CP1631B153 EIS/FIS
23-004B5 DIBOL 1
23-005B5 DIBOL 2
23-008A5-01 CP1631B-10 STD INST 1
23-007A5-01 CP1631B-07 STD INST 2

DEC M7270 LSI-11 – 1982 – All WD Chips

There are more to be found as DEC and Western Digital made many versions.  In early 1976 Western Digital licensed the MCP1600 chipset design to National Semiconductor, in exchange for some RAM technology licensing.  It is unclear if National actually made any of the MCP1600 chipset.  By 1977 DEC had started to produce the LSI-11 chip itself while continuing to source parts from Western DIgital as well.  It is common to see LSI-11 boards with DEC and WD chips mixed well into 1982.

The popularity of the PDP-11 in the 1970’s resulted in many customers for the LSI-11 based PDP’s, and their use continued well into the 1990’s with many systems continuing to be used today.  As with many such systems, they found use in industrial control and automation, where they continue to work.