Signetics 2650

Signetics 2650 introductory ad, October 30, 1975
Signetics 2650AN
PC1001 evaluation board
Signetics 2650 chip magnified

The Signetics 2650 was an 8-bit microprocessor introduced in mid-1975.[1] According to Adam Osborne's book An Introduction to Microprocessors Vol 2: Some Real Products, it was "the most minicomputer-like" of the microprocessors available at the time.

Signetics sold development boards e.g. the PC1001[2][3] and its successor, the PC1500 "Adaptable Board Computer", ranging in price from A$165 to A$400. The chip by itself sold for around A$20. Several hardware construction projects and programming articles were published in magazines such as Electronics Australia and Elektor and related kits were sold by electronics stores. These factors led to its use by a number of hobbyists in many countries such as Australia, U.S.A.,[4] United Kingdom, the Netherlands[5] and Germany.[6]

The chip contained seven 8-bit general purpose registers, although only four were visible at any time. It was limited to a 15-bit address space (thereby addressing a maximum of 32 KB of memory), since the upper bit of a 16-bit memory reference was reserved to indicate that the indirect memory addressing mode was to be used (a minicomputer-like feature).

The address space was further limited by the use of another two bits of the address to indicate the indexing mode for all logical and arithmetic (i.e. non branch) instructions. This divided the address space into four 8 KB 'pages', only the current (i.e. the page the program was running in) could be addressed directly, the others only by indirect addressing.

While there were nine different addressing modes, the lack of any 16-bit registers and the 13–15-bit address space prevented widespread use. Despite this, an operating system ("2650 DOS") was available, along with 8 KB and 12 KB BASIC interpreters (sold by Central Data Corporation USA), and many games of the Hunt the Wumpus style. Most programs were written in assembly language.

Two types of video game console used the Signetics 2650 or 2650A. The first group of consoles are based on the video display controller 2636 Programmable Video Interface; the 1292 Advanced Programmable Video System and the closely related Interton VC4000 belong to this group. Those were released in Germany in 1976 and 1978 respectively. The second group of consoles were based on the Signetics 2637 as a video display controller; Emerson Arcadia 2001 which was released in 1982 and which used a Signetics 2650 running at 3.58 MHz as a CPU belong to this group together with many other ones software-compatible (Leonardo, Hanimex MPT-03 etc.).

At least four coin-operated video games were released in the 70s which used the 2650 CPU – Atari QUIZ SHOW, Meadows Games 3D BOWLING, Meadows Games GYPSY JUGGLER, and Cinematronics EMBARGO.

The processor was also used in the Signetics Instructor 50, which was a small computer designed to teach the use and programming of the Signetics 2650 CPU.

The 2650 was also used in some large items of equipment such as the Tektronix 8540, a microprocessor software development system which supported various in-circuit emulator, trace memory and logic analyser cards for real-time debugging of microprocessor systems, as practiced in the 1980s. The 2650 provided the base operating system functions, data transfer, and interface to a host computer or serial computer terminal.

The processor was most suited as a microcontroller, due to its extensive I/O support:

Industrial Microcomputer System – IMS

Philips IMS 2650 Eurocard computer system

For a short time starting 1979, Philips sold a modular 2650 computer called the 'IMS' – Industrial Microcomputer System,[7] based on the Eurocard format in a 19" rack. It included CPU, PROM, RAM, input, output and teletype modules. This system was meant as a more intelligent programmable logic controller. For development, they later added DEBUG, DISPLAY, INTERRUPT and MODEST ((E)PROM programmer) modules.

Instruction set

Although the 2650 is basically an 8-bit microprocessor, 64 opcodes are actually 9-bit, and another 32 opcodes are 11-bit (using bits in the address field). Of the remaining 128 8-bit opcodes, 124 (126 in the 2650B) are implemented, giving a total of 444 (446) instructions.

Many more instructions are available as the behavior of the standard instructions can be modified by setting or clearing status bits: WC (with or without carry) and COM (logical or arithmetic compare). This doubled the number of rotate, add, subtract and compare instructions.

The instruction set is strongly orthogonal: all logic and arithmetic instructions can use all nine addressing modes:

The most significant bit of all relative and absolute addresses is used to indicate indirection.

The only exceptions are where the opcodes of meaningless operations are used for other purposes:

Although the instruction LOAD register zero with register zero would appear meaningless, and was officially unsupported, it did set the condition code and was often used to determine the status of this register.

Indexing

With all arithmetic and logical instructions using absolute (direct) addressing, bits 14 and 13 of the address field are used to indicate the indexing mode as follows:

When indexing is specified, the register defined in the instruction becomes the index register, and the source/destination is implicitly Register zero. For indirect indexing, Post indexing is used, i.e. the indirect address is first fetched from memory and then the index is added to it.

Branching

Probably the most mini-computer like aspect of the 2650 is the enormous number (62) of branch (jump) instructions; all these instructions could also use indirection:

Highly unusual are the conditional branch to, and return from, a subroutine:

Only the branch instructions using absolute addressing used all 15 bits of the address field as address. Using such a branch instruction was, therefore, the only way to set the two bits in the page register (controlling bits 14 and 13 of the address bus) and changing the current 8 KB page.

Versions

The 2650B had the following changes and improvements over the 2650A:[8]

Second sources

In 1975, Signetics was sold to Philips and the 2650 was later incorporated into the Philips Semiconductors line. They made a version of the 2650 called the MAB2650A. Valvo, a subsidiary of Philips, sold the 2650 in Germany. Valvo also sold the VA200 single board (Eurocard) 2650 computer with 4 KB PROM/EPROM, 1 KB RAM and four I/O ports.[9]

The most well known licensed copy of the Signetics/Philips 2650 was the 2650-P-02 from Synertek.

Other producers of licensed copies of the chip were Harris and Intersil.

Peripheral chips

The 2650 came with a full complement of peripheral chips:

Many of these peripheral chips were designed so they could also be used with other microprocessors, for example the datasheet of the 2672 suggests using it with an Intel 8048 microcontroller.

Philips Technical Note 083 describes how to interface the 2651 PCI to various other microprocessors, such as the 8080, 8085, Z80, 8048 and 6800

Descendants of the 2651/2661 serial communications chips are still sold as the Philips SC26 series.

2656 System Memory Interface[10]

The 2656 was specifically designed to augment, and interface with, the 2650 and make a 2-chip computer possible. It contained everything the 2650 lacked to make a complete computer:

The I/O pins could be used as an 8-bit I/O port or programmed to generate enable signals for extra RAM, ROM or I/O ports. This was achieved by mask-programming a Programmable Logic Array in the 2656.

To develop and test the design before committing it to production, Philips sold the PC4000, a 2656 emulator board using PROM'S and FPLA's to emulate the ROM and PLA in the 2656.

References

  1. ↑ Microcomputer Digest Vol. 2 No. 1 July 1975
  2. ↑ Signetics Technical Note SP50; 2650 evaluation printed circuit board level system PC1001
  3. ↑ Signetics Technical Note SS50; PC1001 monitor program "PIPBUG"
  4. ↑ Build a 2650 Microcomputer system, Radio Electronics magazine: April, May, June 1977
  5. ↑ Hobby Computer Club (HCC) 2650 user group
  6. ↑ Programmierbeispiele mit dem Mikroprozessor 2650, Johann Hatzenbichler, 1978 OCLC 74475572
  7. ↑ Industrial Microcomputer System; System Specification, Philips Electronic Components and materials, 1980
  8. ↑ Philips 2650 Series microprocessor short-form manual 02-1979; 9398 209 50011
  9. ↑ VALVO VA 200 Mikrocomputer im Europa-Format: VALVO Applikationslaboratorium Marz 1978
  10. ↑ 2650PC-4000 memory interface emulator using PROM's and FPLA's

External links

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