The Signetics 2650 processor has always been described as ‘very mini-computer like’ and for good reason, it truly is very minicomputer like in design. It is an 8-bit processor released in July of 1975 made on an NMOS process. The 2650 has a 15-bit address bus (the upper bit (16) is reserved for specifying indirect addressing) allowing addressing of up to 32K of memory. It has 7 registers, R0, which is used as an accumulator, as well as 2 banks of 3 8-bit registers accessed. The 2650 supports 8 different addressing modes, including direct, and indirect with autoincrement/decrement. Its clearly a mini-computer design and there is a reason for that, it was based on one.
The 2650 is very closely based on the IBM 1130 mini-computer released in 1965. Both use 15-bit addressing, many addressing modes, and a set of 3 registers (Signetics added support for 2 banks of 3, The Signetics 2650 is often noted for its novel use of a 16-bit PSW status register, but this too is from the 1130, which used a 16-bit Device Status Register for talking with various I/O components. So why would Signetics base a processor released in 1975 on a 1965 mini-computer?
Because the 2650 was not designed long before it was released. J. Kessler was hired by Signetics in 1972 in part to help design an 8-bit processor. Kessler was hired by Jack Curtis, (Of Write Only Memory fame) from…IBM. Kessler designed the architecture very similar to the IBM 1130 and Kent Andreas did the silicon layout. The design contains 576 bits of ROM (microcode mainly), ~250 bits of RAM (for registers, stack, etc) and about 900 gates for logic. Clock speed was 1.25MHz (2MHz on the -1 version) on a ion implanted NMOS process, very good for 1972 (this was as fast as the fastest IBM 1130 made), but Signetics was tied up working with Dolby Labs on audio products (noise canceling etc) and didn’t have the resources (or perhaps the desire) to do both, so the 2650 was pushed back to 1975. In 1972 the IBM 1130 it was inspired by was still being made. If the 2650 had been released in 1972 it would have had the Intel 4004 and 8008 as competition, both of which were not easy to use, and had complex power supply and clocking requirements. The 2650 needed a 5V supply, and a simple TTL single phase clock.
Signetics troubles continued with the 2650, not the chip itself, which worked well and was well supported with dev systems by Signetics, but from a marketing side. In March of 1976 Signetics reached a second source agreement with AMS (Advanced Memory Systems Inc.) to make the 2650 and the in development revised 2650A. AMS was already second sourcing the CMOS 1802 from RCA so the NMOS 2650 was seen as a good fit, and in the 1970’s having a reliable second source was very important. In November of 1976, however, AMS was acquired by Intersil, who had the 6100 processor, the 2650 was deemed superfluous and AMS/Intersil chose not to make it. Signetics tried again in 1977 with National Semiconductor. National planned to begin making 2650 series processors in 4Q of 1977. It is likely National never made any however, as none have ever been seen, and only a single (French) reference to the INS2650 is seen after 1978.
In 1977 Signetics released 2 new versions of the 2650. The 2650A was a mask rework (and likely shrink) to improve yields and device operating parameters. Speed was unchanged at 1.25MHz and 2MHz for the -1 speed grade. Also released was the 2650B, this contained the same mask changes as well as a number of enhancements to the processor architecture itself. Four changes were made to make the 2650 more user friendly:
- Pin-Out: 2650B and 2650A differ in 2 pin functions In 2650B pin 15 becomes bus enable and pin 25 becomes cycle last
- Instruction Set: 2 new instructions are added LDPL, and STPL (Load/Store Program Status Word Low byte). THese make handling interrupts much easier.
- Program Status word upper: PSU bits 3 and 4 are settable, testable user flags in the 2650B vs set at 0 for the 2650A, making the PSW register even more helpful
- Instruction Execution Time: LODZ, SUBZ, COMZ, STRZ, IORZ, ADDZ, ANDZ and EORZ now execute in 1 cycle instead of 2. These may greatly enhance performance as they all are Register Zero (Accumulator) based.
Despite these changes the 2650 had limited market success. The 2650A found its way into several video game applications and some industrial applications. The 2650B, while a nice enhancement appears to have seen little to no adoption. The 2650B was dropped by the mid-1980’s and Signetics (and Philips in Europe as the MAB2650) stopped actively marketing the 2650A (though still produced it) at around the same time. Devices continued to be up until at least the early 1990’s to fill replacement needs etc, but the 2650 was essentially ‘dead’ by 1985.
Had the 2650 been released closer to when it was actually designed the story may have been a lot different. In 1972 there was only a handful of processors, by 1975 there was dozens of types available. Signetics tried again in the late 70’s with another mini-computer based chip, The SPC-16/10, a chip that is even less common then the 2650, but at least made it to space.