1802. Unfortunately, in November AMS was purchased by Intersil, who had their own Intersil 6100, a single-chip version of the PDP-8 mini. Intersil dropped production of the 2650. Signetics tried again with National Semiconductor in 1977, who planned to introduce versions in the last quarter of the year. For unknown reasons, this appears to have never happened, and only a single example of an NS version, from France, has ever been found. Signetics continued the development of the 2650, introducing two new models in 1977. The 2650A was a reworked version of the original layout intended to improve yield, and thus reduce cost. Speed remained unchanged at 1.25 MHz for the base model and 2 MHz for the -1 versions. The 2650B was based on the A, added a number of new instructions, and improved the performance of a number of existing instructions. Description The overall design of the 2650 was based on the IBM 1130. As such, the 2650 has a number of features that were common on 1960s minicomputers, but rarely found on newly designed microprocessors of the 1970s. Among these, for instance, were status bits that were used to track the status of input/output devices, which makes it simpler to write interfacing code. Another mini-like feature was its use of vectored interrupts, which allowed devices to call the correct interrupt handler code by putting its memory location on the data bus and then forcing an interrupt. This avoids the need to write a centralized interrupt handler that reads additional data from the bus, determines which device driver is being invoked and then calls it; the 2650 can jump directly to the correct code, potentially stored on the device itself. Like the 1130, the 2650's processor registers were divided into sets, with a single global register R0 used as the accumulator, and two sets of three index registers, both named R1, R2 and R3, for a total of seven registers. At any one time, one of the two sets of indexes were visible to the CPU. Which set was visible was controlled by a bit in the status register, PSW. One could easily switch between the two sets of registers with a single instruction. This allowed rapid switching of values during subroutine calls, operating system switches, or handling interrupts. Unlike the 1130, the registers were only 8-bit wide rather than 16-bit, and there were only two sets rather