Signetics 8X300

It was designed to be a fast microcontroller and signal processor, and because of this differs considerably from conventional NMOS logic microprocessors of the time.

Perhaps the major difference was that it was implemented with bipolar Schottky transistor technology, and could fetch, decode and execute an instruction in only 250 ns.

An external pass transistor is required to complete an on-chip voltage regulator, which supplies 3V to selected areas of the chip.

Two control signals, WC (write command) and SC (select command), determine the state of the IV bus as follows: A further two signals, LB (left bank select) and RB (right bank select), effectively double the IV bus address space and were most often used to switch between RAM memory in one bank and I/O ports in the other.

Another unusual feature is that rather than execute mask, rotate, shift and merge instructions in the arithmetic logic unit (ALU), as is the case with most microprocessors, the 8X300 has separate mask, rotate, shift and merge units.

Data can therefore be rotated, masked, modified, shifted and merged (in that order), all in one instruction cycle.

The following combinations are possible, depending on the source and destination: Transferring data to and from the 8X300 is a two-step process: Because the I/O address is output separately, the I/O ports must hold (latch) the selection.

In an extensive application note, Signetics showed how to use the 8X300 as a floppy disk controller.