National Semiconductor INS8900 CPU family

National Semiconductor INS8900 is an NMOS version of 16-bit PACE microprocessor family. Released more than two years after the PACE family, the INS8900 offers nearly the same functionality and performance, and has the same architecture as PACE microprocessors:

• direct addressing of 64 KB of memory,
• instruction set allows many basic operations: data load/store, arithmetic and logical calculations, subroutine calls, conditional and unconditional jumps/branches, and miscellaneous operations.
• four general-purpose accumulators that can be used for 16-bit arithmetic and logical operations, and as index registers,
• six-level interrupt support,
• 10-level on-chip stack.

The INS8900 includes one new instruction: NOP (no operation). The microprocessor operates at the same frequency as the PACE CPUs, and requires the same number of machine cycles to execute instructions. The fastest INS8900 instruction can be executed in 4 machine cycles, and require 1 read cycle. Since each machine cycle takes 4 clock cycles, it takes at least 17 clock cycles to execute one instruction. INS8900 CPU, running at 2 MHz, has 0.5 microsecond cycle, and thus can execute at most 117,600 instructions per second. That is much slower than instruction per second rate of many popular 8-bit processors of the time - Intel 8080, MOS 6502, or Motorola 6800. Then again, only half of INS8900 instructions require more than 4 machine cycles, and the processor may perform twice as much work as 8-bit microprocessors during each operation.

National Semiconductor INS8900 processors were fabricated in 40-pin ceramic package. There were no second-source manufacturers of this microprocessor.