National Semiconductor PACE CPU family

National Semiconductor PACE, abbreviated from "Processing And Control Element", was the first 16-bit microprocessor. The PACE family was announced at the end of 1974, at about the same time when Intel and Motorola released such popular 8-bit microprocessors as 8080 and MC6800. PACE CPUs run at frequencies up to 2 MHz, have 16-bit address and data buses, can address up to 64 KB memory, and provide 5 maskable interrupts and one non-maskable interrupt. Internally the processors have four 16-bit general registers, which can be used for arithmetic and logical operations, and as index registers. PACE instruction set includes all basic types of operations - data transfer, arithmetic, logical, jumps and branches (including SKIP instructions that conditionally skip the next instruction), and subroutine calls. The PACE doesn't have special 8- and 16-bit versions of instructions. Instead, a program can set or clear a special BYTE status flag, that instructs the CPU to operate on 8- or 16-bit data respectively. To support subroutine calls and interrupt handling, the microprocessor incorporates 10-level stack. If enabled, the CPU may generate an interrupt every time when the stack becomes full or empty, which allows user programs to extend stack size by swapping stack data to and from main memory. While the CPU may operate at frequencies up to 2 MHz, it requires on average 10 microseconds to execute one instruction. This is much slower than speed, in instructions per second, of the above mentioned 8080 and 6800 microprocessors. PACE microprocessor was built on P-channel MOS technology. National Semiconductor eventually released NMOS version of the PACE CPU - INS8900.

National Semiconductor PACE microprocessors were manufactured in 40-pin ceramic package. There were no second-source manufacturers of this microprocessor.