WD16
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The WD16 is a 16-bit microprocessor introduced by Western Digital in October 1976. It is based on the MCP-1600 chipset, a general-purpose design that was also used to implement the DEC LSI-11 low-end minicomputer and the Pascal MicroEngine processor. The three systems differed primarily in their microcode, giving each system a unique instruction set architecture (ISA).
WD16 microprocessor in five 40-pin packages installed on an Alpha Microsystems AM-100 CPU board which connects to a S-100 bus.
Microcode
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In processor design, microcode serves as an intermediary layer situated between the central processing unit (CPU) hardware and the programmer-visible instruction set architecture of a computer, also known as its machine code. It consists of a set of hardware-level instructions that implement the higher-level machine code instructions or control internal finite-state machine sequencing in many digital processing components. While microcode is utilized in general-purpose CPUs in contemporary desktops, it also functions as a fallback path for scenarios that the faster hardwired control unit is unable to manage.
The microcode (and "nanocode") of the Motorola 68000 is stored in the two large square blocks in the upper right and the controlled by circuitry to the right of it. It takes up a significant amount of the total chip surface.