This handbook is a complete guide to micro programming for the Hewlett-Packward 2100 Computer. With the information given here, you will be able to expand the already powerful capability of your 2100 by adding custom-tailored instructions to the existing set of micro programmed operations. This capability of expanding the Central Processing Unit, in addition to the extraordinary expansion features of the memory and I/O sections, contributes to the total flexibility and unusual adaptability of the 2100. Essentially, this handbook is self-contained. Micro-assembler documentation is required in order to format and assemble micro-programs correctly.