Last updated June 22 2018. Written by Herb Johnson, (c) Herb Johnson, except for content quoted extensively from others. This is a one of a series of Web pages supporting the COSMAC 1802 microprocessor, as used in the 1802 Membership Card kit. Go to the linked Web page for details.

In the ZIP file

The UT20 ROM ZIP file at this Web linkcontains UT20 source typed, assembled, verified by Herb Johnson March 2014


Dave Ruske contacted me in Aug 2015 and said "Steve Gemeny provided me with a PDF of the "Operator Manual for the RCA COSMAC Development System II CDP18S005 [MPM-216]" and asked that I pass along the UT20 listing from that manual." The UT20 ROM monitor is listed in Appendix G. In Aug-Sept 2015, I OCR'ed that source and hand-edited it. I made some modifications to assemble under the A18 1802 cross assembler and the assembled result matches the hex codes in the manual's listing.

Some time later, Dave made the MPM-216 manual available on the "cosmacelf.com" Web site. Look there for a PDF copy. Here's a PDF exerpt of the UT20 description but there's more information for use of UT20 in the manual.

UT20 is similar in use to UT4, and has the same user-program entry addresses for serial in and out, timer and so on. But UT20 has more commands and code to deal with floppy disk and cassette tape files. Also, UT20 serial output is bit 0 of the OUT 7 instruction; UT4 uses Q for serial output. Both UT20 and UT4 use EF4 for serial input.

- Herb Johnson

Other Related Web pages

I have some dates for and documents about 1802 development on this Web page.

Summaries and links to 1973, 1974, 1975 RCA Research Reports

The A18 MS-DOS 16 bit/32-bit cross assembler I used.

Lee Hart's IDIOT monitor, roughly based on the UT4 monitor and with similar features.

UT4 ROM monitor

- Herb Johnson

Contact information:
Herb Johnson
New Jersey, USA
To email @ me, see
see my home Web page.

This page and edited content is copyright Herb Johnson (c) 2018. Copyright of other contents beyond brief quotes, is held by those authors. Contact Herb at www.retrotechnology.com, an email address is available on that page..