Last updated Aug 13 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 UT4 ROM ZIP file at this Web linkcontains UT4 source typed, verified by Herb Johnson in 2016 and Robert Coward July 2018, assembled July 6 2018. There's also a PDF of the RCA manual and source listing of UT4, and an OCR'ed manual for UT4, as described below.


In April 2013, Bill Degnan posted on his site and in various public vintage-computing groups, about his recently-acquired RCA COSMAC Microkit. This was a 1974-75 product of RCA for development of their new COSMAC microprocessor, later called the COSMAC 1802. Most of these kits were used internally to RCA; this kit came from an RCA engineer. Bill "dumped" the two 1702 ROM contents and asked around for assistance.

In Nov 2013, I examined the ROM dump, inverted the data as 1702's are used that way, and disassembled the 1802 code. Look at this Web page for details about that.

In March 2014, I discovered how close the COSMAC "UT4" ROM monitor is, to the MicroTutor code. But I had to hand-type in the UT4 source from RCA's "Design Ideas Book for the CDP1802 COSMAC Microprocessor" BMP 802 book. Then assemble and verify that code against the hex codes in the book listing. YOu can check my work, as I've included the PDF of the code listing. If you add more comments to the ASM file and or have any corrections, please send me your results.

On the MicroKit Web page mentioned above, I also modifed the UT4 source to "match" the hex code in the MicroTutor ROMs. You might want to look to see how different that monitor is from UT4, but the majority of the code is the same.

During July 2014, Lee Hart and I were discussing his IDIOT monitor program, which is based on UT4 (see link below). He explained how RCA's 1802 assemblers produced a listing file which was also loadable by UT4, and by his IDIOT monitor. Here's Lee's description of that assembler's listing scheme.

Early Oct 2016, I OCR'ed the BMP-802's descriptions of UT4, and added that text file to the ZIP file.

June 2018: based on experiences with the IDIOT monitor, which is based on UT4, UT4 supports 7-bit serial communications, not 8-bit; no parity; one or two stop bits. On serial recieve, IDIOT ignores the parity or 8th bit. On serial transmit, the 8th bit may be "set" or have "mark" parity. But on echoing characters coming in to UT4, the 8th bit may be "set" or not. So, set your terminal or comm program for either "7 bit, mark parity, one stop" or "8 bit, no parity, one stop AND ignore/strip the 8th bit on recieve". On many computer-based communications programs, if the 8th bit is set the characters displayed may look like old-school "IBM PC graphic characters".

In July 2018, I got an email from Robert Coward. He found three instruction errors in the UT4 source I provide. This was after he disassembled a UT4 ROM and only later found this UT4 source on the Web. Robert kindly contacted me; I confirmed his corrections in the PDF'ed RCA source and updated the source I provide here. Here's Robert's disassembled UT4 source with his commentary. He provides another useful view of the operation of UT4's 1802 software. - Herb

- 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.

UT20 ROM monitor.

UT5 ROM monitor which operates from a calculator-like device.

- Herb Johnson

Contact information:
Herb Johnson
New Jersey, USA
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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..