RCA UT71 and UT70 ROM

RCA UT71 and UT70 ROM

Last updated May 21 2020. 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 UT71 ZIP file

The UT71 ROM ZIP file at this Web linkcontains UT71 sources in A18 (Intel-like) and A8 (RCA assembler-like) syntax, a text document description; and hex and binary versions *not* from these assembly sources. The sources were extracted from the RCA publication: "RCA CMOS Microsystems - User Manual for the RCA MicroDisk Development System MS2000" MPM-241, published May 1984. The manual was scanned and PDF'ed by members of the groups.io-hosted cosmacelf discussion group and as of Dec 2019 was at the link given here.

As of May 2020 there was an inconsistency among the sources; there's a "note" document in my ZIP file of the problem near the "MSGE1" label. My sources *at that point*, are consistent with the binary and hex versions given, and consistent with each other, and the PDF of that source in MPM-241. I cannot confirm that my A18 and A8 compatible sources are identical, or produce the same binary and hex as given.

Included with the UT71 source, is an adapted set of SCRT programs for RCA's staandard call and return. - read about SCRT on the linked Web page.

Dave Schultz and n1ltv in Aug-Sept 2016, produced the UT71 RCA source listing from the manual as a assembly file, and converted it to A18 syntax (a conventional 1802 assembler). I OCRed the manual sections describing the UT71 monitor, and edited those results into a flat-text file, as a convenient small sized document. If you find document errors as compared to the PDF image, please let me know. I believe the UT71 sources are correct, but let me know if you find an issue with those too. - Herb Johnson

Dave Schultz's work on RCA MicroDOS is on his Web site.

- Herb Johnson


In Mar 2020, UT70 was found as a source listing, on a "preliminary" paper copy "P1" of the MPM-241 User Manual for the MS2000 RCA MicroDisk Development System. This was discussed in the groups.ip cosmacelf email list, during March 19 2020 and following days. The cosmacelf files section contains the .PDF UT70 source listing and the UT70 .bin file. The discussion did not determine if the two MPM-241 documents available, referred to different hardware or different versions of MS2000 "DOS". Neither UT70 nor UT71 seemed to work completly well with the available MicroDOS software in emulation or on hardware. UT70 is the earlier version. There's considerable discussion in the cosmacelf list, look there for details.

I did a rough eyeball compare, between the UT70 and UT71 sources. Here's my notes. It appears UT70 contained earlier and different code to handle the disk controller than UT71. Among others working on UT70, Marcel van Tongeren of EMMA 02 was adding UT70 to his emulator and was working on its emulated operation.

David Schultz comments that UT70 must be run in read-only memory, because "it executes the INP instruction with X=P. (CMD1 in the disk code.) This will attempt to write the input data to the location following the INP instruction. UT70 has a "SEX SP" just before the equivalent point in its code."

Other Related Web pages

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

UT20 ROM monitor

Microboard Computer Development System with UT62 ROM monitor and BASIC 3 in ROM. I have the binary dump of the ROM, but not source. This system was built in 1982, and it's a good example of cassette-based development tools of the late 1970's.

On that MCDS Web page, I show how I used the UT71 source, to compare to the UT62 binary in the form of a run-time disassembly in EMMA 2, an 1802 emulator. Read the details there and obtain the synthesized UT62 ROM monitor source. - Herb

- Herb Johnson

Contact information:
Herb Johnson
New Jersey, USA

This page and edited content is copyright Herb Johnson (c) 2020. 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..