Hi Peter The TM section (4.11) on the read-write register looks to me like a comprehensive descrioption of it's functionality; esp if read with an LS299 data sheet. There may be second order points which study of the maintenance (schematic) prints will reveal - but I would be surprised, this is intened to be vanilla. The (useful) student guide you identified gives a good precis of R/W register function on page BD-15ff. The R/W register looks like a 16 bit register, writable at byte granularity for testing / exercising minimal systems. Very useful for low level diagnostics / faultfinding of DATI DATO DATOB bus cycles. HtH; Martin -----Original Message----- From: Peter Ekstrom via cctalk &lt;cctalk(a)classiccmp.org&gt; Sent: 23 November 2025 16:52 To: General Discussion: On-Topic and Off-Topic Posts &lt;cctalk(a)classiccmp.org&gt; Cc: Peter Ekstrom &lt;epekstrom(a)gmail.com&gt; Subject: [cctalk] Re: BDV11 programming information Hi Glen, Yes, and I have looked over it several times. It does have quite a bit of info, but it doesn't go into much detail for the R/W register. I also found a student guide that includes the BDV11 and it help shed some light on how the code in the ROMs is segmented. It helped me visualize more how it is done. But I am still missing things like the details for the R/W register. https://wwcm.synology.me/pdf/EY-DX056-SP-002&J6269-A&EY-DX052-SG-00… - Peter On Sun, Nov 23, 2025 at 10:52 AM Glen Slick via cctalk < cctalk(a)classiccmp.org&gt; wrote: