We visited the Computer History Museum which has a pair of core rope modules containing the Retread 50 software. These are part of their exhibition of an Apollo Guidance Computer, but we arranged to archive the contents of the modules using our working AGC, in support of their software history effort and to deepen understanding of what is in their artifacts.
Before we connected these to our AGC, it was important to verify that there were no open circuits on the strand and module select lines or other inputs as our computer could be damaged if that was the case. Following a long test plan, we determined that the rope modules were safe to insert.
We first started the AGC in single instruction mode, turned on the core rope access and tried a couple of accesses to see if they appeared to be delivering valid contents, based on the known archive of the predecessor Retread 44 software. It matched.
Mike then used his test monitor tool to read through all of the core rope modules and save their contents. We did spot some parity errors which were confine to specific parts of the modules. In fact, all errors were in the first (B1) module and they were in strands 1 and 9. Modules have 12 strands each holding 512 words of data (thus 6K capacity per module).
For strand 1, a particular data bit was always reading as a 1 value. This appears to be from a failed component inside the core rope module. We did not open the modules as they are artifacts that should not be disturbed. When a word legitimately had a 1 value in that bit, no parity error ensued, but every location that intended a 0 value would trigger a parity alarm.
Strand 9 had more erratic results, but again for just a single bit location in every word. That meant it too could be recovered by looking at the parity bit value.
We wrote a quick post-processing routine to use the parity bit to interplate the correct value of our bad bit. In early programs such as Retread, they left any unused words unwired - thus they would legitimately produce a parity error with all zero results. We had to find those unused words since we had improperly corrected it to have a non-zero word value.
With the data recovered, it could be shared and analyzed. Many of the expected changes were there - since Retread 44 was written on the emulator before the first block II hardware was built, it had some code that failed on real hardware due to inaccuracies in the emulator. All of these flaws in code were corrected, many as were expected but some through alternate changes.
There is some added code, parts that are seen in later (Aurora) software but some that are unique to this previously lost software version. Analysis continues
We hope to archive other lost software versions as owners of other rope modules permit us to plug them in to our system.