I don't know about the Sinclair development specifically, but for the HP-35, they wrote a register-level simulator in software that let them develop the algorit

I don't know about the Sinclair development specifically, but for the HP-35, they wrote a register-level simulator in software that let them develop the algorithms and single-step through for debugging. My favorite quote from the article: "Correcting a problem was a simple matter of changing a punched card or two." http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1972-06.pdf http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1972-06.pdf The article goes into a lot of detail on the HP-35 internals, so it's worth a look.