Interview with Inventor of HP’s First Calculator Hits 2000 Views!

MH - You would have found it very difficult to program the HP 9825 in assembly language. The machine booted directly into the ROM-based HPL interpreter. You needed a special binary to get access to the bare machine and we didn't sell it. Things were different by the time the HP 9835 showed up.

Thanks... the key is "The indirect bit was not active during subsequent memory accesses". Probably because a 21xx was used during development, and it wouldn''t have taken kindly to the top bit of the indirection target being set. And most likely by the time the extra address space was needed it would have taken a lot of engineering work to revise the processor, all for a few "big" machines. The use of the 21xx architecture came as a big surprise when reading the 9825 yesterday... had I known it at the time I would have been sorely tempted to try 21xx assembly language instead of HPL.

MH, your question is about a point that often confuses me, so I went back to the source, the "How they do dat" manual, that documents HP''s 16-bit hybrid microprocessor from the 1970s. It turns out that HP made two versions of this processor for the HP 9825 (and even more for the HP 9835 and 9845, but that''s yet another story). Both processors had single-level indirect capability. The HP 2116''s multi-level indirect capability was omitted to save on-chip circuitry after careful examination of HP 2116 code showed that it was used only rarely. The first version of the HP hybrid microprocessor was the 15-bit version, referring to its 15-bit address space. This version of the hybrid microprocessor could address 32K words or 64K bytes. The second version, called the 16-bit version, had full 16-bit addressing and could address 64K words or 128K bytes. It was this processor that enabled the HP 9825S, which could have all the optional language ROMs in place and still support a maximum of 62K bytes of user RAM (the rest of the 64K RAM space went to internal uses). Again, both versions of the hybrid microprocessor still only supported single-level indirection. The indirect bit was active during the initial instruction fetch but not during subsequent memory fetches initiated by the setting of the indirect bit in the fetched instruction. Subsequent versions of the hybrid microprocessor added memory block switching to further expand the memory space.

Sorry to be posting here, but there doesn't seem to be an appropriate place on the 9825 site. I must be missing something regarding the 9825 address space limitation. If the 2116 instruction set was modified to eliminate multi-level indirection, that should give a 64 Kword = 128Kbyte address space. Yet I'm sure that up to 32 kbytes of RAM was the norm on 9825s. On multi-level indirection, one of the fun things that could go wrong on the HP1000s was two memory locations could have indirect references to each other... resulting in instructions that never completed.