71B vs 75D

[Martin Hepperle]

In my view, the Capricorn CPU is remarkable technology and the 75 is an interesting bridge between the pocket calculators and the Series-80 desktop computers.

Unfortunately the RAM capability of the 75 is severely limited - the RAM also serves as a file system, like on the 71. They could have done better by adding an additional memory controller.

I think the CPU it is underestimated - one has to compare with CPUs like 8080, Z80, 6809, 6502 and the like.

For its time, the CPU offers some very innovative features, the only drawback in my view is that it did not provide multiplication and division op codes.

On the other hand, which CPU from this era gives you a large set of 16 and 64 bit registers and almost symmetric (all opcodes allowed with all registers) operations?

Similar features can be found later in the 68000 family.

You can extend the BASIC with new keywords or write "standalone programs" in assembler (these are in in principle BASIC keywords). This is no problem as in the Series-80 machines. Documentation is sparse, but the ROM listings and associated do0cuemntation contain all the information and together with the HP-85 Assembler ROM manual it is not really difficult, only time consuming.

Martin

[J-F Garnier]

(04-15-2021 11:10 AM)Martin Hepperle Wrote:  In my view, the Capricorn CPU is remarkable technology and the 75 is an interesting bridge between the pocket calculators and the Series-80 desktop computers.
[...]
I think the CPU it is underestimated - one has to compare with CPUs like 8080, Z80, 6809, 6502 and the like.

For its time, the CPU offers some very innovative features, the only drawback in my view is that it did not provide multiplication and division op codes.

On the other hand, which CPU from this era gives you a large set of 16 and 64 bit registers and almost symmetric (all opcodes allowed with all registers) operations?

I concur. I discovered the Capricorn CPU with the Series 80 Assembler ROM (around 1982-1983), and found it very powerful, with its indirect/indexed addressing modes and the capability to operate on 1 to 8 bytes with a single opcode.

One weakness of the Capricorn was the lack of relative call and long jump, requiring to use the index addressing mode to build relocatable (position-independent) code as needed for LEXs, using one register and extra overhead. But I believe this didn't affect the overall performance so much.

I don't see where the Capricorn architecture comes from. It doesn't come from the calculator CPU family, nor the HP desktop computer CPUs. Strange that such CPU was designed from scratch by a calculator division (Corvallis) for the Series 80.
In contrast, the Saturn architecture clearly comes from the Coconut and predecessors.
Any information on the origin of the Capricorn?

J-F

[Sylvain Cote]

(04-15-2021 12:30 PM)J-F Garnier Wrote:  Any information on the origin of the Capricorn?

HP Journal V31N8 August 1980 Article → A Custom LSI Approach to a Personal Computer

[hth]

(04-15-2021 11:10 AM)Martin Hepperle Wrote:  In my view, the Capricorn CPU is remarkable technology and the 75 is an interesting bridge between the pocket calculators and the Series-80 desktop computers.

Unfortunately the RAM capability of the 75 is severely limited - the RAM also serves as a file system, like on the 71. They could have done better by adding an additional memory controller.

Yes, this was my thinking also. I am very much into ISAs and architectures and Capricorn is really interesting. I think the HP-75 is let down by the lack of RAM and I am not sure if it is possible to replace ROMs, there seems to be no plug-in feature.

Which leads me to think about the series 80 computers, but I do not fancy hunting down old hardware that is bulky and may need repair. Not even sure if they are collectable and expensive as well.

I wonder if there is any FPGA implementation of this ISA, someone could replicate something new and "modern" retro around it? I think that is what I really want, then perhaps an HP-75 to complement it.

[Martin Hepperle]

(04-15-2021 04:32 PM)hth Wrote:  Yes, this was my thinking also. I am very much into ISAs and architectures and Capricorn is really interesting. I think the HP-75 is let down by the lack of RAM and I am not sure if it is possible to replace ROMs, there seems to be no plug-in feature.

You have 3 ROM plugin ports - ROMs are bankswitched, all very similar to the Series-80s.
One could also make something like the ROM emulator which was available from HP and plugged via cable to the ROM ports or something like the "Modem/RAM-Disk Pod".
The ROMs have some "intelligence" as they monitor a certain address and switch themselves on/off as needed.
The difficulty is probably more in finding or making proper connectors. Today it could be easier with 3D printing and individual contact springs?

Martin

[Sylvain Cote]

(04-15-2021 06:10 PM)Martin Hepperle Wrote:  One could also make something like the ROM emulator which was available from HP and plugged via cable to the ROM ports or something like the "Modem/RAM-Disk Pod".

The 82173A Plug-in Module Simulator (PMS) implemented in RAM a 2 banks of 8KB ROM. (16K total, RAM was battery-backed)
You could connect up to three PMS in the front ports.

[Didier Lachieze]

(04-15-2021 04:32 PM)hth Wrote:  I wonder if there is any FPGA implementation of this ISA, someone could replicate something new and "modern" retro around it? I think that is what I really want, then perhaps an HP-75 to complement it.

There is an HP 86B FPGA implementation available here.