hp41 vs hp42 memory


UI would like to know how big is the hp42 memory in comparison with a hp41cx with 600 registers (xmemory x 2 module)
Thank you


If I recall right, 600 registers of 7 bytes =~4.1Kbytes. The 42S have almost 6.7KB (I don't remenber well, mine has 31KB now!).

Best regards,



Hi, Fulcrum;

the HP42S comes with 8KBytes of direct use RAM, I mean, no need for "extended" memory handling. Anyway, it can be upgraded to 32Kbytes "direct" RAM and up to 64Kbytes, whre the upper 32K is not "seem" as user RAM; instead, there is a "hidden" XFCN (thanks to J. F. Garnier) that allows system routines to be run there (I know Nelson is dealing with this, too).

As you may see, the HP41 has:

320 × 7 bytes main RAM (2240B) and

600 × 7 bytes maximum extended memory (4200B)

what gives us a maximum of 6440 bytes, fewer than the standard HP42S. Well, many other things must be also considered: resident functions (HP42S) against ROM expandability (HP41), but this is another history. Iprefer the HP42S, mostly because I use complex and matrices a lot.

Hope this helps. Cheers.

Luiz (Brazil)

Edited: 26 Apr 2004, 4:36 p.m.



HP41 can easily be expanded to 47 KByte by inserting two hepax modules.

HP41 standard ram = 2240 bytes + internal registers

HP41 ext memory (e.g. one double xmem module) = 4200 bytes

two hepax-modules = 40000 bytes

(32 kwords à 10 bits each: 32 kbytes with user code, 40 kbytes with machine code with higher complexity command space)


With addressing via bank switching more is possible.
Available rambox systems allow

a] extra 80000 bytes (64 kwords à 10 bit/word)

b] extra 160000 bytes (128 kwords à 10 bit/word)

This extra ram is not made by HP but only made by Eramco and by Corvallis, CMT. Maybe there were more unknown small maunfacturers but I do not know.

In principle the architecture of HP41 allows even more rambox space to use if not only regular pages 8,9,A,B,C,D,E,F are used but also pages 0,1,2,3,4,5,6,7 via four times bank switching.

In principle 297 kbytes is maximum: 64 pages minus 6 pages for operating system of HP41CX (each page is 5 kbyte or 4 kword) plus 7 kbyte of standard ram + ext memory ram + internal register ram. But there was never built any real application.


So, the maximum ram configuration that can be realized with a real product is:

167 kbytes !

HP41 standard ram = 2240 bytes + internal registers

HP41 ext memory (e.g. one double xmem module) = 4200 bytes

rambox = 160000 bytes

That is much more than 64 kbytes of HP42s !


Edited: 27 Apr 2004, 8:39 a.m.


How much do you spend to get this amount of RAM on the 41? I have my 42S with 32K+32K for about nothing, using chips "retired" from junk hard disks... And a bit of soldering...


Best regards,



I don't own that "mega"-configuration (e.g. I have no hepax) but I wanted to show what is principally possible.

But I own a 64 kword Eramco rambox that cost me only $200 and this item alone already gives me already 87 kbyte of total ram ! And that for less money than one standard 8-kbyte-HP42s costs at ebay these days !!!

So what ?



Edited: 27 Apr 2004, 11:13 a.m.


Have you checked the prices for RAM boxes on ebay lately. In the last two years I haven't seen one for less than $500. Of course, if you are willing to sell yours for $200 I will gladly take it ;>



e.g. for only 290 EURO:


(oh I forgot W&W. This is another rambox manufacturer with top products)

There were more ramboxes from Eramco for less than this price during the last months!

So prices have fallen due to new ram modules for HP41 like CLONIX, NOVRAM and MLDL2000.

With the new development MLDL2000 from Meindert Kuipers you will have something around 256 kBytes of ram !!!! Or even more ??

And he will sell it for only 200 Dollars or something around this price !!!

And last not least ...
the MLDL 2000 has USB connection to PC. So you can load and save your ram and programs.

But how to save the stuff you typed into a HP42s ?? Yes, you can only load data via keyboard !!!
So 32 kbyte or even 64 kbyte are completely useless ! You need years for filling 64 kbytes by keystrokes. And then suddenly your battery fails ...

oh wow, let's begin typing in the stuff again ... :-)

This all really blows away that tiny HP42. HP42 is nothing against the hyper calc HP41 !!!


Edited: 27 Apr 2004, 12:52 p.m.


I have a project to build a serial RS-232 and MMC card interface to my trusty HP-42S(X?) and replacing one of the 32K SRAM chips with 64K SRAM or FLASH ROM. I'll publish any news here in the next month(s).

As soon I learn the Saturn Assembler I will "upgrade" the OS of the 42S to allow the use of I/O and storage.

Best regards,



That is a great project, congratulations !!!
Maybe this can make HP42s to become a rival to HP41.

Still one question:

Does HP42s also have the absolutely great capabilities to be programmed in a very complex way of synthetic programming which is extremely efficient ?

And can it be programmed in machine code like HP41 ?

In principle the latter should be possible with any computing device, but I am not sure if the easy access to do so is guaranteed with HP42s in the same great way as with the HP41.




I don't know yet if the 42S is able to use any type of "synthetic" instructions, as they are "collateral" instructions based on "bugs" of the ROM. But assembler is the way to go, and I have tested some small routines on my 42S to prove that is possible. I have to learn the internal OS functions of the 42S to be able to extend it and add more functions. I know that the Saturn CPU is much more capable than the NUT CPU (from 41, voyagers). This can take a long time...

Maybe someone else can help me with this challenge? I'm already committed with the "OpenRPN" project, and this can be another good project to work with.

Best regards,




the HP42S has an already built-in memory browser. Try this:

- start the interactive self-test with [ON]/[LOG]
- press [<-] (back space)

What you see is something like:

If you knwo what you're doing, you can change anything that's in RAM and produce most combinations. Some of them mean nothing. J.F. Garnier found that [XFCN] (a hidden instruction) allows the execution of "X"tra functions (maybe this is sort of a XROM version for the HP42S). Nelson also has gone a lot deeper into this subject.

I'm following form an observer distance for now, but I'm trying to get as close as I can to participate. I prefer listenning before talking... Teacher's point of view.

Best regards.

Luiz (Brazil)


Hi Luiz,

Judging by the level of technical wondermazement I see here, do you think Brazilians get more silicon in their diets, or what?! ;^)




Hey Bill, thanks!

I'm flattered! And I think all my "countrymates" as well...

Now I'm also curious... Do we? ;^)


Luiz (Brazil)


The ZENROM provides the same kind of thing for the HP-41.


Correct amounts of memory: 512 KByte SRAM, 512 KByte FLASH eprom.

Both memory types are to be used for ROM images, but some ROMs support the SRAM banks to be used for storage as well. Eventually all of the memory will be accessible with a special mcode sequence. The final pricing is not yet fixed however.

And yes, the fortyone is by far the best thing that HP has ever done, I think the HP41 averages about one machine per day on ebay changing hands.



Wow! one Meg of RAM !!!

HP41 seems to always be one step ahead ...

(thanks to pioneers from nowadays)



Now I see yet one more reason that the HP41 system fails to die.....it has almost unbelievable capabilities---especially considering all of the basic capability was developed in 1978! Imagine, a handheld personal computing device, produced in the same time period as double-knit slacks, that was in principle capable of using more memory than any desktop microcomputer produced through roughly the mid-80's. That is extraordinary.

No wonder so many scientists and engineers put so much effort into programming it--it really did save valuable mainframe time, and could actually process meaningful amounts of data with suitably large and complex programs!

Bear in mind I am not an experienced 41 user--in fact never even held one until last year, and have not bothered to use it to capacity.

I am just pleasantly amazed by all this.

Edited: 27 Apr 2004, 12:23 p.m.

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