THe 41CL ROM library contains two versions of the CL extra functions ROM. They differ only in their ROM labels and in their XROM numbers. The YFNZ ROM has XROM 15, and is the default version. (i.e. it is loaded when the MMU is disabled, hardwired into port 7.) The"alternate" version is called YFNS. It has an XROM number of 31.

In order to play with flash memory on the 41CL, it is necessary to map one or the other of these ROMs into RAM. For some reason, people writing code like that tend to map the YFNS version at ROM address 0x062000 into RAM, rather than the default YFNZ version at ROM address 0x070000. They often also "restore" the port mapping of the YFNS version when they are through. This has caused me some confusion, because code I wrote using the YFNZ version fails to work if the YFNS version is loaded.

Is there an XROM collision that leads people to prefer YFNS over YFNZ? Or perhaps this is a legacy of the beta test era?


I think that this is probably an artifact of the Beta testing.

The 41CL hardware does not allow the first sector of Flash

memory to be written, to protect the operating system. The

YFNZ version is located in this sector. So, when patching is

required the YFNS version was used, because it can be written

back to the Flash.


To that end we were also patching the YFNS to read ROM id #15 using a ypoke command.

Cheers, Geoff

p.s. I encourage all CL users to sign into Bruce Bergmans wiki in which is a dedicated CL forum including PDFs and etc:

HP 41CL wiki

Also there are some beta pdf's created which are available with an email to me.

geoffqf@xxxtelus.net (remove the xxx of course.)

Edited: 21 Dec 2011, 10:10 p.m.


One reason to use the XROM#15 one is CLUTILS - the function launchers expect that id# to work.

The drawback is HEPAX RAM tends to use the same id# in 16k configurations, so a manual adjustment is required.

I have re-flashed my YFNS#31 with a patched version of the YFNZ#15, so I'm totally commited to that id number.

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