Re: [PATCH 03/13] ARM: remove riscpc

[Ethan Nelson-Moore]

Hi, Karl,

On Fri, Sep 11, 2026 at 11:43 AM Karl Mehltretter
<kmehltretter@xxxxxxxxx> wrote:
> I've made a RiscPC emulator in the browser using QEMU and WASM. It boots
> Linux kernels and currently emulates a StrongARM RiscPC:

That's awesome. Are you planning to add support for the NetWinder to the b?

A couple things that I didn't see when I glanced through the code:

* QEMU needs to model the broken ldrh/strh instructions for the Risc
PC (but not the NetWinder). I am not sure exactly what their behavior
is on real hardware (though I have seen forum posts where people
investigated this - I'll let you know when I find them). For now, you
could just have them load/store a random value (:p). It would also be
helpful if it could print a warning when one of these instructions is
executed, to help people detect and remove instances of them in e.g.
JITs.

* On ARMv3, CP15 register c0 is write-only, and you don't model this.
This caused Linux on ARMv3 to be broken for a very long time before
support was eventually removed - it was broken even in 2.4.18-rmk4. A
patch to run Linux on the CL-PS7500FE (= ARM710 Risc PC on a chip,
used in the Acorn A7000+) works around this by keeping a copy of it:
http://webpal.bigbrd.com/patch-2.4.18-rmk4-wp3.gz. According to a
comment in later 2.4 versions, that approach won't work, but I don't
understand why. Also, the patch fails to fix other reads to the
register outside cpu_get_pgd() (I forget which ones).

A few things I know about early ARM hardware and Linux that might help you:

Before 2.6, ARM support in the mainline kernel was very incomplete
and, IIRC, unusable. To build a working kernel, you needed to apply a
patch from http://ftp.armlinux.org.uk/pub/linux/arm/kernel/. There are
no patches for versions newer than 2.4.27, but the 2.4.27 patch mostly
applies correctly to 2.4.37.11 (I didn't try to fully fix it up
though). The last patch that was released was 2.6.0-rmk2 - it has some
relevant changes like the CL-PS7500 PS/2 mouse driver. There is also
an unreleased 2.6.4 patch - I should ask RMK if he still has it.

The NetWinder development team shipped patches to their kernel with
additional features for the NetWinder (and other ARM hardware,
including Acorn). This is the last one:
http://www.netwinder.org/mirror/users/r/ralphs/kernel/patch-2.4.21-rmk1-nw1.gz
I don't think most of those changes ended up in mainline.

The Risc PC and NetWinder lost their audio drivers when OSS was
removed in commit 727dede0ba8afbd8d19116d39f2ae8d19d00033d. RMK was
working on an ALSA driver for the NetWinder but never finished it
(IIRC it is in 2.6.0-rmk2 but probably doesn't work).

> I used LLMs a lot while building it.

I don't use LLMs for environmental reasons, because they are often
inaccurate, and because I like using my own brain and don't want it to
atrophy, but we are all entitled to our own opinion.

> Your GCC work looks useful for this. I will look at building the RiscPC
> Linux kernel with it, and at emulating the original ARMv3 CPU too.

Great. Let me know if it works.

P.S. I have a few UI suggestions for the emulator:
* Make the display bigger and able to lock the mouse/keyboard and go
into fullscreen.
* Remove the LK201 keyboard - the NetWinder used a normal PC keyboard
(if you add it), and the Risc PC used an Acorn PS/2 keyboard (a normal
British PC keyboard but with no Windows key):
https://chrisacorns.computinghistory.org.uk/32bit_UpgradesA2G/Acorn_RiscPC_Keyboard.html.
Just keep virtual keys for the keys that you can't input in a web
browser.
* It should be possible to middle click using a modifier key if the
emulator ever runs Risc OS, because middle clicking is heavily used in
Risc OS.
* I think some of the explanatory text is unnecessary.
* The skeuomorphism is cute but inaccurately represents a Risc PC, and
I think it clutters the UI unnecessarily.

Ethan