486-linux-busybox-low-mem

Experiments in running Linux on a low RAM 486, simulated with QEMU

To recreate these experiments as tested:

Grab a copy of the i486 musl cross compiler https://musl.cc/i486-linux-musl-cross.tgz

Busybox https://www.busybox.net/downloads/busybox-1.32.0.tar.bz2

Linux kernel https://git.kernel.org/torvalds/t/linux-5.8-rc1.tar.gz

qemu-system-i386 Download/install using your distro's package manager

qemu-static-i386 (only if you want to test your busybox/other user binaries first) Download/install using your distro's package manager

Extract the musl compiler somewhere convenient, then add i486-linux-musl-cross/bin to your path eg

export PATH=$PATH:/opt/i486-linux-musl-cross/bin

Extract all the sources, clone/download this repo somewhere (i'll refer to it as being in ~/Downloads)

Building Busybox

cd to the busybox folder, then copy the Busybox config into it

cd ~/Downloads/busybox-1.32.0
cp ~/Downloads/486-linux-busybox-low-mem/busybox_config/.config ./

CROSS_COMPILE=i486-linux-musl- ARCH=x86 make menuconfig
CROSS_COMPILE=i486-linux-musl- ARCH=x86 make

Building Linux

cd ~/Downloads/linux-5.8-rc1
cp ~/Downloads/486-linux-busybox-low-mem/linux_config/.config ./
cp ~/Downloads/486-linux-busybox-low-mem/rootfs ./ -R
cp ~/Downloads/busybox-1.32.0/busybox initramfs
CROSS_COMPILE=i486-linux-musl- ARCH=x86 make menuconfig
CROSS_COMPILE=i486-linux-musl- ARCH=x86 make

Running your newly built system

From within the linux folder

qemu-system-i386 -cpu 486 -kernel arch/x86/boot/bzImage -m 8M -append 'console=ttyS0,115200' -serial mon:stdio -nographic

or, if you're looking to see how little ram you can get away with

qemu-system-i386 -cpu 486 -kernel arch/x86/boot/bzImage -m 3666K -append 'console=ttyS0,115200' -serial mon:stdio -nographic

You can also boot qemu in graphical mode, but I find that less useful

qemu-system-i386 -cpu 486 -kernel arch/x86/boot/bzImage -m 8M

Notes:

Compressing the initramfs requires more memory - since it's so small anyway, I recommend not bothering

Compressing the kernel does not affect runtime memory usage at all - if there's sufficient memory for the uncompressed kernel and the decompression buffers, then you can use any compression method you like. I did not run into issues with the kernel sizes I was getting with a minimal config. XZ should give best compression, and therefore best boot speeds from slow media like floppy disk.

ProcFS as configured here uses about 100K of ram.

printk support (currently disabled) uses about 200K of ram

###Useful commands for checking memory usage:

cat /proc/meminfo
cat /proc/iomem

9names / 486-linux-busybox-low-mem