How is kernel loaded from .vmdk image?

Question

I am using poky at warrior branch to generate root images. Yocto can generate the final image in various formats. One of them is .hddimg, which looks like this when extracted:

EFI
bzImage
initrd
libcom32.c32
libutil.c32
rootfs
startup.nsh
syslinux
vesamenu.c32

Following this link: http://embeddedguruji.blogspot.com/2018/11/run-your-intels-yocto-hddimg-in-virtual.html, I convert the .hddimg file to .vmdk file. When I use 7zip to view the contents of the .vmdk file, it is same as above. I was curious to know how the boot process works, and how the kernel is loaded. And how I can switch default kernels when multiple kernels are installed in rootfs.

I base this question from my previous unanswered question: https://stackoverflow.com/questions/57611557/switch-kernel-in-yocto-root-image

Answer 1

Both .hddimg and .vmdk are disk images, so they boot exactly like ordinary (physical) HDDs. That is, they don't just contain a root filesystem but a whole partitioned disk layout.

(Note that 7zip will only show the first partition it recognizes.)

Like real disks, these images have a boot sector (for BIOS) and/or an EFI system partition (for UEFI), and there is a bootloader installed (in your case that's syslinux). The virtual machine software emulates a full BIOS, which starts syslinux from the virtual disk, which then reads its menu configuration (syslinux.cfg) and starts the kernel configured in that file.