On which is the open boot prom available?

No OpenBoot Environment on the Intel Platform. The Intel environment has no OpenBoot PROM or NVRAM. On Intel systems, before the kernel is started, the system is controlled by the basic input/output system (BIOS), the firmware interface on a PC. Therefore, many features provided by OpenBoot are not available on Intel systems.
Note: The Open Boot PROM (OBP) bootloader only exists within SPARC. Before Solaris 10 01/06, the bootloader for Solaris x86 was a Sun customized bootstrap software. After Solaris 10 01/06, it uses GRUB, a well known bootloader that's commonly used in the Linux world. With GRUB, it's much easier to make the system dual-boot Linux and Solaris. GRUB extends the capabilities of the bootloader that was not available previously such as the ability to boot from a USB DVD drive. Those who have used Linux will be quite familiar with GRUB and its options.