Yes and no. The prom only knows how to load a bootloader from the beginning of a cylinder. This means that your bootable partitions need to start on cylinder boundaries. The cylinder size in your disklabel does not have to bear any relation to the actual physical cylinder size of the disk. You can't just set your cylinder size to 1 (i.e. the number of cylinders = the number of sectors), since the number of cylinders is stored as a 16 bit number (i.e. your disk could only be about 32 MB).