Create UEFI/BIOS chip dump from within Linux

Some people recommend using flashrom to create backup dump of a PC's UEFI/BIOS SPI flash contents. However, flashrom does not work with all platforms. Sometimes people recommend finding the address range of the SPI flash from /dev/iomem and then using that in combination with dd to extract it from /dev/mem. The problem with this is that /dev/iomem pretty much always reports that the System ROM resides in the range 0x000f0000-0x000fffff, which is 64 KiB whilst the SPI flash is almost always larger than that. Thus, how can one extract the contents of the SPI flash while running Linux on the target system and without using a SPI flash clip? I know that there is a way to do it because I found it once and I used it to extract a firmware image which UEFITools was able to open, but I've forgotten what that method is.