A computer system boots the system from a bootup-memory-image stored in a nonvolatile memory unit forming part of a main memory device. When the first write access occurs after system operation, the system copies the data of a block of the bootup-memory-image to a readable/writable main memory unit forming another part of the main memory device for each block of the bootup-memory image. The system performs write operation on the copy, and subsequently switches access to an area of the bootup-memory-image to which data has already been copied to access to the copy. A bootup method for a computer system and program recording medium are also disclosed.