COMPUTER SYSTEM

PURPOSE:To protect a basic I/O system(BIOS) program from runaway destruction due to software and to efficiently rewrite a necessary part by providing the title device with write control means for inhibiting or permitting writing operation to a BIOS program are written a random access memory(RAM). CONSTITUTION:At the time of a reading access to a read-only memory(ROM) 1, a BIOS program writing means 3 writes a BIOS program read out at a prescribed timing in the RAM 2. After ending said writing, the write control means 3, 8, 10-12 inhibit or permit writing operation to the BIOS program area written in the RAM 2. Consequently, an illegal access to the RAM 2 can be inhibited, the rewriting of the contents of the RAM 2 can be previously prevented and a necessary part of the BIOS program can be efficiently rewritten without executing system resetting processing.