1. A computer system comprising: a central processor unit; a main memory having a stack region device connected to said central processor unit; a subsidiary memory device connected to said central processor unit and said main memory device for delivering a translated real address and protection codes including ordinary protection codes and a special protection code; an error detection means responsive to said ordinary protection codes from said subsidiary memory device and an access instruction from said central processor unit for checking said ordinary protection codes; and a special protection code check means responsive to said special protection code from said subsidiary memory device and an access instruction from said central processor unit for checking said special protection code; error detection in said error detection means being carried out when said ordinary protection codes from said subsidiary memory device do not coincide with said access instruction from said central processor unit; checking of said special protection code in said special protection code check means being carried out when said special protection code from said subsidiary memory device is present and said access instruction from said central processor unit is a write instruction, whereby writing into a region beyond said stack region in said main memory device is possible and the information of execution of the write operation to said region beyond said stack region in said main memory device is transmitted to said central processor unit.