| 摘要 |
PURPOSE:To prevent erroneous read permission of segments other than instruction segment by permitting data read independently of presence/absence of the read permission attribute of segment attributes at the time of referring to data of the instruction segment. CONSTITUTION:When the instruction segment which is stored in an instruction base register 33 from a base register 21 and is in course of instruction execution is selected by a selector 34 to refer to data of the instruction segment, an access check circuit 35 checks a write permission attribute W of segment attributes of the instruction segment. If the attribute W is '0', read is permitted whether the read permission attribute of the instruction segment is '1' or '0'. Consequently, erroneous read permission of segments other than the instruction segment which may occur at the time of setting the read permission attribute to '1' by a subroutine to give read permission based on this attribute is prevented.
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