determined by the map. This is done through the protection attribute bits discussed later in this section. Two type s of maps reside in the MMU, one for the oper ating system and one for user programs. In user mode, a user map is selected and the processor may not execute priviledged I/O. These are instructions whose execution could compromise the integrity of the operating system. The set of priviledged instructions only includes I/O instructions. The operating system always changes the CPU to be in user mode before transferring control to a user program. Therefore, if the MMU maps are accessed by I/O instructions, a user program cannot access the maps.
Of course, all other I/O must also be done by the operating system, since only it can execute I/O instruction~ Table 5-9 Memory Manageaent Re1ocation Registers I LOGICAL I BLOCK ADDRESS ATTRIBUTE AND MAPPING NUMBER RANGE PORT ADDRESS OFFSETS 00 00000-00FFF 000 01 01000-01FFF 002 02 02000-02FFF 004 03 03000-03FFF 006 04 04000-04FFF 008 05 05000-05FFF 00A 06 06000-06FFF 00C 07 07000-07 FFF 00E 08 08000-08FFF 010 09 09000-09FFF 012 0A 0A000-0AFFF 014 0B 0B000-0BFFF 016 0C 0C000-0CFFF 018 0D 0D000-0DFFF 01A 0E 0E000-0EFFF 01C 0F 0F000-0FFFF 01E 10 10000-10FFF 020 11 11000-11FFF 022 12 12000-12FFF 024 13 13000-13FFF 026 14 14000-14FFF 028 15 15000-15FFF 02A 16 16000-16FFF 02C 17 17000-17FFF 02E 5-14