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ARM System Developer’s Guide phần 8

Một thiết kế bộ nhớ cache khác nhau được sử dụng để hỗ trợ hai kiến trúc. Trong các lõi xử lý bằng cách sử dụng kiến trúc Von Neumann, có một bộ nhớ cache duy nhất được sử dụng để được hướng dẫn và dữ liệu. | 478 Chapter 13 Memory Protection Units Table 13.9 Protection unit enable bits in CP15 control register 1. Bit Function enabled Value 0 MPU 0 disabled 1 enabled 2 data cache 0 disabled 1 enabled 12 instruction cache 0 disabled 1 enabled bit in the control register and a 0 leaves the bit value unchanged regardless of the bit state in the first parameter. For example to enable the MPU and I-cache and disable the D-cache set bit 12 to 1 bit 2 to 0 and bit 0 to 1. The value of the first parameter should be 0x00001001 the remaining unchanged bits should be zero. To select only bit 12 bit 2 and bit 0 as the values to change set the mask value to 0x00001005. Example This routine reads the control register and places the value in a holding register. Then it 13.5 clears all the changing bits using the mask input and assigns them the desired state using the value input. The routine completes by writing the new control values to the CP15 c1 c0 register. void controlSet unsigned value unsigned mask unsigned int clf __asm MRC p15 0 clf cl c0 0 read control register clf c1f mask mask off bit that change c1f c1f value set bits that change __asm MCR p15 0 c1f c1 c0 0 write control register 13.3 Demonstration of an MPU system We have provided a set of routines to use as building blocks to initialize and control a protected system. This section uses the routines described to initialize and control a simple protected system using a fixed memory map. Here is a demonstration that uses the examples presented in the previous sections of this chapter to create a functional protection system. It provides an infrastructure that enables the running of three tasks in a simple protected multi-tasking system. We believe it provides a suitable demonstration of the concepts underlying the ARM MPU hardware. It is written in C and uses standard access permission. 13.3 Demonstration of an MPU system 479 13.3.1 SysTEM Requirements The demonstration system has the following hardware characteristics An