RAM 6.0 BUX II Series Guía de usuario descargar pdf (Pagina 141)

A Common Development Issues

A.3 Cache and MMU

133

If your system becomes unresponsive, verify that the RAM addresses the code is

actually loaded at match the addresses used in the code.

Position-Independent Code

One exception to the matching RAM addresses rule is the PIC instructions at the

beginning of the flash image.

In VxWorks, there is a short section of PIC that copies the image from flash to RAM

and then transfers execution to the copy in RAM. This includes the romInit( ) and

romStart( ) routines discussed in 2.2.4 Detailed Boot Sequence, p.15. The OS image

code must be linked to reside at the RAM address. However, when the image code

is programmed into flash, the addresses are different.

When verifying the addresses of the code in flash, the romInit( ) address must

match the address of the processor's reset vector. This does not match the address

in the bootrom image file.

A.3 Cache and MMU

A BSP that is running correctly without cache often encounters problems when

cache is enabled for the first time. In most cases, these problems are not the result

of problems with the cache library.

When accessing device registers or other shared memory, the processor should

invalidate the cache line before reading from the register, and it should flush the

cache line immediately after writing to the register. Failure to perform these steps

in the appropriate places in a driver or BSP can cause cache coherency problems.

When reading from a device register, the main processor may find the register data

in cache. In this case, it does not actually check the hardware unless the cache line

has been invalidated. Therefore, the value being read may no longer be valid.

When writing to a device register, the main processor puts information into a cache

line, but does not write it to RAM. Meanwhile, before the processor writes the

cached information to RAM, some other device can modify the memory that the

NOTE: In this discussion, the terms device register and register may refer to the

actual registers on some peripheral device or, they may refer to structures in RAM

that are manipulated by both the processor and the device.

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RAM 6.0 BUX II Series Guía de usuario Pagina 141