RTEMS 4.6.1 On-Line Library

Board Support Packages Processor Initialization

RTEMS Motorola MC68xxx Applications Supplement

6.3: Processor Initialization

The address of the application's initialization code should be stored in the first vector of the EVT which will allow the immediate vectoring to the application code. If the application requires that the VBR be some value besides zero, then it should be set to the required value at this point. All tasks share the same MC68020's VBR value. Because interrupts are enabled automatically by RTEMS as part of the initialize executive directive, the VBR MUST be set before this directive is invoked to insure correct interrupt vectoring. If processor caching is to be utilized, then it should be enabled during the reset application initialization code.

In addition to the requirements described in the Board Support Packages chapter of the Applications User's Manual for the reset code which is executed before the call to initialize executive, the MC68020 version has the following specific requirements:

Must leave the S bit of the status register set so that the MC68020 remains in the supervisor state.
Must set the M bit of the status register to remove the MC68020 from the interrupt state.
Must set the master stack pointer (MSP) such that a minimum stack size of MINIMUM_STACK_SIZE bytes is provided for the initialize executive directive.
Must initialize the MC68020's vector table.

Note that the BSP is not responsible for allocating or installing the interrupt stack. RTEMS does this automatically as part of initialization. If the BSP does not install an interrupt stack and -- for whatever reason -- an interrupt occurs before initialize_executive is invoked, then the results are unpredictable.

RTEMS Motorola MC68xxx Applications Supplement