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(2) Debug Breakpoint exception
• Cause
− A Debug Breakpoint exception is raised when an SDBBP instruction is executed.
• Exception masking
− The Breakpoint exception cannot be masked.
(Note : Its behavior during another debug exception is undefined.)
• Instruction causing this exception
SDBBP
• Processing
− When this exception is raised, processing jumps to a special debug exception handler at 0xBFC0
0200. (In the R3900 Processor Core, the debug exception vector is located in an uncacheable
address space.)
− The DBp bit in the Debug register is set to 1.
− The DEPC register points to the SDBBP instruction, unless that instruction is in the branch delay
slot, in which case the DEPC register points to the branch instruction and the Debug register
DBD bit is set to 1.
• Servicing
The unused bits of the SDBBP instruction (bits 26 to 6) can be used for passing additional
information to the exception handler. In order to allow these bits to be looked at, the user
program should load the contents of the memory word containing this instruction, using the
DEPC register. When Cause register BD bit is set to 1 (the SDBBP instruction is in the branch
delay slot), you should add +4 to the value in EPC register.