Instruction Issue Delay

Sometimes, specific leaf PCs appear more frequently because the instruction that they represent is delayed before issue. This can occur for a number of reasons, some of which are listed below:

• The previous instruction takes a long time to execute and is not interruptible, for example when an instruction traps into the kernel.

• An arithmetic instruction needs a register that is not available because the register contents were set by an earlier instruction that has not yet completed. An example of this sort of delay is a load instruction that has a data cache miss.

• A floating-point arithmetic instruction is waiting for another floating-point instruction to complete. This situation occurs for instructions that cannot be pipelined, such as square root and floating-point divide.

• The instruction cache does not include the memory word that contains the instruction (I-cache miss).

• On UltraSPARC® III processors, a cache miss on a load instruction blocks all instructions that follow it until the miss is resolved, regardless of whether these instructions use the data item that is being loaded. UltraSPARC II processors only block instructions that use the data item that is being loaded.