4.3.1. Instruction Cache
The Instruction cache parameters provide the following options for the Nios® II/f core:
- Size—Specifies the size of the instruction cache. Valid sizes are from 512 bytes to 64 KBytes, or None.
Choosing None disables the instruction cache. The Avalon® -MM instruction master port from the Nios® II processor will still available. In this case, you must include a tightly-coupled instruction memory.
- Add burstcount signal to instruction_master —The Nios® II processor can fill its instruction cache lines using burst transfers. Usually you enable bursts on the processor's instruction master when instructions are stored in DRAM, and disable bursts when instructions are stored in SRAM.
Bursting to DRAM typically improves memory bandwidth, but might consume additional FPGA resources. Be aware that when bursts are enabled, accesses to slaves might go through additional hardware (called burst adapters) which might decrease your fMAX.
When the Nios® II processor transfers execution to the first word of a cache line, the processor fills the line by executing a sequence of word transfers that have ascending addresses, such as 0, 4, 8, 12, 16, 20, 24, 28.
However, when the Nios® II processor transfers execution to an instruction that is not the first word of a cache line, the processor fetches the required (or “critical”) instruction first, and then fills the rest of the cache line. The addresses of a burst increase until the last word of the cache line is filled, and then continue with the first word of the cache line. For example, with a 32-byte cache line, transferring control to address 8 results in a burst with the following address sequence: 8, 12, 16, 20, 24, 28, 0, 4.
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