Instruction Fetch
rtl/cve2_if_stage.sv.
The Instruction Fetch (IF) stage of the core is able to supply one instruction to the Instruction-Decode (ID) stage per cycle if the instruction cache or the instruction memory is able to serve one instruction per cycle.
Instructions are fetched into a prefetch buffer (rtl/cve2_prefetch_buffer.sv
) for optimal performance and timing closure reasons.
This buffer simply fetches instructions linearly until it is full.
The instructions themselves are stored along with the Program Counter (PC) they came from in the fetch FIFO (rtl/cve2_fetch_fifo.sv
).
The fetch FIFO has a feedthrough path so when empty a new instruction entering the FIFO is immediately made available on the FIFO output.
A localparam DEPTH
gives a configurable depth which is set to 3 by default.
The top-level of the instruction fetch controls the prefetch buffer (in particular flushing it on branches/jumps/exception and beginning prefetching from the appropriate new PC) and supplies new instructions to the ID/EX stage along with their PC.
Compressed instructions are expanded by the IF stage so the decoder can always deal with uncompressed instructions (the ID stage still receives the compressed instruction for placing into mtval
on an illegal instruction exception).
Instruction-Side Memory Interface
The following table describes the signals that are used to fetch instructions. This interface is a simplified version of the interface used on the data interface as described in Load-Store Unit. The main difference is that the instruction interface does not allow for write transactions and thus needs less signals.
Signal |
Direction |
Description |
---|---|---|
|
output |
Request valid, must stay high until
|
|
output |
Address, word aligned |
|
input |
The other side accepted the request.
|
|
input |
|
|
input |
Data read from memory |
|
input |
Data integrity bits from memory |
|
input |
Memory access error |
Misaligned Accesses
Externally, the IF interface performs word-aligned instruction fetches only. Misaligned instruction fetches are handled by performing two separate word-aligned instruction fetches. Internally, the core can deal with both word- and half-word-aligned instruction addresses to support compressed instructions. The LSB of the instruction address is ignored internally.
Protocol
The protocol used to communicate with the instruction cache or the instruction memory is very similar to the protocol used by the LSU on the data interface of Ibex. See the description of the LSU in LSU Protocol for details about this protocol.