Instruction Fetch

The Instruction Fetch (IF) stage of the CV32E40S is able to supply one instruction to the Instruction Decode (ID ) stage per cycle if the external bus interface is able to serve one instruction per cycle. In case of executing compressed instructions, on average less than one 32-bit instruction fetch will we needed per instruction in the ID stage.

For optimal performance and timing closure reasons, a prefetcher is used which fetches instructions via the external bus interface from for example an externally connected instruction memory or instruction cache.

The prefetch unit performs word-aligned 32-bit prefetches and stores the fetched words in an alignment buffer with three entries. As a result of this (speculative) prefetch, CV32E40S can fetch up to three words outside of the code region and care should therefore be taken that no unwanted read side effects occur for such prefetches outside of the actual code region.

Table 5 describes the signals that are used to fetch instructions. This interface is a simplified version of the interface that is used by the LSU, which is described in Load-Store-Unit (LSU). The difference is that no writes are possible and thus it needs fewer signals.

Table 5 Instruction Fetch interface signals

Signal	Direction	Description
instr_req_o	output	Request valid, will stay high until instr_gnt_i is high for one cycle
instr_reqpar_o	output	Odd parity signal for instr_req_o
instr_gnt_i	input	The other side accepted the request. instr_addr_o, instr_memtype_o and instr_prot_o may change in the next cycle.
instr_gntpar_i	input	Odd parity signal for instr_gnt_i
instr_addr_o[31:0]	output	Address, word aligned
instr_memtype_o[1:0]	output	Memory Type attributes (cacheable, bufferable)
instr_prot_o[2:0]	output	Protection attributes
instr_achk_o[12:0]	output	Checksum for address phase signals
instr_dbg_o	output	Debug mode access
instr_rvalid_i	input	instr_rdata_i and instr_err_i are valid when instr_rvalid_i is high. This signal will be high for exactly one cycle per request.
instr_rvalidpar_i	input	Odd parity signal for instr_rvalid_i
instr_rdata_i[31:0]	input	Data read from memory
instr_err_i	input	An instruction interface error occurred
instr_rchk_i[4:0]	input	Checksum for response phase signals

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 instruction bus interface is compliant to the OBI protocol (see [OPENHW-OBI] for detailed signal and protocol descriptions). The CV32E40S instruction fetch interface does not implement the following optional OBI signals: we, be, wdata, auser, wuser, aid, mid, rready, rreadypar, ruser, rid, atop, exokay. These signals can be thought of as being tied off as specified in [OPENHW-OBI]. The CV32E40S instruction fetch interface can cause up to two outstanding transactions.

Figure 3 and Figure 5 show example timing diagrams of the protocol.

Figure 3 Back-to-back Memory Transactions

Figure 4 Back-to-back Memory Transactions with bus errors on A2/RD2 and A4/RD4

Figure 5 Multiple Outstanding Memory Transactions

Figure 6 Multiple Outstanding Memory Transactions with bus error on A1/RD1

Interface integrity

The CV32E40S implements interface integrity by the instr_reqpar_o, instr_gntpar_i, instr_rvalidpar_i, instr_achk_o and instr_rchk_i signals (see see Interface integrity and [OPENHW-OBI] for further details).