Auxiliary Processing Unit (APU)

Auxiliary Processing Unit Interface

Table 6 describes the signals of the Auxiliary Processing Unit interface.

Table 6 Auxiliary Processing Unit interface signals

Signal

Direction

Description

apu_req_o

output

Request valid, will stay high until apu_gnt_i is high for one cycle

apu_gnt_i

input

The other side accepted the request. apu_operands_o, apu_op_o, apu_flags_o may change in the next cycle.

apu_operands_o[2:0][31:0]

output

APU’s operands

apu_op_o[5:0]

output

APU’s operation

apu_flags_o[14:0]

output

APU’s flags

apu_rvalid_i

input

apu_result_i holds valid data when apu_valid_i is high. This signal will be high for exactly one cycle per request

apu_result_i[31:0]

input

APU’s result

apu_flags_i[4:0]

input

APU’s flag result

Protocol

The apu bus interface is derived from to the OBI (Open Bus Interface) protocol. See https://github.com/openhwgroup/core-v-docs/blob/master/cores/obi/OBI-v1.2.pdf for details about the protocol. The CV32E41P apu interface uses the apu_operands_o, apu_op_o, and apu_flags_o as the address signal during the Address phase, indicating its validity with the apu_req_o signal. It uses the apu_result_i and apu_flags_i as the rdata of the response phase. It does not implement the OBI signals: we, be, wdata, auser, wuser, aid, rready, err, ruser, rid. These signals can be thought of as being tied off as specified in the OBI specification. The CV32E41P apu interface can cause up to two outstanding transactions.

Connection with the FPU

The CV32E41P sends FP operands over the apu_operands_o bus; the decoded RV32F operation as ADD, SUB, MUL, etc through the apu_op_o bus; the cast, destination and source formats as well as rounding mode through the apu_flags_o bus. The response is the FPU result and relative output flags as Overflow, Underflow, etc.

APU Tracer

The module cv32e41p_apu_tracer can be used to create a log of the APU interface. It is a behavioral, non-synthesizable, module instantiated in the example testbench that is provided for the cv32e41p_core. It can be enabled during simulation by defining CV32E41P_APU_TRACE.

Output file

The APU trace is written to a log file which is named apu_trace_core_<HARTID>.log, with <HARTID> being the 32 digit hart ID of the core being traced.

Trace output format

The trace output is in tab-separated columns.

  1. Time: The current simulation time.

  2. Register: The register file write address.

  3. Result: The register file write data.