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.

Time: The current simulation time.
Register: The register file write address.
Result: The register file write data.