From b4777b378d256e83d64b56a2ae686bc3c09d77c9 Mon Sep 17 00:00:00 2001 From: Marcus Comstedt Date: Sun, 2 Aug 2020 13:29:45 +0200 Subject: [PATCH] Add big-endian support Signed-off-by: Marcus Comstedt --- README.md | 15 ++++++++ picorv32.v | 110 ++++++++++++++++++++++++++++++++++++----------------- 2 files changed, 90 insertions(+), 35 deletions(-) diff --git a/README.md b/README.md index 3f41f0c..a77084c 100644 --- a/README.md +++ b/README.md @@ -320,6 +320,21 @@ uninitialized.) Note that the RISC-V calling convention requires the stack point to be aligned on 16 bytes boundaries (4 bytes for the RV32I soft float calling convention). +#### BIG_ENDIAN_OPERANDS (default 0) + +Set this to 1 to enable big endian addressing for sub-word memory accesses, +i.e. the most significant bits at the lowest address. + +#### BIG_ENDIAN_INSNS (default 0) + +Set this to 1 to enable big endian instruction streams, i.e. the parcel in +`mem_rdata[31:16]` logically precedes the one in `mem_rdata[15:0]`, +and provides lower numbered bits in case the parcels are part of the same +instruction. +This is how the official RISC-V documents say instructions should be encoded +on big endian, but note that current toolchains do not support generating +code in this format. + Cycles per Instruction Performance ---------------------------------- diff --git a/picorv32.v b/picorv32.v index 6364cbe..8966734 100644 --- a/picorv32.v +++ b/picorv32.v @@ -85,7 +85,9 @@ module picorv32 #( parameter [31:0] LATCHED_IRQ = 32'h ffff_ffff, parameter [31:0] PROGADDR_RESET = 32'h 0000_0000, parameter [31:0] PROGADDR_IRQ = 32'h 0000_0010, - parameter [31:0] STACKADDR = 32'h ffff_ffff + parameter [31:0] STACKADDR = 32'h ffff_ffff, + parameter [ 0:0] BIG_ENDIAN_OPERANDS = 0, + parameter [ 0:0] BIG_ENDIAN_INSNS = 0 ) ( input clk, resetn, output reg trap, @@ -366,6 +368,7 @@ module picorv32 #( reg clear_prefetched_high_word; reg [15:0] mem_16bit_buffer; + wire [31:0] mem_rdata_insn; wire [31:0] mem_rdata_latched_noshuffle; wire [31:0] mem_rdata_latched; @@ -381,7 +384,13 @@ module picorv32 #( (COMPRESSED_ISA && mem_xfer && (!last_mem_valid ? mem_la_firstword : mem_la_firstword_reg) && !mem_la_secondword && &mem_rdata_latched[1:0])); assign mem_la_addr = (mem_do_prefetch || mem_do_rinst) ? {next_pc[31:2] + mem_la_firstword_xfer, 2'b00} : {reg_op1[31:2], 2'b00}; - assign mem_rdata_latched_noshuffle = (mem_xfer || LATCHED_MEM_RDATA) ? mem_rdata : mem_rdata_q; + generate if (BIG_ENDIAN_INSNS) begin + assign mem_rdata_insn = { mem_rdata[15:0], mem_rdata[31:16] }; + end else begin + assign mem_rdata_insn = mem_rdata; + end endgenerate + + assign mem_rdata_latched_noshuffle = (mem_xfer || LATCHED_MEM_RDATA) ? mem_rdata_insn : mem_rdata_q; assign mem_rdata_latched = COMPRESSED_ISA && mem_la_use_prefetched_high_word ? {16'bx, mem_16bit_buffer} : COMPRESSED_ISA && mem_la_secondword ? {mem_rdata_latched_noshuffle[15:0], mem_16bit_buffer} : @@ -398,39 +407,70 @@ module picorv32 #( end end - always @* begin - (* full_case *) - case (mem_wordsize) - 0: begin - mem_la_wdata = reg_op2; - mem_la_wstrb = 4'b1111; - mem_rdata_word = mem_rdata; - end - 1: begin - mem_la_wdata = {2{reg_op2[15:0]}}; - mem_la_wstrb = reg_op1[1] ? 4'b1100 : 4'b0011; - case (reg_op1[1]) - 1'b0: mem_rdata_word = {16'b0, mem_rdata[15: 0]}; - 1'b1: mem_rdata_word = {16'b0, mem_rdata[31:16]}; - endcase - end - 2: begin - mem_la_wdata = {4{reg_op2[7:0]}}; - mem_la_wstrb = 4'b0001 << reg_op1[1:0]; - case (reg_op1[1:0]) - 2'b00: mem_rdata_word = {24'b0, mem_rdata[ 7: 0]}; - 2'b01: mem_rdata_word = {24'b0, mem_rdata[15: 8]}; - 2'b10: mem_rdata_word = {24'b0, mem_rdata[23:16]}; - 2'b11: mem_rdata_word = {24'b0, mem_rdata[31:24]}; - endcase - end - endcase - end + generate if (BIG_ENDIAN_OPERANDS) begin + always @* begin + (* full_case *) + case (mem_wordsize) + 0: begin + mem_la_wdata = reg_op2; + mem_la_wstrb = 4'b1111; + mem_rdata_word = mem_rdata; + end + 1: begin + mem_la_wdata = {2{reg_op2[15:0]}}; + mem_la_wstrb = reg_op1[1] ? 4'b0011 : 4'b1100; + case (reg_op1[1]) + 1'b0: mem_rdata_word = {16'b0, mem_rdata[31:16]}; + 1'b1: mem_rdata_word = {16'b0, mem_rdata[15: 0]}; + endcase + end + 2: begin + mem_la_wdata = {4{reg_op2[7:0]}}; + mem_la_wstrb = 4'b1000 >> reg_op1[1:0]; + case (reg_op1[1:0]) + 2'b00: mem_rdata_word = {24'b0, mem_rdata[31:24]}; + 2'b01: mem_rdata_word = {24'b0, mem_rdata[23:16]}; + 2'b10: mem_rdata_word = {24'b0, mem_rdata[15: 8]}; + 2'b11: mem_rdata_word = {24'b0, mem_rdata[ 7: 0]}; + endcase + end + endcase + end + end else begin // if (BIG_ENDIAN_OPERANDS) + always @* begin + (* full_case *) + case (mem_wordsize) + 0: begin + mem_la_wdata = reg_op2; + mem_la_wstrb = 4'b1111; + mem_rdata_word = mem_rdata; + end + 1: begin + mem_la_wdata = {2{reg_op2[15:0]}}; + mem_la_wstrb = reg_op1[1] ? 4'b1100 : 4'b0011; + case (reg_op1[1]) + 1'b0: mem_rdata_word = {16'b0, mem_rdata[15: 0]}; + 1'b1: mem_rdata_word = {16'b0, mem_rdata[31:16]}; + endcase + end + 2: begin + mem_la_wdata = {4{reg_op2[7:0]}}; + mem_la_wstrb = 4'b0001 << reg_op1[1:0]; + case (reg_op1[1:0]) + 2'b00: mem_rdata_word = {24'b0, mem_rdata[ 7: 0]}; + 2'b01: mem_rdata_word = {24'b0, mem_rdata[15: 8]}; + 2'b10: mem_rdata_word = {24'b0, mem_rdata[23:16]}; + 2'b11: mem_rdata_word = {24'b0, mem_rdata[31:24]}; + endcase + end + endcase + end + end endgenerate // else: !if(BIG_ENDIAN_OPERANDS) always @(posedge clk) begin if (mem_xfer) begin - mem_rdata_q <= COMPRESSED_ISA ? mem_rdata_latched : mem_rdata; - next_insn_opcode <= COMPRESSED_ISA ? mem_rdata_latched : mem_rdata; + mem_rdata_q <= COMPRESSED_ISA ? mem_rdata_latched : mem_rdata_insn; + next_insn_opcode <= COMPRESSED_ISA ? mem_rdata_latched : mem_rdata_insn; end if (COMPRESSED_ISA && mem_done && (mem_do_prefetch || mem_do_rinst)) begin @@ -602,13 +642,13 @@ module picorv32 #( mem_valid <= 1; mem_la_secondword <= 1; if (!mem_la_use_prefetched_high_word) - mem_16bit_buffer <= mem_rdata[31:16]; + mem_16bit_buffer <= mem_rdata_insn[31:16]; end else begin mem_valid <= 0; mem_la_secondword <= 0; if (COMPRESSED_ISA && !mem_do_rdata) begin - if (~&mem_rdata[1:0] || mem_la_secondword) begin - mem_16bit_buffer <= mem_rdata[31:16]; + if (~&mem_rdata_insn[1:0] || mem_la_secondword) begin + mem_16bit_buffer <= mem_rdata_insn[31:16]; prefetched_high_word <= 1; end else begin prefetched_high_word <= 0;