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Add big-endian support #172

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Add big-endian support #172

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b4777b3
Add big-endian support
zeldin Aug 2, 2020
cc1c843
Update big endian instruction encoding
zeldin Oct 20, 2020
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11 changes: 11 additions & 0 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -320,6 +320,17 @@ 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 byteswapping of the instruction stream.
This is required on big endian because instructions are stored in little
endian byteorder.


Cycles per Instruction Performance
----------------------------------
Expand Down
110 changes: 75 additions & 35 deletions picorv32.v
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Expand Up @@ -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,
Expand Down Expand Up @@ -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;

Expand All @@ -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[7:0], mem_rdata[15:8], mem_rdata[23:16], mem_rdata[31:24] };
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} :
Expand All @@ -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
Expand Down Expand Up @@ -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;
Expand Down