decode shift suck less
authorChristopher Lu <lu@nona.hsd1.pa.comcast.net>
Sun, 28 Dec 2008 08:09:20 +0000 (03:09 -0500)
committerChristopher Lu <lu@nona.hsd1.pa.comcast.net>
Sun, 28 Dec 2008 08:09:20 +0000 (03:09 -0500)
Decode.v

index 42ae993..7d7f95b 100644 (file)
--- a/Decode.v
+++ b/Decode.v
@@ -31,13 +31,13 @@ module Decode(
        assign regs1 = (read_1 == 4'b1111) ? rpc : rdata_1;
        assign regs2 = rdata_2; /* use regs2 for things that cannot be r15 */
 
-       IHATEARMSHIFT blowme(.insn(insn),
-                            .operand(regs1),
-                            .reg_amt(regs2),
-                            .cflag_in(incpsr[`CPSR_C]),
-                            .res(shift_res),
-                            .cflag_out(shift_cflag_out));
-       
+       IREALLYHATEARMSHIFT blowme(.insn(insn),
+                                  .operand(regs1),
+                                  .reg_amt(regs2),
+                                  .cflag_in(incpsr[`CPSR_C]),
+                                  .res(shift_res),
+                                  .cflag_out(shift_cflag_out));
+
        always @(*)
                casez (insn)
                32'b????000000??????????????1001????,   /* Multiply -- must come before ALU, because it pattern matches a specific case of ALU */
@@ -226,7 +226,7 @@ module Decode(
 
 endmodule
 
-module IHATEARMSHIFT(
+module IREALLYHATEARMSHIFT(
        input [31:0] insn,
        input [31:0] operand,
        input [31:0] reg_amt,
@@ -235,41 +235,84 @@ module IHATEARMSHIFT(
        output cflag_out
 );
        wire [5:0] shift_amt;
-       wire elanus;
+       wire rshift_cout, is_arith, is_rot;
+       wire [31:0] rshift_res;
+
+       assign shift_amt = insn[4] ? {|reg_amt[7:5], reg_amt[4:0]}     /* reg-specified shift */
+                                  : {insn[11:7] == 5'b0, insn[11:7]}; /* immediate shift */
 
+       SuckLessShifter biteme(.oper(operand),
+                              .carryin(cflag_in),
+                              .amt(shift_amt),
+                              .is_arith(is_arith),
+                              .is_rot(is_rot),
+                              .res(rshift_res),
+                              .carryout(rshift_cout));
 
-       /* might want to write our own damn shifter that does arithmetic/logical efficiently and stuff */
        always @(*)
-               if(insn[4]) begin
-                       shift_amt = {|reg_amt[7:5], reg_amt[4:0]};
-                       elanus = 1'b1;
-               end else begin
-                       shift_amt = {insn[11:7] == 5'b0, insn[11:7]};
-                       elanus = 1'b0;
+               case (insn[6:5])
+               `SHIFT_LSL: begin
+                       is_rot = 1'b0;
+                       is_arith = 1'b0;
                end
-       
+               `SHIFT_LSR: begin
+                       is_rot = 1'b0;
+                       is_arith = 1'b0;
+               end
+               `SHIFT_ASR: begin
+                       is_rot = 1'b0;
+                       is_arith = 1'b1;
+               end
+               `SHIFT_ROR: begin
+                       is_rot = 1'b1;
+                       is_arith = 1'b0;
+               end
+               endcase
+
        always @(*)
                case (insn[6:5]) /* shift type */
-               `SHIFT_LSL: begin
-                       {cflag_out, res} = {cflag_in, operand} << {elanus & shift_amt[5], shift_amt[4:0]};
-               end
+               `SHIFT_LSL:
+                       {cflag_out, res} = {cflag_in, operand} << {insn[4] & shift_amt[5], shift_amt[4:0]};
                `SHIFT_LSR: begin
-                       {res, cflag_out} = {operand, cflag_in} >> shift_amt;
+                       res = rshift_res;
+                       cflag_out = rshift_cout;
                end
                `SHIFT_ASR: begin
-                       {res, cflag_out} = {operand, cflag_in} >> shift_amt | (operand[31] ? ~(33'h1FFFFFFFF >> shift_amt) : 33'b0);
+                       res = rshift_res;
+                       cflag_out = rshift_cout;
                end
                `SHIFT_ROR: begin
-                       if(!elanus && shift_amt[4:0] == 5'b0) begin /* RRX x.x */
+                       if(!insn[4] && shift_amt[4:0] == 5'b0) begin /* RRX x.x */
                                res = {cflag_in, operand[31:1]};
                                cflag_out = operand[0];
-                       end else if(shift_amt == 6'b0) begin
-                               res = operand;
-                               cflag_out = cflag_in;
-                       end else begin
-                               res = operand >> shift_amt[4:0] | operand << (5'b0 - shift_amt[4:0]);
-                               cflag_out = operand[shift_amt[4:0] - 5'b1];
+                       else
+                               res = rshift_res;
+                               cflag_out = rshift_cout;
                        end
                end
                endcase
 endmodule
+
+module SuckLessShifter(
+       input [31:0] oper,
+       input carryin,
+       input [5:0] amt,
+       input is_arith,
+       input is_rot,
+       output [31:0] res,
+       output carryout
+);
+
+       wire [32:0] stage1, stage2, stage3, stage4, stage5;
+
+       wire pushbits = is_arith & operand[31];
+
+       /* do a barrel shift */
+       assign stage1 = amt[5] ? {is_rot ? oper : {32{pushbits}}, oper[31]} : {oper, carryin};
+       assign stage2 = amt[4] ? {is_rot ? stage1[15:0] : {16{pushbits}}, stage1[31:16], stage1[16]} : stage1;
+       assign stage3 = amt[3] ? {is_rot ? stage2[7:0] : {8{pushbits}}, stage2[31:8], stage2[8]} : stage2;
+       assign stage4 = amt[2] ? {is_rot ? stage3[3:0] : {4{pushbits}}, stage3[31:4], stage3[4]} : stage3;
+       assign stage5 = amt[1] ? {is_rot ? stage4[1:0] : {2{pushbits}}, stage4[31:2], stage4[2]} : stage4;
+       assign {res, carryout} = amt[0] ? {is_rot ? stage4[0] : pushbits, stage5[31:1], stage5[1]} : stage5;
+
+endmodule
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