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////////////////////////////////////////////////////////////////////////////////
//
// Filename:	idecode.v
//
// Project:	Zip CPU -- a small, lightweight, RISC CPU soft core
//
// Purpose:	This RTL file specifies how instructions are to be decoded
//		into their underlying meanings.  This is specifically a version
//	designed to support a "Next Generation", or "Version 2" instruction
//	set as (currently) activated by the OPT_NEW_INSTRUCTION_SET option
//	in cpudefs.v.
//
//	I expect to (eventually) retire the old instruction set, at which point
//	this will become the default instruction set decoder.
//
//
// Creator:	Dan Gisselquist, Ph.D.
//		Gisselquist Technology, LLC
//
////////////////////////////////////////////////////////////////////////////////
//
// Copyright (C) 2015-2017, Gisselquist Technology, LLC
//
// This program is free software (firmware): you can redistribute it and/or
// modify it under the terms of  the GNU General Public License as published
// by the Free Software Foundation, either version 3 of the License, or (at
// your option) any later version.
//
// This program is distributed in the hope that it will be useful, but WITHOUT
// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
// for more details.
//
// You should have received a copy of the GNU General Public License along
// with this program.  (It's in the $(ROOT)/doc directory.  Run make with no
// target there if the PDF file isn't present.)  If not, see
// <http://www.gnu.org/licenses/> for a copy.
//
// License:	GPL, v3, as defined and found on www.gnu.org,
//		http://www.gnu.org/licenses/gpl.html
//
//
////////////////////////////////////////////////////////////////////////////////
//
//
//
`define	CPU_SP_REG	4'hd
`define	CPU_CC_REG	4'he
`define	CPU_PC_REG	4'hf
//
`include "cpudefs.v"
//
//
//
module	idecode(i_clk, i_rst, i_ce, i_stalled,
		i_instruction, i_gie, i_pc, i_pf_valid,
			i_illegal,
		o_valid,
		o_phase, o_illegal,
		o_pc, o_gie,
		o_dcdR, o_dcdA, o_dcdB, o_I, o_zI,
		o_cond, o_wF,
		o_op, o_ALU, o_M, o_DV, o_FP, o_break, o_lock,
		o_wR, o_rA, o_rB,
		o_early_branch, o_branch_pc, o_ljmp,
		o_pipe,
		o_sim, o_sim_immv
		);
	parameter	ADDRESS_WIDTH=24, IMPLEMENT_MPY=1, EARLY_BRANCHING=1,
			IMPLEMENT_DIVIDE=1, IMPLEMENT_FPU=0, AW = ADDRESS_WIDTH;
	input			i_clk, i_rst, i_ce, i_stalled;
	input	[31:0]		i_instruction;
	input			i_gie;
	input	[(AW-1):0]	i_pc;
	input			i_pf_valid, i_illegal;
	output	wire		o_valid, o_phase;
	output	reg		o_illegal;
	output	reg	[AW:0]	o_pc;
	output	reg		o_gie;
	output	reg	[6:0]	o_dcdR, o_dcdA, o_dcdB;
	output	wire	[31:0]	o_I;
	output	reg		o_zI;
	output	reg	[3:0]	o_cond;
	output	reg		o_wF;
	output	reg	[3:0]	o_op;
	output	reg		o_ALU, o_M, o_DV, o_FP, o_break;
	output	wire		o_lock;
	output	reg		o_wR, o_rA, o_rB;
	output	wire		o_early_branch;
	output	wire	[(AW-1):0]	o_branch_pc;
	output	wire		o_ljmp;
	output	wire		o_pipe;
	output	reg		o_sim		/* verilator public_flat */;
	output	reg	[22:0]	o_sim_immv	/* verilator public_flat */;
 
	wire	dcdA_stall, dcdB_stall, dcdF_stall;
	wire			o_dcd_early_branch;
	wire	[(AW-1):0]	o_dcd_branch_pc;
	reg	o_dcdI, o_dcdIz;
`ifdef	OPT_PIPELINED
	reg	r_lock;
`endif
`ifdef	OPT_PIPELINED_BUS_ACCESS
	reg	r_pipe;
`endif
 
 
	wire	[4:0]	w_op;
	wire		w_ldi, w_mov, w_cmptst, w_ldilo, w_ALU, w_brev,
			w_noop, w_lock;
	wire	[4:0]	w_dcdR, w_dcdB, w_dcdA;
	wire		w_dcdR_pc, w_dcdR_cc;
	wire		w_dcdA_pc, w_dcdA_cc;
	wire		w_dcdB_pc, w_dcdB_cc;
	wire	[3:0]	w_cond;
	wire		w_wF, w_mem, w_sto, w_div, w_fpu;
	wire		w_wR, w_rA, w_rB, w_wR_n;
	wire		w_ljmp, w_ljmp_dly, w_cis_ljmp;
	wire	[31:0]	iword;
 
 
`ifdef	OPT_CIS
	reg	[15:0]	r_nxt_half;
	assign	iword = (o_phase)
				// set second half as a NOOP ... but really
				// shouldn't matter
			? { r_nxt_half[15:0], i_instruction[15:0] }
			: i_instruction;
`else
	assign	iword = { 1'b0, i_instruction[30:0] };
`endif
 
	generate
	if (EARLY_BRANCHING != 0)
	begin
`ifdef	OPT_CIS
		reg	r_pre_ljmp;
		always @(posedge i_clk)
		if ((i_rst)||(o_early_branch))
			r_pre_ljmp <= 1'b0;
		else if ((i_ce)&&(i_pf_valid))
			r_pre_ljmp <= (!o_phase)&&(i_instruction[31])
				&&(i_instruction[14:0] == 15'h7cf8);
		else if (i_ce)
			r_pre_ljmp <= 1'b0;
 
		assign	w_cis_ljmp = r_pre_ljmp;
`else
		assign	w_cis_ljmp = 1'b0;
`endif
		// 0.1111.10010.000.1.1111.000000000...
		// 0111.1100.1000.0111.11000....
		assign	w_ljmp = (iword == 32'h7c87c000);
	end else begin
		assign	w_cis_ljmp = 1'b0;
		assign	w_ljmp = 1'b0;
	end
	endgenerate
 
`ifdef	OPT_CIS
`ifdef	VERILATOR
	wire	[4:0]	w_cis_op;
	always @(iword)
		if (!iword[31])
			w_cis_op = w_op;
		else case(iword[26:24])
		3'h0: w_cis_op = 5'h00;
		3'h1: w_cis_op = 5'h01;
		3'h2: w_cis_op = 5'h02;
		3'h3: w_cis_op = 5'h10;
		3'h4: w_cis_op = 5'h12;
		3'h5: w_cis_op = 5'h13;
		3'h6: w_cis_op = 5'h18;
		3'h7: w_cis_op = 5'h0d;
		endcase
`else
	reg	[4:0]	w_cis_op;
	always @(iword,w_op)
		if (!iword[31])
			w_cis_op <= w_op;
		else case(iword[26:24])
		3'h0: w_cis_op <= 5'h00;
		3'h1: w_cis_op <= 5'h01;
		3'h2: w_cis_op <= 5'h02;
		3'h3: w_cis_op <= 5'h10;
		3'h4: w_cis_op <= 5'h12;
		3'h5: w_cis_op <= 5'h13;
		3'h6: w_cis_op <= 5'h18;
		3'h7: w_cis_op <= 5'h0d;
		endcase
`endif
`else
	wire	[4:0]	w_cis_op;
	assign	w_cis_op = w_op;
`endif
 
	assign	w_op= iword[26:22];
	assign	w_mov    = (w_cis_op      == 5'h0d);
	assign	w_ldi    = (w_cis_op[4:1] == 4'hc);
	assign	w_brev   = (w_cis_op      == 5'h8);
	assign	w_cmptst = (w_cis_op[4:1] == 4'h8);
	assign	w_ldilo  = (w_cis_op[4:0] == 5'h9);
	assign	w_ALU    = (!w_cis_op[4]) // anything with [4]==0, but ...
				&&(w_cis_op[3:1] != 3'h7); // not the divide
 
 
	// w_dcdR (4 LUTs)
	//
	// What register will we be placing results into (if at all)?
	//
	// Two parts to the result register: the register set, given for
	// moves in iword[18] but only for the supervisor, and the other
	// four bits encoded in the instruction.
	//
	assign	w_dcdR = { ((!iword[31])&&(w_mov)&&(~i_gie))?iword[18]:i_gie,
				iword[30:27] };
	// 2 LUTs
	//
	// If the result register is either CC or PC, and this would otherwise
	// be a floating point instruction with floating point opcode of 0,
	// then this is a NOOP.
	assign	w_lock   = (!iword[31])&&(w_op[4:0]==5'h1d)&&(
				((IMPLEMENT_FPU>0)&&(w_dcdR[3:1]==3'h7))
				||(IMPLEMENT_FPU==0));
`ifdef	OPT_PIPELINED
	assign	w_noop   = (!iword[31])&&(w_op[4:0] == 5'h1f)&&(
			((IMPLEMENT_FPU>0)&&(w_dcdR[3:1] == 3'h7))
			||(IMPLEMENT_FPU==0));
`else
	// Allow w_lock's to set the w_noop bit in the case of no pipelining
	assign	w_noop   = (!iword[31])
			&&((w_op[4:0] == 5'h1f)||(w_op[4:0]==5'h1d))
			&&(
				((IMPLEMENT_FPU>0)&&(w_dcdR[3:1] == 3'h7))
				||(IMPLEMENT_FPU==0));
`endif
 
	// dcdB - What register is used in the opB?
	//
	assign w_dcdB[4] = ((!iword[31])&&(w_mov)&&(~i_gie))?iword[13]:i_gie;
	assign w_dcdB[3:0]= (iword[31])
				? (((!iword[23])&&(iword[26:25]==2'b10))
					? `CPU_SP_REG : iword[22:19])
				: iword[17:14];
 
	// 0 LUTs
	assign	w_dcdA = w_dcdR;	// on ZipCPU, A is always result reg
	// 2 LUTs, 1 delay each
	assign	w_dcdR_pc = (w_dcdR == {i_gie, `CPU_PC_REG});
	assign	w_dcdR_cc = (w_dcdR == {i_gie, `CPU_CC_REG});
	// 0 LUTs
	assign	w_dcdA_pc = w_dcdR_pc;
	assign	w_dcdA_cc = w_dcdR_cc;
	// 2 LUTs, 1 delays each
	assign	w_dcdB_pc = (w_rB)&&(w_dcdB[3:0] == `CPU_PC_REG);
	assign	w_dcdB_cc = (w_rB)&&(w_dcdB[3:0] == `CPU_CC_REG);
 
	// Under what condition will we execute this
	// instruction?  Only the load immediate instruction
	// is completely unconditional.
	//
	// 3+4 LUTs
	assign	w_cond = ((w_ldi)||(iword[31])) ? 4'h8 :
			{ (iword[21:19]==3'h0), iword[21:19] };
 
	// 1 LUT
	assign	w_mem    = (w_cis_op[4:3] == 2'b10)&&(w_cis_op[2:1] !=2'b00);
	assign	w_sto     = (w_mem)&&( w_cis_op[0]);
	// 1 LUT
	assign	w_div     = (!iword[31])&&(w_op[4:1] == 4'h7);
	// 2 LUTs
	assign	w_fpu   = (!iword[31])&&(w_op[4:3] == 2'b11)
				&&(w_dcdR[3:1] != 3'h7)&&(w_op[2:1] != 2'b00);
	//
	// rA - do we need to read register A?
	assign	w_rA = // Floating point reads reg A
			((w_fpu)&&(w_cis_op[4:1] != 4'hf))
			// Divide's read A
			||(w_div)
			// ALU ops read A,
			//	except for MOV's and BREV's which don't
			||((w_ALU)&&(!w_brev)&&(!w_mov))
			// STO's read A
			||(w_sto)
			// Test/compares
			||(w_cmptst);
	// rB -- do we read a register for operand B?  Specifically, do we
	// add the registers value to the immediate to create opB?
	assign	w_rB     = (w_mov)
				||((!iword[31])&&(iword[18])&&(!w_ldi))
				||(( iword[31])&&(iword[23])&&(!w_ldi))
				// If using compressed instruction sets,
				// we *always* read on memory operands.
				||(( iword[31])&&(w_mem));
	// wR -- will we be writing our result back?
	// wR_n = !wR
	// 1 LUT: All but STO, NOOP/BREAK/LOCK, and CMP/TST write back to w_dcdR
	assign	w_wR_n   = (w_sto)
				||((!iword[31])&&(w_cis_op[4:3]==2'b11)
					&&(w_cis_op[2:1]!=2'b00)
					&&(w_dcdR[3:1]==3'h7))
				||(w_cmptst);
	assign	w_wR     = ~w_wR_n;
	//
	// wF -- do we write flags when we are done?
	//
	assign	w_wF     = (w_cmptst)
			||((w_cond[3])&&((w_fpu)||(w_div)
				||((w_ALU)&&(!w_mov)&&(!w_ldilo)&&(!w_brev)
					&&(w_dcdR[3:1] != 3'h7))));
 
	// Bottom 13 bits: no LUT's
	// w_dcd[12: 0] -- no LUTs
	// w_dcd[   13] -- 2 LUTs
	// w_dcd[17:14] -- (5+i0+i1) = 3 LUTs, 1 delay
	// w_dcd[22:18] : 5 LUTs, 1 delay (assuming high bit is o/w determined)
	reg	[22:0]	r_I;
	wire	[22:0]	w_I, w_fullI;
	wire		w_Iz;
 
	assign	w_fullI = (w_ldi) ? { iword[22:0] } // LDI
			// MOVE immediates have one less bit
			:((w_mov) ?{ {(23-13){iword[12]}}, iword[12:0] }
			// Normal Op-B immediate ... 18 or 14 bits
			:((~iword[18]) ? { {(23-18){iword[17]}}, iword[17:0] }
			: { {(23-14){iword[13]}}, iword[13:0] }
			));
 
`ifdef	OPT_CIS
	wire	[7:0]	w_halfbits;
	assign	w_halfbits = iword[23:16];
 
	wire	[7:0]	w_halfI;
	assign	w_halfI = (iword[26:24]==3'h6) ? w_halfbits[7:0]
			:(w_halfbits[7])?
				{ {(6){w_halfbits[2]}}, w_halfbits[1:0]}
				:{ w_halfbits[6], w_halfbits[6:0] };
	assign	w_I  = (iword[31])?{{(23-8){w_halfI[7]}}, w_halfI }:w_fullI;
`else
	assign	w_I  = w_fullI;
`endif
	assign	w_Iz = (w_I == 0);
 
 
`ifdef	OPT_CIS
	//
	// The o_phase parameter is special.  It needs to let the software
	// following know that it cannot break/interrupt on an o_phase asserted
	// instruction, lest the break take place between the first and second
	// half of a CIS instruction.  To do this, o_phase must be asserted
	// when the first instruction half is valid, but not asserted on either
	// a 32-bit instruction or the second half of a 2x16-bit instruction.
	reg	r_phase;
	initial	r_phase = 1'b0;
	always @(posedge i_clk)
		if ((i_rst) // When no instruction is in the pipe, phase is zero
			||(o_early_branch)||(w_ljmp_dly))
			r_phase <= 1'b0;
		else if ((i_ce)&&(i_pf_valid))
			r_phase <= (o_phase)? 1'b0
				: ((i_instruction[31])&&(i_pf_valid));
		else if (i_ce)
			r_phase <= 1'b0;
	// Phase is '1' on the first instruction of a two-part set
	// But, due to the delay in processing, it's '1' when our output is
	// valid for that first part, but that'll be the same time we
	// are processing the second part ... so it may look to us like a '1'
	// on the second half of processing.
 
	assign	o_phase = r_phase;
`else
	assign	o_phase = 1'b0;
`endif
 
 
	initial	o_illegal = 1'b0;
	always @(posedge i_clk)
		if (i_rst)
			o_illegal <= 1'b0;
		else if (i_ce)
		begin
`ifdef	OPT_CIS
			o_illegal <= (i_illegal);
`else
			o_illegal <= ((i_illegal) || (i_instruction[31]));
`endif
			if ((IMPLEMENT_MPY==0)&&((w_cis_op[4:1]==4'h5)||(w_cis_op[4:0]==5'h0c)))
				o_illegal <= 1'b1;
 
			if ((IMPLEMENT_DIVIDE==0)&&(w_div))
				o_illegal <= 1'b1;
			else if ((IMPLEMENT_DIVIDE!=0)&&(w_div)&&(w_dcdR[3:1]==3'h7))
				o_illegal <= 1'b1;
 
 
			if ((IMPLEMENT_FPU==0)&&(w_fpu))
				o_illegal <= 1'b1;
 
			if ((w_cis_op[4:3]==2'b11)&&(w_cis_op[2:1]!=2'b00)
				&&(w_dcdR[3:1]==3'h7)
				&&(
					(w_cis_op[2:0] != 3'h4)	// BREAK
// `ifdef	OPT_PIPELINED
					&&(w_cis_op[2:0] != 3'h5)	// LOCK
// `endif
					// SIM instructions are always illegal
					&&(w_cis_op[2:0] != 3'h7)))	// NOOP
				o_illegal <= 1'b1;
		end
 
 
	always @(posedge i_clk)
		if (i_ce)
		begin
`ifdef	OPT_CIS
			if (!o_phase)
				o_gie<= i_gie;
 
			if (iword[31])
			begin
				if (o_phase)
					o_pc <= o_pc + 1'b1;
				else if (i_pf_valid)
					o_pc <= { i_pc, 1'b1 };
			end else begin
				// The normal, non-CIS case
				o_pc <= { i_pc + 1'b1, 1'b0 };
			end
`else
			o_gie<= i_gie;
			o_pc <= { i_pc + 1'b1, 1'b0 };
`endif
 
			// Under what condition will we execute this
			// instruction?  Only the load immediate instruction
			// is completely unconditional.
			o_cond <= w_cond;
			// Don't change the flags on conditional instructions,
			// UNLESS: the conditional instruction was a CMP
			// or TST instruction.
			o_wF <= w_wF;
 
			// Record what operation/op-code (4-bits) we are doing
			//	Note that LDI magically becomes a MOV
			// 	instruction here.  That way it's a pass through
			//	the ALU.  Likewise, the two compare instructions
			//	CMP and TST becomes SUB and AND here as well.
			// We keep only the bottom four bits, since we've
			// already done the rest of the decode necessary to
			// settle between the other instructions.  For example,
			// o_FP plus these four bits uniquely defines the FP
			// instruction, o_DV plus the bottom of these defines
			// the divide, etc.
			o_op <= ((w_ldi)||(w_noop))? 4'hd : w_cis_op[3:0];
 
			// Default values
			o_dcdR <= { w_dcdR_cc, w_dcdR_pc, w_dcdR};
			o_dcdA <= { w_dcdA_cc, w_dcdA_pc, w_dcdA};
			o_dcdB <= { w_dcdB_cc, w_dcdB_pc, w_dcdB};
			o_wR  <= w_wR;
			o_rA  <= w_rA;
			o_rB  <= w_rB;
			r_I    <= w_I;
			o_zI   <= w_Iz;
 
			// Turn a NOOP into an ALU operation--subtract in
			// particular, although it doesn't really matter as long
			// as it doesn't take longer than one clock.  Note
			// also that this depends upon not setting any registers
			// or flags, which should already be true.
			o_ALU  <=  (w_ALU)||(w_ldi)||(w_cmptst)||(w_noop);
			o_M    <=  w_mem;
			o_DV   <=  w_div;
			o_FP   <=  w_fpu;
 
			o_break <= (!iword[31])&&(w_op[4:0]==5'h1c)&&(
				((IMPLEMENT_FPU>0)&&(w_dcdR[3:1]==3'h7))
				||(IMPLEMENT_FPU==0));
`ifdef	OPT_PIPELINED
			r_lock  <= w_lock;
`endif
`ifdef	OPT_CIS
			r_nxt_half <= { iword[31], iword[14:0] };
`endif
 
`ifdef	VERILATOR
			// Support the SIM instruction(s)
			o_sim <= (!iword[31])&&(w_op[4:1] == 4'hf)
				&&(w_dcdR[3:1] == 3'h7);
`else
			o_sim <= 1'b0;
`endif
			o_sim_immv <= iword[22:0];
		end
 
`ifdef	OPT_PIPELINED
	assign	o_lock = r_lock;
`else
	assign	o_lock = 1'b0;
`endif
 
	generate
	if (EARLY_BRANCHING!=0)
	begin
		reg			r_early_branch, r_ljmp;
		reg	[(AW-1):0]	r_branch_pc;
 
		initial r_ljmp = 1'b0;
		always @(posedge i_clk)
			if (i_rst)
				r_ljmp <= 1'b0;
`ifdef	OPT_CIS
			else if ((i_ce)&&(o_phase))
				r_ljmp <= w_cis_ljmp;
`endif
			else if ((i_ce)&&(i_pf_valid))
				r_ljmp <= (w_ljmp);
		assign	o_ljmp = r_ljmp;
 
		always @(posedge i_clk)
		if (i_rst)
			r_early_branch <= 1'b0;
		else if ((i_ce)&&(i_pf_valid))
		begin
			if (r_ljmp)
				// LOD (PC),PC
				r_early_branch <= 1'b1;
			else if ((!iword[31])&&(iword[30:27]==`CPU_PC_REG)
					&&(w_cond[3]))
			begin
				if ((w_op[4:0]==5'h02)&&(!iword[18]))
					// Add x,PC
					r_early_branch     <= 1'b1;
				else
					r_early_branch     <= 1'b0;
			end else
				r_early_branch <= 1'b0;
		end else if (i_ce)
			r_early_branch <= 1'b0;
 
		always @(posedge i_clk)
			if (i_ce)
			begin
				if (r_ljmp)
					r_branch_pc <= iword[(AW+1):2];
				else // Add x,PC
				r_branch_pc <= i_pc
					+ {{(AW-15){iword[17]}},iword[16:2]}
					+ {{(AW-1){1'b0}},1'b1};
			end
 
		assign	w_ljmp_dly         = r_ljmp;
		assign	o_early_branch     = r_early_branch;
		assign	o_branch_pc        = r_branch_pc;
	end else begin
		assign	w_ljmp_dly         = 1'b0;
		assign	o_early_branch = 1'b0;
		assign	o_branch_pc = {(AW){1'b0}};
		assign	o_ljmp = 1'b0;
	end endgenerate
 
 
	// To be a pipeable operation there must be ...
	//	1. Two valid adjacent instructions
	//	2. Both must be memory operations, of the same time (both lods
	//		or both stos)
	//	3. Both must use the same register base address
	//	4. Both must be to the same address, or the address incremented
	//		by one
	// Note that we're not using iword here ... there's a lot of logic
	// taking place, and it's only valid if the new word is not compressed.
	//
	reg	r_valid;
`ifdef	OPT_PIPELINED_BUS_ACCESS
	initial	r_pipe = 1'b0;
	always @(posedge i_clk)
		if (i_ce)
			r_pipe <= (r_valid)&&((i_pf_valid)||(o_phase))
				// Both must be memory operations
				&&(w_mem)&&(o_M)
				// Both must be writes, or both stores
				&&(o_op[0] == w_cis_op[0])
				// Both must be register ops
				&&(w_rB)
				// Both must use the same register for B
				&&(w_dcdB[3:0] == o_dcdB[3:0])
				// But ... the result can never be B
				&&((o_op[0])
					||(w_dcdB[3:0] != o_dcdA[3:0]))
				// Needs to be to the mode, supervisor or user
				&&(i_gie == o_gie)
				// Same condition, or no condition before
				&&((i_instruction[21:19]==o_cond[2:0])
					||(o_cond[2:0] == 3'h0))
				// Same immediate
				&&((w_I[13:2]==r_I[13:2])
					||({1'b0, w_I[13:2]}==(r_I[13:2]+12'h1)));
	assign o_pipe = r_pipe;
`else
	assign o_pipe = 1'b0;
`endif
 
	always @(posedge i_clk)
		if (i_rst)
			r_valid <= 1'b0;
		else if (i_ce)
			r_valid <= ((i_pf_valid)||(o_phase)||(i_illegal))
					&&(!o_ljmp)&&(!o_early_branch);
		else if (!i_stalled)
			r_valid <= 1'b0;
 
	assign	o_valid = r_valid;
 
 
	assign	o_I = { {(32-22){r_I[22]}}, r_I[21:0] };
 
endmodule