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//=============================================================================

// This module implements the instruction execute state logic.

//

//  Copyright (C) 2014-2016  Goran Devic

//

//  This program is free software; 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 2 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 MERCHANTABILITY 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; if not, write to the Free Software Foundation, Inc.,

//  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.

//=============================================================================

// Using a compiled format will include files generated by "gencompile.py" script

// These files are a processed version of "exec_matrix_compiled.vh"

// You would define this on Xilinx and undefine (comment out) on Altera

`define USE_COMPILED_FORMAT

module execute

(

    //----------------------------------------------------------

    // Control signals generated by the instruction execution

    //----------------------------------------------------------

    `include "exec_module.vh"

    output reg nextM,                   // Last M cycle of any instruction

    output reg setM1,                   // Last T clock of any instruction

    output reg fFetch,                  // Function: opcode fetch cycle ("M1")

    output reg fMRead,                  // Function: memory read cycle

    output reg fMWrite,                 // Function: memory write cycle

    output reg fIORead,                 // Function: IO Read cycle

    output reg fIOWrite,                // Function: IO Write cycle

    //----------------------------------------------------------

    // Inputs from the instruction decode PLA

    //----------------------------------------------------------

    input wire [104:0] pla,             // Statically decoded instructions

    //----------------------------------------------------------

    // Inputs from various blocks

    //----------------------------------------------------------

    input wire in_intr,                 // Servicing maskable interrupt

    input wire in_nmi,                  // Servicing non-maskable interrupt

    input wire in_halt,                 // Currently in HALT mode

    input wire im1,                     // Interrupt Mode 1

    input wire im2,                     // Interrupt Mode 2

    input wire use_ixiy,                // Special decode signal

    input wire flags_cond_true,         // Flags condition is true

    input wire repeat_en,               // Enable repeat of a block instruction

    input wire flags_zf,                // ZF to test a condition

    input wire flags_nf,                // NF to test for subtraction

    input wire flags_sf,                // SF to test for 8-bit sign of a value

    input wire flags_cf,                // CF to set HF for CCF

    //----------------------------------------------------------

    // Machine and clock cycles

    //----------------------------------------------------------

    input wire M1,                      // Machine cycle #1

    input wire M2,                      // Machine cycle #2

    input wire M3,                      // Machine cycle #3

    input wire M4,                      // Machine cycle #4

    input wire M5,                      // Machine cycle #5

    input wire T1,                      // T-cycle #1

    input wire T2,                      // T-cycle #2

    input wire T3,                      // T-cycle #3

    input wire T4,                      // T-cycle #4

    input wire T5,                      // T-cycle #5

    input wire T6                       // T-cycle #6

);

// Detects unknown instructions by signalling the known ones

reg validPLA;                           // Valid PLA asserts this reg

// Activates a state machine to compute WZ=IX+d; takes 5T cycles

reg ixy_d;                              // Compute WX=IX+d

// Signals the setting of IX/IY and CB/ED prefix flags; inhibits clearing them

reg setIXIY;                            // Set IX/IY flag at the next T cycle

reg setCBED;                            // Set CB or ED flag at the next T cycle

// Holds asserted by non-repeating versions of block instructions (LDI/CPI,...)

reg nonRep;                             // Non-repeating block instruction

// Suspends incrementing PC through address latch unless in HALT or interrupt mode

reg pc_inc_hold;                        // Normally 0 unless in one of those modes

//--------------------------------------------------------------

// Define various shortcuts to field naming

//--------------------------------------------------------------

`define GP_REG_BC       2'h0

`define GP_REG_DE       2'h1

`define GP_REG_HL       2'h2

`define GP_REG_AF       2'h3

`define PFSEL_P         2'h0

`define PFSEL_V         2'h1

`define PFSEL_IFF2      2'h2

`define PFSEL_REP       2'h3

//--------------------------------------------------------------

// Make available different bits and sections of the opcode byte

//--------------------------------------------------------------

wire op0 = pla[99];

wire op1 = pla[100];

wire op2 = pla[101];

wire op3 = pla[102];

wire op4 = pla[103];

wire op5 = pla[104];

wire [1:0] op21 = { pla[101], pla[100] };

wire [1:0] op54 = { pla[104], pla[103] };

//--------------------------------------------------------------

// 8-bit register selections needs to swizzle mux for A and F

//--------------------------------------------------------------

wire rsel0 = op0 ^ (op1 & op2);

wire rsel3 = op3 ^ (op4 & op5);

`ifdef USE_COMPILED_FORMAT

`include "temp_wires.vh"                // Define all temp wires used with compiled equations

`endif

always @(*) // always_comb

begin

    //-----------------------------------------------------------------------------

    // Default assignment of all control outputs to 0 to prevent generating latches

    //-----------------------------------------------------------------------------

    `include "exec_zero.vh"             // Initial assignment to all ctl wires to zero

    // Reset internal control regs

    validPLA = 0;                       // Will be set by every *valid* PLA entry

    nextM = 0;                          // Will be set to advance to the next M cycle

    setM1 = 0;                          // Will be set on a last M/T cycle of an instruction

    // Reset global machine cycle functions

    fFetch = M1;                        // Fetch is aliased to M1

    fMRead = 0; fMWrite = 0; fIORead = 0; fIOWrite = 0;

    ixy_d = 0;

    setIXIY = 0;

    setCBED = 0;

    nonRep = 0;

    pc_inc_hold = 0;

    //-------------------------------------------------------------------------

    // State-based signal assignment; code generated from Timings spreadsheet

    //-------------------------------------------------------------------------

`ifdef USE_COMPILED_FORMAT

    `include "exec_matrix_compiled.vh"  // Compiled execution equations

`else

    `include "exec_matrix.vh"           // Execution statements in the original nested-if format

`endif

    // Needed by data bus 0 override logic, make only one bus writer active at any time

    ctl_bus_db_oe = ctl_bus_db_oe & ~(ctl_bus_zero_oe | ctl_bus_ff_oe);

end

endmodule