// asic processor plus memory
module system;
//declaration of parameters
parameter 
  step = 10,        // execution time of one instruction
  width = 32,       // data bus width
  addrsize = 12,    // address bus width
  memsize = 1<<addrsize,  // memory size
  sbits = 6;        // number of status bits
reg[width-1:0] MEM[0:memsize-1],  // memory declaration
               IR;                // instruction register declaration
reg[sbits-1:0] SR;                // status register
reg[addrsize-1:0] PC;             // program counter

//definition of instruction fields
'define OPCODE IR[31:28]          // opcode
'define AA IR[23:12]              // operand AA
'define BB IR[11:0]               // operand BB
'define IM IR[27]                 // operand type
				  // 1: immediate 
				  // 0: memory 
'define CCODE IR[27:24]           // branch condition
'define SHL IR[27]                // shift direction
				  // 1: left 
				  // 0: right 
'define SHD IR[16:12]             // shift distance

//instruction codes
'define HLT 4'b0000             // halt 
'define BRA 4'b0001             // branch 
'define NOP 4'b0010             // no-op 
'define STR 4'b0011             // store 
'define SHF 4'b0100             // shift 
'define CPL 4'b0101             // complement 
'define ADD 4'b0110             // add 
'define MUL 4'b0111             // multiply

//condition code and status bit position
'define ALWAYS    0             
'define CARRY     1             
'define EVEN      2            
'define PARITY    3            
'define ZERO      4            
'define NEG       5            

//set flags
task setcondcode;
input [width:0] RES;
begin
SR['ALWAYS]     = 1;
SR['CARRY]      = RES[width];
SR['EVEN]       = ~RES[0];
SR['PARITY]     = ^RES;
SR['ZERO]       = ~(|RES);
SR['NEG]        = RES[width-1];
end;
end task;

//reset: start module
initial
  PC = 0;            // start program
//main cycle for one instruction
always begin         // execution time of one instruction
  #step
  IR = MEM[PC];      //instruction fetch
  PC = PC + 1;       //increment program counter
  case('OPCODE)
  'NOP:;             //nothing to do
  'BRA: if (SR['CCODE])     //branch
	PC = 'BB;
  'STR: if ('IM)                //store
	MEM['BB] = 'AA;         //immediate
	else
	MEM['BB] = MEM['AA];    //direct
  'ADD: begin
	MEM['BB] = MEM['AA] + MEM['BB];
	setcondcode(MEM['BB]);
	end
  'MUL: begin
	MEM['BB] = MEM['AA] * MEM['BB];
	setcondcode(MEM['BB]);
	end
  'CPL: begin
	  if ('IM)                 //complement store
	  MEM['BB] = ~'AA;         //immediate
	  else
	  MEM['BB] = ~MEM['AA];    //direct
	  setcondcode(MEM['BB]);
	  end
  'SHF: begin
	  if ('SHL)                 //shift 
	  MEM['BB] = MEM['BB] << 'SHD;         //left
	  else
	  MEM['BB] = MEM['BB] >> 'SHD;         //right
	  setcondcode(MEM['BB]);
	  end
  'HLT: $finish                    // halt
  default: $display("erreur: mauvaise valeur dans op-code");
  endcase;
  end
endmodule; 




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