hp-saturn/saturn_decoder_block_8.v

  if (do_block_8x) begin
    $display("block_8x %h | op %d", i_nibble, o_alu_op);
    case (i_nibble)
      4'h0:       // 
        block_80x  <= 1;
      4'h2: 
        block_82x  <= 1;
      4'h4, 4'h5: // ST=[01] n
      begin 
        o_alu_op       <= i_nibble[0]?`ALU_OP_SET_BIT:`ALU_OP_RST_BIT;
        block_sr_bit   <= 1;
        `ifdef SIM
        o_unimplemented <= 0;
        `endif
      end
      4'hA: block_8Ax <= 1;
      4'hC, 4'hD, 
      4'hE, 4'hF: // GOLONG, GOVLNG, GOSUBL, GOSBVL
      begin
        o_alu_no_stall <= 1;
        o_alu_op       <= i_nibble[0]?`ALU_OP_JMP_ABS5:`ALU_OP_JMP_REL4;
        // is it a gosub ?
        o_push         <= i_nibble[1];
        o_alu_debug    <= i_nibble[1];
        mem_load_max   <= i_nibble[0]?4:3;
        o_mem_pos      <= 0;
        block_jmp      <= 1;
        // debug for cases not tested
        o_alu_debug    <= !i_nibble[0];  
        `ifdef SIM
        o_unimplemented <= 0;
        `endif   
      end
      default: begin
        $display("block_8x %h error", i_nibble);
        o_dec_error    <= 1;
      end
    endcase
    block_8x           <= 0;
  end

  if (do_block_80x) begin
    $display("block_80x %h | op %d", i_nibble, o_alu_op);
    case (i_nibble)
      4'h5: begin // CONFIG
        next_nibble   <= 0;
        o_ins_decoded <= 1;
      end
      4'hA: begin // RESET
        next_nibble   <= 0;
        o_ins_decoded <= 1;
      end
      4'hC: block_80Cx <= 1;
      default: begin
        $display("block_80x %h error", i_nibble);
        o_dec_error    <= 1;
      end
    endcase
    block_80x          <= 0;
  end

  if (do_block_80Cx) begin
    o_ins_alu_op   <= 1;
    o_alu_op       <= `ALU_OP_COPY;
    next_nibble    <= 0;
    o_ins_decoded  <= 1;
    block_80Cx     <= 0;
  end

  // 821 XM=0
  // 822 SB=0
  // 824 SR=0
  // 828 MP=0
  // 82F CLRHST
  // 82x CLRHST     x
  if (do_block_82x) begin
    o_ins_alu_op    <= 1;
    o_alu_op        <= `ALU_OP_CLR_MASK;
    o_imm_value     <= i_nibble;
    next_nibble     <= 0;
    o_ins_decoded   <= 1;
    `ifdef SIM
    o_unimplemented <= 0;
    `endif
    block_82x       <= 0;
  end

  if (do_block_8Ax) begin
    o_fields_table <= `FT_TABLE_f;
    o_ins_alu_op <= 1;
    o_alu_op <= i_nibble[2]?`ALU_OP_TEST_NEQ:`ALU_OP_TEST_EQ;
   // o_alu_debug <= 1;
    o_mem_pos <= 0;
    mem_load_max <= 1;
    o_ins_decoded <= 1;
    next_nibble <= 0;
    block_jump_test <= 1;
    
    // lauch the ALU into test_go mode
    o_ins_test_go    <= 1;
    block_8Ax <= 0;
  end