/* Main instruction fetch/decode loop */

while (reason == 0) {                                   /* loop until halted */

    // clean SIMH loop exit code here

    // SIMH clock queue code here

/* ---PiDP add--------------------------------------------------------------------------------------------- */

    // handle PiDP-8/i Sing Step and Sing Inst switch behavior here

/* ---PiDP end---------------------------------------------------------------------------------------------- */

    this_Major_State = next_Major_State;
    switch (this_Major_State) {

        case FETCH_state:
            // fetch state for all instructions, regardless of op code
            // perform actual instruction fetch here

            // handle SIMH breakpoint here

            // handle SIMH history here   

            op_code = (IR >> 9) & 07;
            switch (op_code) {
                case 4: 
                    // special JMS-specific on top of AND, TAD, DCA, ISZ action here
                    // intentional fall-through for JMS
                case 0:case 1:case 2:case 3:
                    // Fetch state for MRIs: AND, TAD, ISZ, DCA, JMS

                    // handle current page / zero page with direct / indirect here

                    if (IR & 0400)                                  /* indirect or direct? */
                        next_Major_State = DEFER_state;             /* indirect */
                    else
                        next_Major_State = EXECUTE_state;           /* direct */
                    break;
                    // end of case op_code 0..4: AND, TAD, ISZ, DCA, JMS

                case 5:
                    // Fetch state for JMP, including emulating timeshare control

                    // handle current page / zero page with direct / indirect here

                    if (IR & 0400)                                  /* direct or indirect? */
                        next_Major_State = DEFER_state;             /* indirect JMP */
                    else {
                        // handle direct jump and timeshare control
                        }                                           /* end direct JMP */
                    break;
                    // end of case op_code 5: JMP

                case 6:
                    // Fetch state for IOTs

                    // handle all IOTs including timeshare control card

                    break;
                    // end of case op_code 6: IOT

                case 7:
                    // Fetch state for OPRs

                    // handle all Operate group 1, 2, 3 standard, and 3 EAE variations

                    // group 3 EAE includes Mode A and Mode B emulation

                    break;
                    // end of case op_code 7

                } // end of switch (op_code)
                break;

            // end of case FETCH_state


        case DEFER_state:

            // handle DEFER major state for MRIs and JMP

            if (((IR >> 9) & 07) != 5)                      /* MRI or JMP? */
                next_Major_State = EXECUTE_state;           /* it's a MRI */
            else {
                if (UF) {                                   /* it's a JMP, user mode? */
                    // handle timeshare control
                    }
                }
                next_Major_State = FETCH_state;
            }
            break;
            // end of case DEFER_state


        case EXECUTE_state:

            // handle EXECUTE major state for MRIs and JMS

            if (((IR >> 9) & 07) < 4) {                     /* AND .. DCA, or is it JMS? */
                if (IR & 00400)                             /* it is AND .. DCA, direct or indirect? */
                    MA = DF | (MA & 07777);                 /* indirect, use DF */
                else
                    MA = IF | (MA & 07777);                 /* direct, use IF */
                MB = M[MA];                                 /* get the data word */
                switch ((IR >> 9) & 07) {
                    case 0:                                 /* AND */
                        // execute AND here
                        break;
                    case 1:                                 /* TAD */
                        // execute TAD here
                        break;
                    case 2:                                 /* ISZ */
                        // execute ISZ here
                        break;
                    case 3:                                 /* DCA */
                        // execute DCA here
                        break;
                    }  // end of switch ((IR >> 9) & 07)
                }
            else {
                // execute JMS here, including timeshare control
                }
            next_Major_State = FETCH_state;
            break;
            // end of case EXECUTE_state


        }  // end of switch (Major_State)


/* ---PiDP add--------------------------------------------------------------------------------------------- */

    // Update the front panel with this Major State's ending state.

/* ---PiDP end---------------------------------------------------------------------------------------------- */

    // handle hardware device interrupt here

    }                                                   /* end while (reason == 0) */