PDP8/pdp8_cpu.c

/* pdp8_cpu.c: PDP-8 CPU simulator

   Copyright (c) 1993-2003, Robert M Supnik

   Permission is hereby granted, free of charge, to any person obtaining a
   copy of this software and associated documentation files (the "Software"),
   to deal in the Software without restriction, including without limitation
   the rights to use, copy, modify, merge, publish, distribute, sublicense,
   and/or sell copies of the Software, and to permit persons to whom the
   Software is furnished to do so, subject to the following conditions:

   The above copyright notice and this permission notice shall be included in
   all copies or substantial portions of the Software.

   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
   ROBERT M SUPNIK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
   IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
   CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

   Except as contained in this notice, the name of Robert M Supnik shall not
   be used in advertising or otherwise to promote the sale, use or other dealings
   in this Software without prior written authorization from Robert M Supnik.

   cpu		central processor

   12-Mar-03	RMS	Added logical name support
   04-Oct-02	RMS	Revamped device dispatching, added device number support
   06-Jan-02	RMS	Added device enable/disable routines
   30-Dec-01	RMS	Added old PC queue
   16-Dec-01	RMS	Fixed bugs in EAE
   07-Dec-01	RMS	Revised to use new breakpoint package
   30-Nov-01	RMS	Added RL8A, extended SET/SHOW support
   16-Sep-01	RMS	Fixed bug in reset routine, added KL8A support
   10-Aug-01	RMS	Removed register from declarations
   17-Jul-01	RMS	Moved function prototype
   07-Jun-01	RMS	Fixed bug in JMS to non-existent memory
   25-Apr-01	RMS	Added device enable/disable support
   18-Mar-01	RMS	Added DF32 support
   05-Mar-01	RMS	Added clock calibration support
   15-Feb-01	RMS	Added DECtape support
   14-Apr-99	RMS	Changed t_addr to unsigned

   The register state for the PDP-8 is:

   AC<0:11>		accumulator
   MQ<0:11>		multiplier-quotient
   L			link flag
   PC<0:11>		program counter
   IF<0:2>		instruction field
   IB<0:2>		instruction buffer
   DF<0:2>		data field
   UF			user flag
   UB			user buffer
   SF<0:6>		interrupt save field

   The PDP-8 has three instruction formats: memory reference, I/O transfer,
   and operate.  The memory reference format is:

     0  1  2  3  4  5  6  7  8  9 10 11
   +--+--+--+--+--+--+--+--+--+--+--+--+
   |   op   |in|zr|    page offset     |	memory reference
   +--+--+--+--+--+--+--+--+--+--+--+--+

   <0:2>	mnemonic	action

    000		AND		AC = AC & M[MA]
    001		TAD		L'AC = AC + M[MA]
    010		DCA		M[MA] = AC, AC = 0
    011		ISZ		M[MA] = M[MA] + 1, skip if M[MA] == 0
    100		JMS		M[MA] = PC, PC = MA + 1
    101		JMP		PC = MA

   <3:4>	mode		action
    00 	page zero		MA = IF'0'IR<5:11>
    01	current page		MA = IF'PC<0:4>'IR<5:11>
    10	indirect page zero	MA = xF'M[IF'0'IR<5:11>]
    11	indirect current page	MA = xF'M[IF'PC<0:4>'IR<5:11>]

   where x is D for AND, TAD, ISZ, DCA, and I for JMS, JMP.

   Memory reference instructions can access an address space of 32K words.
   The address space is divided into eight 4K word fields; each field is
   divided into thirty-two 128 word pages.  An instruction can directly
   address, via its 7b offset, locations 0-127 on page zero or on the current
   page.  All 32k words can be accessed via indirect addressing and the
   instruction and data field registers.  If an indirect address is in
   locations 0010-0017 of any field, the indirect address is incremented
   and rewritten to memory before use.
*/

/* The I/O transfer format is as follows:

     0  1  2  3  4  5  6  7  8  9 10 11
   +--+--+--+--+--+--+--+--+--+--+--+--+
   |   op   |      device     | pulse  |	I/O transfer
   +--+--+--+--+--+--+--+--+--+--+--+--+

   The IO transfer instruction sends the the specified pulse to the
   specified I/O device.  The I/O device may take data from the AC,
   return data to the AC, initiate or cancel operations, or skip on
   status.

   The operate format is as follows:

   +--+--+--+--+--+--+--+--+--+--+--+--+
   | 1| 1| 1| 0|  |  |  |  |  |  |  |  |	operate group 1
   +--+--+--+--+--+--+--+--+--+--+--+--+
	        |  |  |  |  |  |  |  |
	        |  |  |  |  |  |  |  +--- increment AC	3
	        |  |  |  |  |  |  +--- rotate 1 or 2	4
	        |  |  |  |  |  +--- rotate left		4
	        |  |  |  |  +--- rotate right		4
	        |  |  |  +--- complement L		2
	        |  |  +--- complement AC		2
	        |  +--- clear L				1
	        +-- clear AC				1

   +--+--+--+--+--+--+--+--+--+--+--+--+
   | 1| 1| 1| 1|  |  |  |  |  |  |  | 0|	operate group 2
   +--+--+--+--+--+--+--+--+--+--+--+--+
	        |  |  |  |  |  |  |
	        |  |  |  |  |  |  +--- halt		3
	        |  |  |  |  |  +--- or switch register	3
	        |  |  |  |  +--- reverse skip sense	1
	        |  |  |  +--- skip on L != 0		1
	        |  |  +--- skip on AC == 0		1
	        |  +--- skip on AC < 0			1
	        +-- clear AC				2

   +--+--+--+--+--+--+--+--+--+--+--+--+
   | 1| 1| 1| 1|  |  |  |  |  |  |  | 1|	operate group 3
   +--+--+--+--+--+--+--+--+--+--+--+--+
		|  |  |  | \______/
		|  |  |  |     |
		|  |  +--|-----+--- EAE command		3
		|  |     +--- AC -> MQ, 0 -> AC		2
		|  +--- MQ v AC --> AC			2
		+-- clear AC				1

  The operate instruction can be microprogrammed to perform operations
  on the AC, MQ, and link.
*/

/* This routine is the instruction decode routine for the PDP-8.
   It is called from the simulator control program to execute
   instructions in simulated memory, starting at the simulated PC.
   It runs until 'reason' is set non-zero.

   General notes:

   1. Reasons to stop.  The simulator can be stopped by:

	HALT instruction
	breakpoint encountered
	unimplemented instruction and stop_inst flag set
	I/O error in I/O simulator

   2. Interrupts.  Interrupts are maintained by three parallel variables:

	dev_done 	device done flags
	int_enable	interrupt enable flags
	int_req		interrupt requests

      In addition, int_req contains the interrupt enable flag, the
      CIF not pending flag, and the ION not pending flag.  If all
      three of these flags are set, and at least one interrupt request
      is set, then an interrupt occurs.

   3. Non-existent memory.  On the PDP-8, reads to non-existent memory
      return zero, and writes are ignored.  In the simulator, the
      largest possible memory is instantiated and initialized to zero.
      Thus, only writes outside the current field (indirect writes) need
      be checked against actual memory size.

   3. Adding I/O devices.  These modules must be modified:

	pdp8_defs.h	add device number and interrupt definitions
	pdp8_sys.c	add sim_devices table entry
*/

#include "pdp8_defs.h"

#define PCQ_SIZE	64				/* must be 2**n */
#define PCQ_MASK	(PCQ_SIZE - 1)
#define PCQ_ENTRY	pcq[pcq_p = (pcq_p - 1) & PCQ_MASK] = PC
#define UNIT_V_NOEAE	(UNIT_V_UF)			/* EAE absent */
#define UNIT_NOEAE	(1 << UNIT_V_NOEAE)
#define UNIT_V_MSIZE	(UNIT_V_UF+1)			/* dummy mask */
#define UNIT_MSIZE	(1 << UNIT_V_MSIZE)

uint16 M[MAXMEMSIZE] = { 0 };				/* main memory */
int32 saved_LAC = 0;					/* saved L'AC */
int32 saved_MQ = 0;					/* saved MQ */
int32 saved_PC = 0;					/* saved IF'PC */
int32 saved_DF = 0;					/* saved Data Field */
int32 IB = 0;						/* Instruction Buffer */
int32 SF = 0;						/* Save Field */
int32 emode = 0;					/* EAE mode */
int32 gtf = 0;						/* EAE gtf flag */
int32 SC = 0;						/* EAE shift count */
int32 UB = 0;						/* User mode Buffer */
int32 UF = 0;						/* User mode Flag */
int32 OSR = 0;						/* Switch Register */
int16 pcq[PCQ_SIZE] = { 0 };				/* PC queue */
int32 pcq_p = 0;					/* PC queue ptr */
REG *pcq_r = NULL;					/* PC queue reg ptr */
int32 dev_done = 0;					/* dev done flags */
int32 int_enable = INT_INIT_ENABLE;			/* intr enables */
int32 int_req = 0;					/* intr requests */
int32 stop_inst = 0;					/* trap on ill inst */
int32 (*dev_tab[DEV_MAX])(int32 IR, int32 dat);		/* device dispatch */

extern int32 sim_interval;
extern int32 sim_int_char;
extern int32 sim_brk_types, sim_brk_dflt, sim_brk_summ;	/* breakpoint info */
extern DEVICE *sim_devices[];
extern FILE *sim_log;
extern UNIT clk_unit, ttix_unit;

t_stat cpu_ex (t_value *vptr, t_addr addr, UNIT *uptr, int32 sw);
t_stat cpu_dep (t_value val, t_addr addr, UNIT *uptr, int32 sw);
t_stat cpu_reset (DEVICE *dptr);
t_stat cpu_set_size (UNIT *uptr, int32 val, char *cptr, void *desc);
t_bool build_dev_tab (void);

/* CPU data structures

   cpu_dev	CPU device descriptor
   cpu_unit	CPU unit descriptor
   cpu_reg	CPU register list
   cpu_mod	CPU modifier list
*/

UNIT cpu_unit = { UDATA (NULL, UNIT_FIX + UNIT_BINK, MAXMEMSIZE) };

REG cpu_reg[] = {
	{ ORDATA (PC, saved_PC, 15) },
	{ ORDATA (AC, saved_LAC, 12) },
	{ FLDATA (L, saved_LAC, 12) },
	{ ORDATA (MQ, saved_MQ, 12) },
	{ ORDATA (SR, OSR, 12) },
	{ GRDATA (IF, saved_PC, 8, 3, 12) },
	{ GRDATA (DF, saved_DF, 8, 3, 12) },
	{ GRDATA (IB, IB, 8, 3, 12) },
	{ ORDATA (SF, SF, 7) },
	{ FLDATA (UB, UB, 0) },
	{ FLDATA (UF, UF, 0) },
	{ ORDATA (SC, SC, 5) },
	{ FLDATA (GTF, gtf, 0) },
	{ FLDATA (EMODE, emode, 0) },
	{ FLDATA (ION, int_req, INT_V_ION) },
	{ FLDATA (ION_DELAY, int_req, INT_V_NO_ION_PENDING) },
	{ FLDATA (CIF_DELAY, int_req, INT_V_NO_CIF_PENDING) },
	{ FLDATA (PWR_INT, int_req, INT_V_PWR) },
	{ FLDATA (UF_INT, int_req, INT_V_UF) },
	{ ORDATA (INT, int_req, INT_V_ION+1), REG_RO },
	{ ORDATA (DONE, dev_done, INT_V_DIRECT), REG_RO },
	{ ORDATA (ENABLE, int_enable, INT_V_DIRECT), REG_RO },
	{ BRDATA (PCQ, pcq, 8, 15, PCQ_SIZE), REG_RO+REG_CIRC },
	{ ORDATA (PCQP, pcq_p, 6), REG_HRO },
	{ FLDATA (STOP_INST, stop_inst, 0) },
	{ ORDATA (WRU, sim_int_char, 8) },
	{ NULL }  };

MTAB cpu_mod[] = {
	{ UNIT_NOEAE, UNIT_NOEAE, "no EAE", "NOEAE", NULL },
	{ UNIT_NOEAE, 0, "EAE", "EAE", NULL },
	{ UNIT_MSIZE, 4096, NULL, "4K", &cpu_set_size },
	{ UNIT_MSIZE, 8192, NULL, "8K", &cpu_set_size },
	{ UNIT_MSIZE, 12288, NULL, "12K", &cpu_set_size },
	{ UNIT_MSIZE, 16384, NULL, "16K", &cpu_set_size },
	{ UNIT_MSIZE, 20480, NULL, "20K", &cpu_set_size },
	{ UNIT_MSIZE, 24576, NULL, "24K", &cpu_set_size },
	{ UNIT_MSIZE, 28672, NULL, "28K", &cpu_set_size },
	{ UNIT_MSIZE, 32768, NULL, "32K", &cpu_set_size },
	{ 0 }  };

DEVICE cpu_dev = {
	"CPU", &cpu_unit, cpu_reg, cpu_mod,
	1, 8, 15, 1, 8, 12,
	&cpu_ex, &cpu_dep, &cpu_reset,
	NULL, NULL, NULL,
	NULL, 0 };

t_stat sim_instr (void)
{
int32 IR, MB, IF, DF, LAC, MQ;
uint32 PC, MA;
int32 device, pulse, temp, iot_data;
t_stat reason;

/* Restore register state */

if (build_dev_tab ()) return SCPE_STOP;			/* build dev_tab */
PC = saved_PC & 007777;					/* load local copies */
IF = saved_PC & 070000;
DF = saved_DF & 070000;
LAC = saved_LAC & 017777;
MQ = saved_MQ & 07777;
int_req = INT_UPDATE;
reason = 0;
sim_rtcn_init (clk_unit.wait, TMR_CLK);			/* init clk calib */
sim_rtcn_init (ttix_unit.wait, TMR_TTX);		/* init ttx calib */

/* Main instruction fetch/decode loop */

while (reason == 0) {					/* loop until halted */
if (sim_interval <= 0) {				/* check clock queue */
	if (reason = sim_process_event ()) break;  }

if (int_req > INT_PENDING) {				/* interrupt? */
	int_req = int_req & ~INT_ION;			/* interrupts off */
	SF = (UF << 6) | (IF >> 9) | (DF >> 12);	/* form save field */
	IF = IB = DF = UF = UB = 0;			/* clear mem ext */
	PCQ_ENTRY;					/* save old PC */
	M[0] = PC;					/* save PC in 0 */
	PC = 1;  }					/* fetch next from 1 */

MA = IF | PC;						/* form PC */
if (sim_brk_summ && sim_brk_test (MA, SWMASK ('E'))) {	/* breakpoint? */
	reason = STOP_IBKPT;				/* stop simulation */
	break;  }

IR = M[MA];						/* fetch instruction */
PC = (PC + 1) & 07777;					/* increment PC */
int_req = int_req | INT_NO_ION_PENDING;			/* clear ION delay */
sim_interval = sim_interval - 1;

/* Instruction decoding.

   The opcode (IR<0:2>), indirect flag (IR<3>), and page flag (IR<4>)
   are decoded together.  This produces 32 decode points, four per
   major opcode.  For IOT, the extra decode points are not useful;
   for OPR, only the group flag (IR<3>) is used.

   The following macros define the address calculations for data and
   jump calculations.  Data calculations return a full 15b extended
   address, jump calculations a 12b field-relative address.

   Autoindex calculations always occur within the same field as the
   instruction fetch.  The field must exist; otherwise, the instruction
   fetched would be 0000, and indirect addressing could not occur.

   Note that MA contains IF'PC.
*/

#define ZERO_PAGE	MA = IF | (IR & 0177)
#define CURR_PAGE	MA = (MA & 077600) | (IR & 0177)
#define INDIRECT	if ((MA & 07770) != 00010) MA = DF | M[MA]; \
			else MA = DF | (M[MA] = (M[MA] + 1) & 07777)

#define ZERO_PAGE_J	MA = IR & 0177
#define CURR_PAGE_J	MA = (MA & 007600) | (IR & 0177)
#define INDIRECT_J	if ((MA & 07770) != 00010) MA = M[MA]; \
			else MA = (M[MA] = (M[MA] + 1) & 07777)
#define CHANGE_FIELD	IF = IB; UF = UB; \
			int_req = int_req | INT_NO_CIF_PENDING

switch ((IR >> 7) & 037) {				/* decode IR<0:4> */

/* Opcode 0, AND */

case 000:						/* AND, dir, zero */
	ZERO_PAGE;
	LAC = LAC & (M[MA] | 010000);
	break;
case 001:						/* AND, dir, curr */
	CURR_PAGE;
	LAC = LAC & (M[MA] | 010000);
	break;
case 002:						/* AND, indir, zero */
	ZERO_PAGE;
	INDIRECT;
	LAC = LAC & (M[MA] | 010000);
	break;
case 003:						/* AND, indir, curr */
	CURR_PAGE;
	INDIRECT;
	LAC = LAC & (M[MA] | 010000);
	break;

/* Opcode 1, TAD */

case 004:						/* TAD, dir, zero */
	ZERO_PAGE;
	LAC = (LAC + M[MA]) & 017777;
	break;
case 005:						/* TAD, dir, curr */
	CURR_PAGE;
	LAC = (LAC + M[MA]) & 017777;
	break;
case 006:						/* TAD, indir, zero */
	ZERO_PAGE;
	INDIRECT;
	LAC = (LAC + M[MA]) & 017777;
	break;
case 007:						/* TAD, indir, curr */
	CURR_PAGE;
	INDIRECT;
	LAC = (LAC + M[MA]) & 017777;
	break;

/* Opcode 2, ISZ */

case 010:						/* ISZ, dir, zero */
	ZERO_PAGE;
	M[MA] = MB = (M[MA] + 1) & 07777;		/* field must exist */
	if (MB == 0) PC = (PC + 1) & 07777;
	break;
case 011:						/* ISZ, dir, curr */
       	CURR_PAGE;
	M[MA] = MB = (M[MA] + 1) & 07777;		/* field must exist */
	if (MB == 0) PC = (PC + 1) & 07777;
	break;
case 012:						/* ISZ, indir, zero */
	ZERO_PAGE;
	INDIRECT;
	MB = (M[MA] + 1) & 07777;
	if (MEM_ADDR_OK (MA)) M[MA] = MB;
	if (MB == 0) PC = (PC + 1) & 07777;
	break;
case 013:						/* ISZ, indir, curr */
	CURR_PAGE;
	INDIRECT;
	MB = (M[MA] + 1) & 07777;
	if (MEM_ADDR_OK (MA)) M[MA] = MB;
	if (MB == 0) PC = (PC + 1) & 07777;
	break;

/* Opcode 3, DCA */

case 014:						/* DCA, dir, zero */
	ZERO_PAGE;
	M[MA] = LAC & 07777;
	LAC = LAC & 010000;
	break;
case 015:						/* DCA, dir, curr */
	CURR_PAGE;
	M[MA] = LAC & 07777;
	LAC = LAC & 010000;
	break;
case 016:						/* DCA, indir, zero */
	ZERO_PAGE;
	INDIRECT;
	if (MEM_ADDR_OK (MA)) M[MA] = LAC & 07777;
	LAC = LAC & 010000;
	break;
case 017:						/* DCA, indir, curr */
	CURR_PAGE;
	INDIRECT;
	if (MEM_ADDR_OK (MA)) M[MA] = LAC & 07777;
	LAC = LAC & 010000;
	break;

/* Opcode 4, JMS */

case 020:						/* JMS, dir, zero */
	ZERO_PAGE_J;
	CHANGE_FIELD;
	MA = IF | MA;
	PCQ_ENTRY;
	if (MEM_ADDR_OK (MA)) M[MA] = PC;
	PC = (MA + 1) & 07777;
	break;
case 021:						/* JMS, dir, curr */
	CURR_PAGE_J;
	CHANGE_FIELD;
	MA = IF | MA;
	PCQ_ENTRY;
	if (MEM_ADDR_OK (MA)) M[MA] = PC;
	PC = (MA + 1) & 07777;
	break;
case 022:						/* JMS, indir, zero */
	ZERO_PAGE;
	INDIRECT_J;
	CHANGE_FIELD;
	MA = IF | MA;
	PCQ_ENTRY;
	if (MEM_ADDR_OK (MA)) M[MA] = PC;
	PC = (MA + 1) & 07777;
	break;
case 023:						/* JMS, indir, curr */
	CURR_PAGE;
	INDIRECT_J;
	CHANGE_FIELD;
	MA = IF | MA;
	PCQ_ENTRY;
	if (MEM_ADDR_OK (MA)) M[MA] = PC;
	PC = (MA + 1) & 07777;
	break;

/* Opcode 5, JMP */

case 024:						/* JMP, dir, zero */
	ZERO_PAGE_J;
	CHANGE_FIELD;
	PCQ_ENTRY;
	PC = MA;
	break;
case 025:						/* JMP, dir, curr */
	CURR_PAGE_J;
	CHANGE_FIELD;
	PCQ_ENTRY;
	PC = MA;
	break;
case 026:						/* JMP, indir, zero */
	ZERO_PAGE;
	INDIRECT_J;
	CHANGE_FIELD;
	PCQ_ENTRY;
	PC = MA;
	break;
case 027:						/* JMP, indir, curr */
	CURR_PAGE;
	INDIRECT_J;
	CHANGE_FIELD;
	PCQ_ENTRY;
	PC = MA;
	break;

/* Opcode 7, OPR group 1 */

case 034:case 035:					/* OPR, group 1 */
	switch ((IR >> 4) & 017) {			/* decode IR<4:7> */
	case 0:						/* nop */
	    break;
	case 1:						/* CML */
	    LAC = LAC ^ 010000;
	    break;
	case 2:						/* CMA */
	    LAC = LAC ^ 07777;
	    break;
	case 3:						/* CMA CML */
	    LAC = LAC ^ 017777;
	    break;
	case 4:						/* CLL */
	    LAC = LAC & 07777;
	    break;
	case 5:						/* CLL CML = STL */
	    LAC = LAC | 010000;
	    break;
	case 6:						/* CLL CMA */
	    LAC = (LAC ^ 07777) & 07777;
	    break;
	case 7:						/* CLL CMA CML */
	    LAC = (LAC ^ 07777) | 010000;
	    break;
	case 010:					/* CLA */
	    LAC = LAC & 010000;
	    break;
	case 011:					/* CLA CML */
	    LAC = (LAC & 010000) ^ 010000;
	    break;
	case 012:					/* CLA CMA = STA */
	    LAC = LAC | 07777;
	    break;
	case 013:					/* CLA CMA CML */
	    LAC = (LAC | 07777) ^ 010000;
	    break;
	case 014:					/* CLA CLL */
	    LAC = 0;
	    break;
	case 015:					/* CLA CLL CML */
	    LAC = 010000;
	    break;
	case 016:					/* CLA CLL CMA */
	    LAC = 07777;
	    break;
	case 017:					/* CLA CLL CMA CML */
	    LAC = 017777;
	    break;  }					/* end switch opers */

/* OPR group 1, continued */

	if (IR & 01) LAC = (LAC + 1) & 017777;		/* IAC */
	switch ((IR >> 1) & 07) {			/* decode IR<8:10> */
	case 0: 					/* nop */
	    break;
	case 1:						/* BSW */
	    LAC = (LAC & 010000) | ((LAC >> 6) & 077) | ((LAC & 077) << 6);
	    break;
	case 2:						/* RAL */
	    LAC = ((LAC << 1) | (LAC >> 12)) & 017777;
	    break;
	case 3:						/* RTL */
	    LAC = ((LAC << 2) | (LAC >> 11)) & 017777;
	    break;
	case 4:						/* RAR */
	    LAC = ((LAC >> 1) | (LAC << 12)) & 017777;
	    break;
	case 5:						/* RTR */
	    LAC = ((LAC >> 2) | (LAC << 11)) & 017777;
	    break;
	case 6:						/* RAL RAR - undef */
	    LAC = LAC & (IR | 010000);			/* uses AND path */
	    break;
	case 7:						/* RTL RTR - undef */
	    LAC = (LAC & 010000) | (MA & 07600) | (IR & 0177);
	    break;  }					/* uses address path */
	break;						/* end group 1 */

/* OPR group 2 */

case 036:case 037:					/* OPR, groups 2, 3 */
	if ((IR & 01) == 0) {				/* group 2 */
	    switch ((IR >> 3) & 017) {			/* decode IR<6:8> */
	    case 0: 					/* nop */
		break;
	    case 1:					/* SKP */
		PC = (PC + 1) & 07777;
		break;
	    case 2: 					/* SNL */
		if (LAC >= 010000) PC = (PC + 1) & 07777;
		break;
	    case 3: 					/* SZL */
		if (LAC < 010000) PC = (PC + 1) & 07777;
		break;
	    case 4: 					/* SZA */
		if ((LAC & 07777) == 0) PC = (PC + 1) & 07777;
		break;
	    case 5: 					/* SNA */
		if ((LAC & 07777) != 0) PC = (PC + 1) & 07777;
		break;
	    case 6:					/* SZA | SNL */
		if ((LAC == 0) || (LAC >= 010000))
		    PC = (PC + 1) & 07777;
		break;
	    case 7:					/* SNA & SZL */
		if ((LAC != 0) && (LAC < 010000)) PC = (PC + 1) & 07777;
		break;
	    case 010: 					/* SMA */
		if ((LAC & 04000) != 0) PC = (PC + 1) & 07777;
		break;
	    case 011: 					/* SPA */
		if ((LAC & 04000) == 0) PC = (PC + 1) & 07777;
		break;
	    case 012:					/* SMA | SNL */
		if (LAC >= 04000) PC = (PC + 1) & 07777;
		break;
	    case 013:					/* SPA & SZL */
		if (LAC < 04000) PC = (PC + 1) & 07777;
		break;
	    case 014:					/* SMA | SZA */
		if (((LAC & 04000) != 0) || ((LAC & 07777) == 0))
		    PC = (PC + 1) & 07777;
		break;
	    case 015:					/* SPA & SNA */
		if (((LAC & 04000) == 0) && ((LAC & 07777) != 0))
			PC = (PC + 1) & 07777;
		break;
	    case 016:					/* SMA | SZA | SNL */
		if ((LAC >= 04000) || (LAC == 0)) PC = (PC + 1) & 07777;
		break;
	    case 017:					/* SPA & SNA & SZL */
		if ((LAC < 04000) && (LAC != 0)) PC = (PC + 1) & 07777;
		break;	}				/* end switch skips */
	    if (IR & 0200) LAC = LAC & 010000;		/* CLA */
	    if ((IR & 06) && UF) int_req = int_req | INT_UF;
	    else {
	    	if (IR & 04) LAC = LAC | OSR;		/* OSR */
		if (IR & 02) reason = STOP_HALT;  }	/* HLT */
	    break;  }				  	/* end group 2 */

/* OPR group 3 standard

   MQA!MQL exchanges AC and MQ, as follows:

	temp = MQ;
	MQ = LAC & 07777;
	LAC = LAC & 010000 | temp;
*/

	temp = MQ;					/* group 3 */
	if (IR & 0200) LAC = LAC & 010000;		/* CLA */
	if (IR & 0020) {				/* MQL */
	    MQ = LAC & 07777;
	    LAC = LAC & 010000;  }
	if (IR & 0100) LAC = LAC | temp;		/* MQA */
	if ((IR & 0056) && (cpu_unit.flags & UNIT_NOEAE)) {
	    reason = stop_inst;				/* EAE not present */
	    break;  }

/* OPR group 3 EAE

   The EAE operates in two modes:

	Mode A, PDP-8/I compatible
	Mode B, extended capability

   Mode B provides eight additional subfunctions; in addition, some
   of the Mode A functions operate differently in Mode B.

   The mode switch instructions are decoded explicitly and cannot be
   microprogrammed with other EAE functions (SWAB performs an MQL as
   part of standard group 3 decoding).  If mode switching is decoded,
   all other EAE timing is suppressed.
*/

	if (IR == 07431) {				/* SWAB */
	    emode = 1;					/* set mode flag */
	    break;  }
	if (IR == 07447) {				/* SWBA */
	    emode = gtf = 0;				/* clear mode, gtf */
	    break;  }

/* If not switching modes, the EAE operation is determined by the mode
   and IR<6,8:10>:

   <6:10>	mode A		mode B		comments

   0x000	NOP		NOP
   0x001	SCL		ACS
   0x010	MUY		MUY		if mode B, next = address
   0x011	DVI		DVI		if mode B, next = address
   0x100	NMI		NMI		if mode B, clear AC if
						 result = 4000'0000
   0x101	SHL		SHL		if mode A, extra shift
   0x110	ASR		ASR		if mode A, extra shift
   0x111	LSR		LSR		if mode A, extra shift
   1x000	SCA		SCA
   1x001	SCA + SCL	DAD
   1x010	SCA + MUY	DST
   1x011	SCA + DVI	SWBA            NOP if not detected earlier
   1x100	SCA + NMI	DPSZ		
   1x101	SCA + SHL	DPIC            must be combined with MQA!MQL
   1x110	SCA + ASR	DCM		must be combined with MQA!MQL
   1x111	SCA + LSR	SAM

   EAE instructions which fetch memory operands use the CPU's DEFER
   state to read the first word; if the address operand is in locations
   x0010 - x0017, it is autoincremented.
*/

/* EAE continued */

	if (emode == 0) gtf = 0;			/* mode A? clr gtf */
	switch ((IR >> 1) & 027) {			/* decode IR<6,8:10> */
	case 020:					/* mode A, B: SCA */
	    LAC = LAC | SC;
	    break;
	case 000:					/* mode A, B: NOP */
	    break;
	case 021:					/* mode B: DAD */
	    if (emode) {
		MA = IF | PC;
		INDIRECT;				/* defer state */
		MQ = MQ + M[MA];
		MA = DF | ((MA + 1) & 07777);
		LAC = (LAC & 07777) + M[MA] + (MQ >> 12);
		MQ = MQ & 07777;
		PC = (PC + 1) & 07777;
		break;  }
	    LAC = LAC | SC;				/* mode A: SCA then */
	case 001:					/* mode B: ACS */
	    if (emode) {
		SC = LAC & 037;
		LAC = LAC & 010000;  }
	    else {					/* mode A: SCL */
		SC = (~M[IF | PC]) & 037;
		PC = (PC + 1) & 07777;  }
	    break;
	case 022:					/* mode B: DST */
	    if (emode) {
		MA = IF | PC;
		INDIRECT;				/* defer state */
		if (MEM_ADDR_OK (MA)) M[MA] = MQ & 07777;
		MA = DF | ((MA + 1) & 07777);
		if (MEM_ADDR_OK (MA))  M[MA] = LAC & 07777;
		PC = (PC + 1) & 07777;
		break;  }
	    LAC = LAC | SC;				/* mode A: SCA then */
	case 002:					/* MUY */
	    MA = IF | PC;
	    if (emode) { INDIRECT; }			/* mode B: defer */
	    temp = (MQ * M[MA]) + (LAC & 07777);
	    LAC = (temp >> 12) & 07777;
	    MQ = temp & 07777;
	    PC = (PC + 1) & 07777;
	    SC = 014;					/* 12 shifts */
	    break;

/* EAE continued */

	case 023:					/* mode B: SWBA */
	    if (emode) break;
	    LAC = LAC | SC;				/* mode A: SCA then */
	case 003:					/* DVI */
	    MA = IF | PC;
	    if (emode) { INDIRECT; }			/* mode B: defer */
	    if ((LAC & 07777) >= M[MA]) {		/* overflow? */
		LAC = LAC | 010000;			/* set link */
		MQ = ((MQ << 1) + 1) & 07777;		/* rotate MQ */
		SC = 01;  }				/* 1 shift */
	    else {
	    	temp = ((LAC & 07777) << 12) | MQ;
		MQ = temp / M[MA];
		LAC = temp % M[MA];
		SC = 015;  }				/* 13 shifts */
	    PC = (PC + 1) & 07777;
	    break;
	case 024:					/* mode B: DPSZ */
	    if (emode) {
		if (((LAC | MQ) & 07777) == 0) PC = (PC + 1) & 07777;
		break;  }
	    LAC = LAC | SC;				/* mode A: SCA then */
	case 004:					/* NMI */
	    temp = (LAC << 12) | MQ;			/* preserve link */
	    for (SC = 0; ((temp & 017777777) != 0) &&
		(temp & 040000000) == ((temp << 1) & 040000000); SC++)
		temp = temp << 1;
	    LAC = (temp >> 12) & 017777;
	    MQ = temp & 07777;
	    if (emode && ((LAC & 07777) == 04000) && (MQ == 0))
		LAC = LAC & 010000;			/* clr if 4000'0000 */
	    break;
	case 025:					/* mode B: DPIC */
	    if (emode) {
		temp = (LAC + 1) & 07777;		/* SWP already done! */
		LAC = MQ + (temp == 0);
		MQ = temp;
		break;  }
	    LAC = LAC | SC;				/* mode A: SCA then */
	case 5:						/* SHL */
	    SC = (M[IF | PC] & 037) + (emode ^ 1);	/* shift+1 if mode A */
	    if (SC > 25) temp = 0;			/* >25? result = 0 */
	    else temp = ((LAC << 12) | MQ) << SC;	/* <=25? shift LAC:MQ */
	    LAC = (temp >> 12) & 017777;
	    MQ = temp & 07777;
	    PC = (PC + 1) & 07777;
	    SC = emode? 037: 0;				/* SC = 0 if mode A */
	    break;

/* EAE continued */

	case 026:					/* mode B: DCM */
	    if (emode) {
		temp = (-LAC) & 07777;			/* SWP already done! */
		LAC = (MQ ^ 07777) + (temp == 0);
		MQ = temp;
		break;  }
	    LAC = LAC | SC;				/* mode A: SCA then */
	case 6:						/* ASR */
	    SC = (M[IF | PC] & 037) + (emode ^ 1);	/* shift+1 if mode A */
	    temp = ((LAC & 07777) << 12) | MQ;		/* sext from AC0 */
	    if (LAC & 04000) temp = temp | ~037777777;
	    if (emode && (SC != 0)) gtf = (temp >> (SC - 1)) & 1;
	    if (SC > 25) temp = (LAC & 04000)? -1: 0;
	    else temp = temp >> SC;
	    LAC = (temp >> 12) & 017777;
	    MQ = temp & 07777;
	    PC = (PC + 1) & 07777;
	    SC = emode? 037: 0;				/* SC = 0 if mode A */
	    break;
	case 027:					/* mode B: SAM */
	    if (emode) {
		temp = LAC & 07777;
		LAC = MQ + (temp ^ 07777) + 1;		/* L'AC = MQ - AC */
		gtf = (temp <= MQ) ^ ((temp ^ MQ) >> 11);
		break;  }
	    LAC = LAC | SC;				/* mode A: SCA then */
	case 7:						/* LSR */
	    SC = (M[IF | PC] & 037) + (emode ^ 1);	/* shift+1 if mode A */
	    temp = ((LAC & 07777) << 12) | MQ;		/* clear link */
	    if (emode && (SC != 0)) gtf = (temp >> (SC - 1)) & 1;
	    if (SC > 24) temp = 0;			/* >24? result = 0 */
	    else temp = temp >> SC;			/* <=24? shift AC:MQ */
	    LAC = (temp >> 12) & 07777;
	    MQ = temp & 07777;
	    PC = (PC + 1) & 07777;
	    SC = emode? 037: 0;				/* SC = 0 if mode A */
	    break;  }					/* end switch */
	break;						/* end case 7 */

/* Opcode 6, IOT */

case 030:case 031:case 032:case 033:			/* IOT */
	if (UF) {					/* privileged? */
	    int_req = int_req | INT_UF;
	    break;  }
	device = (IR >> 3) & 077;			/* device = IR<3:8> */
	pulse = IR & 07;				/* pulse = IR<9:11> */
	iot_data = LAC & 07777;				/* AC unchanged */
	switch (device) {				/* decode IR<3:8> */
	case 0:						/* CPU control */
	    switch (pulse) {				/* decode IR<9:11> */
	    case 0:					/* SKON */
		if (int_req & INT_ION) PC = (PC + 1) & 07777;
		int_req = int_req & ~INT_ION;
		break;
	    case 1:					/* ION */
		int_req = (int_req | INT_ION) & ~INT_NO_ION_PENDING;
		break;
	    case 2:					/* IOF */
		int_req = int_req & ~INT_ION;
		break;
	    case 3:					/* SRQ */
		if (int_req & INT_ALL) PC = (PC + 1) & 07777;
		break;
	    case 4:					/* GTF */
		LAC = (LAC & 010000) |
		      ((LAC & 010000) >> 1) | (gtf << 10) |
		      (((int_req & INT_ALL) != 0) << 9) |
		      (((int_req & INT_ION) != 0) << 7) | SF;
		break;
	    case 5:					/* RTF */
		gtf = ((LAC & 02000) >> 10);
		UB = (LAC & 0100) >> 6;
		IB = (LAC & 0070) << 9;
		DF = (LAC & 0007) << 12;
		LAC = ((LAC & 04000) << 1) | iot_data;
		int_req = (int_req | INT_ION) & ~INT_NO_CIF_PENDING;
		break;
	    case 6:					/* SGT */
		if (gtf) PC = (PC + 1) & 07777;
		break;
	    case 7:					/* CAF */
		gtf = 0;
		emode = 0;
		int_req = int_req & INT_NO_CIF_PENDING;
		dev_done = 0;
		int_enable = INT_INIT_ENABLE;
		LAC = 0;
		break;  }				/* end switch pulse */
	    break;					/* end case 0 */

/* IOT, continued: memory extension */

	case 020:case 021:case 022:case 023:
	case 024:case 025:case 026:case 027:		/* memory extension */
	    switch (pulse) {				/* decode IR<9:11> */
	    case 1: 					/* CDF */
		DF = (IR & 0070) << 9;
		break;
	    case 2:					/* CIF */
		IB = (IR & 0070) << 9;
		int_req = int_req & ~INT_NO_CIF_PENDING;
		break;
	    case 3:					/* CDF CIF */
		DF = IB = (IR & 0070) << 9;
		int_req = int_req & ~INT_NO_CIF_PENDING;
		break;
	    case 4:
		switch (device & 07) {			/* decode IR<6:8> */
		case 0:					/* CINT */
		    int_req = int_req & ~INT_UF;
		    break;
		case 1:					/* RDF */
		    LAC = LAC | (DF >> 9);
		    	break;
		case 2:					/* RIF */
		    LAC = LAC | (IF >> 9);
		    break;
		case 3:					/* RIB */
		    LAC = LAC | SF;
		    break;
		case 4:					/* RMF */
		    UB = (SF & 0100) >> 6;
		    IB = (SF & 0070) << 9;
		    DF = (SF & 0007) << 12;
		    int_req = int_req & ~INT_NO_CIF_PENDING;
		    break;
		case 5:					/* SINT */
		    if (int_req & INT_UF) PC = (PC + 1) & 07777;
		    break;
		case 6:					/* CUF */
		    UB = 0;
		    int_req = int_req & ~INT_NO_CIF_PENDING;
		    break;
		case 7:					/* SUF */
		    UB = 1;
		    int_req = int_req & ~INT_NO_CIF_PENDING;
		    break;  } 				/* end switch device */
		break;
	    default:
		reason = stop_inst;
		break;  }				/* end switch pulse */
	    break;					/* end case 20-27 */

/* IOT, continued: other special cases */

	case 010:					/* power fail */
	    switch (pulse) {				/* decode IR<9:11> */
	    case 1:					/* SBE */
		break;
	    case 2: 					/* SPL */
		if (int_req & INT_PWR) PC = (PC + 1) & 07777;
		break;
	    case 3: 					/* CAL */
		int_req = int_req & ~INT_PWR;
		break;
	    default:
		reason = stop_inst;
		break;  }				/* end switch pulse */
	    break;					/* end case 10 */
	default:					/* I/O device */
	    if (dev_tab[device]) {			/* dev present? */
		iot_data = dev_tab[device] (IR, iot_data);
		LAC = (LAC & 010000) | (iot_data & 07777);
		if (iot_data & IOT_SKP) PC = (PC + 1) & 07777;
		if (iot_data >= IOT_REASON)
		    reason = iot_data >> IOT_V_REASON;  }
	    else reason = stop_inst;			/* stop on flag */
	    break;  }					/* end switch device */
	break;						/* end case IOT */
	}						/* end switch opcode */
}							/* end while */

/* Simulation halted */

saved_PC = IF | (PC & 07777);				/* save copies */
saved_DF = DF & 070000;
saved_LAC = LAC & 017777;
saved_MQ = MQ & 07777;
pcq_r->qptr = pcq_p;					/* update pc q ptr */
return reason;
}							/* end sim_instr */

/* Reset routine */

t_stat cpu_reset (DEVICE *dptr)
{
int_req = (int_req & ~INT_ION) | INT_NO_CIF_PENDING;
saved_DF = IB = saved_PC & 070000;
UF = UB = gtf = emode = 0;
pcq_r = find_reg ("PCQ", NULL, dptr);
if (pcq_r) pcq_r->qptr = 0;
else return SCPE_IERR;
sim_brk_types = sim_brk_dflt = SWMASK ('E');
return SCPE_OK;
}

/* Memory examine */

t_stat cpu_ex (t_value *vptr, t_addr addr, UNIT *uptr, int32 sw)
{
if (addr >= MEMSIZE) return SCPE_NXM;
if (vptr != NULL) *vptr = M[addr] & 07777;
return SCPE_OK;
}

/* Memory deposit */

t_stat cpu_dep (t_value val, t_addr addr, UNIT *uptr, int32 sw)
{
if (addr >= MEMSIZE) return SCPE_NXM;
M[addr] = val & 07777;
return SCPE_OK;
}

/* Memory size change */

t_stat cpu_set_size (UNIT *uptr, int32 val, char *cptr, void *desc)
{
int32 mc = 0;
uint32 i;

if ((val <= 0) || (val > MAXMEMSIZE) || ((val & 07777) != 0))
	return SCPE_ARG;
for (i = val; i < MEMSIZE; i++) mc = mc | M[i];
if ((mc != 0) && (!get_yn ("Really truncate memory [N]?", FALSE)))
	return SCPE_OK;
MEMSIZE = val;
for (i = MEMSIZE; i < MAXMEMSIZE; i++) M[i] = 0;
return SCPE_OK;
}

/* Change device number for a device */

t_stat set_dev (UNIT *uptr, int32 val, char *cptr, void *desc)
{
DEVICE *dptr;
DIB *dibp;
uint32 newdev;
t_stat r;

if (cptr == NULL) return SCPE_ARG;
if (uptr == NULL) return SCPE_IERR;
dptr = find_dev_from_unit (uptr);
if (dptr == NULL) return SCPE_IERR;
dibp = (DIB *) dptr->ctxt;
if (dibp == NULL) return SCPE_IERR;
newdev = get_uint (cptr, 8, DEV_MAX - 1, &r);		/* get new */
if ((r != SCPE_OK) || (newdev == dibp->dev)) return r;
dibp->dev = newdev;					/* store */
return SCPE_OK;
}

/* Show device number for a device */

t_stat show_dev (FILE *st, UNIT *uptr, int32 val, void *desc)
{
DEVICE *dptr;
DIB *dibp;

if (uptr == NULL) return SCPE_IERR;
dptr = find_dev_from_unit (uptr);
if (dptr == NULL) return SCPE_IERR;
dibp = (DIB *) dptr->ctxt;
if (dibp == NULL) return SCPE_IERR;
fprintf (st, "devno=%02o", dibp->dev);
if (dibp->num > 1) fprintf (st, "-%2o", dibp->dev + dibp->num - 1);
return SCPE_OK;
}

/* CPU device handler - should never get here! */

int32 bad_dev (int32 IR, int32 AC)
{
return (SCPE_IERR << IOT_V_REASON) | AC;		/* broken! */
}

/* Build device dispatch table */

t_bool build_dev_tab (void)
{
DEVICE *dptr;
DIB *dibp;
uint32 i, j;
static const uint8 std_dev[] =
	{ 000, 010, 020, 021, 022, 023, 024, 025, 026, 027 };

for (i = 0; i < DEV_MAX; i++) dev_tab[i] = NULL;	/* clr table */
for (i = 0; i < ((uint32) sizeof (std_dev)); i++)	/* std entries */
	dev_tab[std_dev[i]] = &bad_dev;
for (i = 0; (dptr = sim_devices[i]) != NULL; i++) {	/* add devices */
	dibp = (DIB *) dptr->ctxt;			/* get DIB */
	if (dibp && !(dptr->flags & DEV_DIS)) {		/* enabled? */
	    for (j = 0; j < dibp->num; j++) {		/* loop thru disp */
		if (dibp->dsp[j]) {			/* any dispatch? */
		    if (dev_tab[dibp->dev + j]) {	/* already filled? */
			printf ("%s device number conflict at %02o\n",
			    sim_dname (dptr), dibp->dev + j);
			if (sim_log) fprintf (sim_log,
			    "%s device number conflict at %02o\n",
			    sim_dname (dptr), dibp->dev + j);
			 return TRUE;  }
		    dev_tab[dibp->dev + j] = dibp->dsp[j];	/* fill */
		    }					/* end if dsp */
		}					/* end for j */
	    }						/* end if enb */
	}						/* end for i */
return FALSE;
}