I1401/i1401_cpu.c

/* i1401_cpu.c: IBM 1401 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.

   12-Jul-03	RMS	Moved ASCII/BCD tables to included file
			Revised fetch to model hardware
			Removed length checking in fetch phase
   16-Mar-03	RMS	Fixed mnemonic, instruction lengths, and reverse
			scan length check bug for MCS
			Fixed MCE bug, BS off by 1 if zero suppress
			Fixed chaining bug, D lost if return to SCP
			Fixed H branch, branch occurs after continue
			Added check for invalid 8 character MCW, LCA
   03-Jun-03	RMS	Added 1311 support
   22-May-02	RMS	Added multiply and divide
   30-Dec-01	RMS	Added old PC queue
   30-Nov-01	RMS	Added extended SET/SHOW support
   10-Aug-01	RMS	Removed register in declarations
   07-Dec-00	RMS	Fixed bugs found by Charles Owen
			-- 4,7 char NOPs are legal
			-- 1 char B is chained BCE
			-- MCE moves whole char after first
   14-Apr-99	RMS	Changed t_addr to unsigned

   The register state for the IBM 1401 is:

   IS		I storage address register (PC)
   AS		A storage address register (address of first operand)
   BS		B storage address register (address of second operand)
   ind[0:63]	indicators
   SSA		sense switch A
   IOCHK	I/O check
   PRCHK	process check

   The IBM 1401 is a variable instruction length, decimal data system.
   Memory consists of 4000, 8000, 12000, or 16000 BCD characters, each
   containing six bits of data and a word mark.  There are no general
   registers; all instructions are memory to memory, using explicit
   addresses or an address pointer from a prior instruction.

   BCD numeric data consists of the low four bits of a character (DIGIT),
   encoded as X, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, X, X, X, X, X.  The high
   two bits (ZONE) encode the sign of the data as +, +, -, +.  Character
   data uses all six bits of a character.  Numeric and character fields are
   delimited by a word mark.  Fields are typically processed in descending
   address order (low-order data to high-order data).

   The 1401 encodes a decimal address, and an index register number, in
   three characters:

	character		zone			digit
	addr + 0		<1:0> of thousands	hundreds
	addr + 1		index register #	tens
	addr + 2		<3:2> of thousands	ones

   Normally the digit values 0, 11, 12, 13, 14, 15 are illegal in addresses.
   However, in indexing, digits are passed through the adder, and illegal
   values are normalized to legal counterparts.

   The 1401 has six instruction formats:

	op			A and B addresses, if any, from AS and BS
	op d			A and B addresses, if any, from AS and BS
	op aaa			B address, if any, from BS
	op aaa d		B address, if any, from BS
	op aaa bbb
	op aaa bbb d

   where aaa is the A address, bbb is the B address, and d is a modifier.
   The opcode has word mark set; all other characters have word mark clear.
*/

/* This routine is the instruction decode routine for the IBM 1401.
   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
	illegal addresses or instruction formats
	I/O error in I/O simulator

   2. Interrupts.  The 1401 has no interrupt structure.

   3. Non-existent memory.  On the 1401, references to non-existent
      memory halt the processor.

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

	i1401_cpu.c	add device dispatching code to iodisp
	i1401_sys.c	add sim_devices table entry
*/

#include "i1401_defs.h"
#include "i1401_dat.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] = saved_IS

/* These macros validate addresses.  If an addresses error is detected,
   they return an error status to the caller.  These macros should only
   be used in a routine that returns a t_stat value.
*/

#define MM(x)		x = x - 1; \
			if (x < 0) { \
				x = BA + MAXMEMSIZE - 1; \
				reason = STOP_WRAP; \
				break;  }
			
#define PP(x)		x = x + 1; \
			if (ADDR_ERR (x)) { \
				x = BA + (x % MAXMEMSIZE); \
				reason = STOP_WRAP; \
				break;  }

#define BRANCH		if (ADDR_ERR (AS)) { \
				reason = STOP_INVBR; \
				break;  } \
			if (cpu_unit.flags & XSA) BS = IS; \
			else BS = BA + 0; \
			PCQ_ENTRY; \
			IS = AS;

uint8 M[MAXMEMSIZE] = { 0 };				/* main memory */
int32 saved_IS = 0;					/* saved IS */
int32 AS = 0;						/* AS */
int32 BS = 0;						/* BS */
int32 D = 0;						/* modifier */
int32 as_err = 0, bs_err = 0;				/* error flags */
int32 hb_pend = 0;					/* halt br pending */
uint16 pcq[PCQ_SIZE] = { 0 };				/* PC queue */
int32 pcq_p = 0;					/* PC queue ptr */
REG *pcq_r = NULL;					/* PC queue reg ptr */
int32 ind[64] = { 0 };					/* indicators */
int32 ssa = 1;						/* sense switch A */
int32 prchk = 0;					/* process check stop */
int32 iochk = 0;					/* I/O check stop */
extern int32 sim_int_char;
extern int32 sim_brk_types, sim_brk_dflt, sim_brk_summ;	/* breakpoint info */

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);
int32 store_addr_h (int32 addr);
int32 store_addr_t (int32 addr);
int32 store_addr_u (int32 addr);
int32 div_add (int32 ap, int32 bp, int32 aend);
int32 div_sub (int32 ap, int32 bp, int32 aend);
void div_sign (int32 dvrc, int32 dvdc, int32 qp, int32 rp);
t_stat iomod (int32 ilnt, int32 mod, const int32 *tptr);
t_stat iodisp (int32 dev, int32 unit, int32 flag, int32 mod);

extern t_stat read_card (int32 ilnt, int32 mod);
extern t_stat punch_card (int32 ilnt, int32 mod);
extern t_stat select_stack (int32 mod);
extern t_stat carriage_control (int32 mod);
extern t_stat write_line (int32 ilnt, int32 mod);
extern t_stat inq_io (int32 flag, int32 mod);
extern t_stat mt_io (int32 unit, int32 flag, int32 mod);
extern t_stat dp_io (int32 fnc, int32 flag, int32 mod);
extern t_stat mt_func (int32 unit, int32 mod);
extern t_stat sim_activate (UNIT *uptr, int32 delay);

/* 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_BCD + STDOPT,
	MAXMEMSIZE) };

REG cpu_reg[] = {
	{ DRDATA (IS, saved_IS, 14), PV_LEFT },
	{ DRDATA (AS, AS, 14), PV_LEFT },
	{ DRDATA (BS, BS, 14), PV_LEFT },
	{ FLDATA (ASERR, as_err, 0) },
	{ FLDATA (BSERR, bs_err, 0) },
	{ ORDATA (D, D, 7) },
	{ FLDATA (SSA, ssa, 0) },
	{ FLDATA (SSB, ind[IN_SSB], 0) },
	{ FLDATA (SSC, ind[IN_SSC], 0) },
	{ FLDATA (SSD, ind[IN_SSD], 0) },
	{ FLDATA (SSE, ind[IN_SSE], 0) },
	{ FLDATA (SSF, ind[IN_SSF], 0) },
	{ FLDATA (SSG, ind[IN_SSG], 0) },
	{ FLDATA (EQU, ind[IN_EQU], 0) },
	{ FLDATA (UNEQ, ind[IN_UNQ], 0) },
	{ FLDATA (HIGH, ind[IN_HGH], 0) },
	{ FLDATA (LOW, ind[IN_LOW], 0) },
	{ FLDATA (OVF, ind[IN_OVF], 0) },
	{ FLDATA (IOCHK, iochk, 0) },
	{ FLDATA (PRCHK, prchk, 0) },
	{ FLDATA (HBPEND, hb_pend, 0) },
	{ BRDATA (ISQ, pcq, 10, 14, PCQ_SIZE), REG_RO+REG_CIRC },
	{ DRDATA (ISQP, pcq_p, 6), REG_HRO },
	{ ORDATA (WRU, sim_int_char, 8) },
	{ NULL }  };

MTAB cpu_mod[] = {
	{ XSA, XSA, "XSA", "XSA", NULL },
	{ XSA, 0, "no XSA", "NOXSA", NULL },
	{ HLE, HLE, "HLE", "HLE", NULL },
	{ HLE, 0, "no HLE", "NOHLE", NULL },
	{ BBE, BBE, "BBE", "BBE", NULL },
	{ BBE, 0, "no BBE", "NOBBE", NULL },
	{ MA, MA, "MA", 0, NULL },
	{ MA, 0, "no MA", 0, NULL },
	{ MR, MR, "MR", "MR", NULL },
	{ MR, 0, "no MR", "NOMR", NULL },
	{ EPE, EPE, "EPE", "EPE", NULL },
	{ EPE, 0, "no EPE", "NOEPE", NULL },
	{ MDV, MDV, "MDV", "MDV", NULL },
	{ MDV, 0, "no MDV", "NOMDV", NULL },
	{ UNIT_MSIZE, 4000, NULL, "4K", &cpu_set_size },
	{ UNIT_MSIZE, 8000, NULL, "8K", &cpu_set_size },
	{ UNIT_MSIZE, 12000, NULL, "12K", &cpu_set_size },
	{ UNIT_MSIZE, 16000, NULL, "16K", &cpu_set_size },
	{ 0 }  };

DEVICE cpu_dev = {
	"CPU", &cpu_unit, cpu_reg, cpu_mod,
	1, 10, 14, 1, 8, 7,
	&cpu_ex, &cpu_dep, &cpu_reset,
	NULL, NULL, NULL };

/* Tables */

/* Opcode table - length, dispatch, and option flags.  This table is also
   used by the symbolic input routine to validate instruction lengths  */

const int32 op_table[64] = {
	0,						/* 00: illegal */
	L1 | L2 | L4 | L5,				/* read */
	L1 | L2 | L4 | L5,				/* write */
	L1 | L2 | L4 | L5,				/* write and read */
	L1 | L2 | L4 | L5,				/* punch */
	L1 | L4,					/* read and punch */
	L1 | L2 | L4 | L5,				/* write and read */
	L1 | L2 | L4 | L5,				/* write, read, punch */
	L1,						/* 10: read feed */
	L1,						/* punch feed */
	0,						/* illegal */
	L1 | L4 | L7 | AREQ | BREQ | MA,		/* modify address */
	L1 | L4 | L7 | AREQ | BREQ | MDV,		/* multiply */
	0,						/* illegal */
	0,						/* illegal */
	0,						/* illegal */
	0,						/* 20: illegal */
	L1 | L4 | L7 | BREQ | NOWM,			/* clear storage */
	L1 | L4 | L7 | AREQ | BREQ,			/* subtract */
	0,						/* illegal */
	L5 | IO,					/* magtape */
	L1 | L8 | BREQ,					/* branch wm or zone */
	L1 | L8 | BREQ | BBE,				/* branch if bit eq */
	0,						/* illegal */
	L1 | L4 | L7 | AREQ | BREQ,			/* 30: move zones */
	L1 | L4 | L7 | AREQ | BREQ,			/* move supress zero */
	0,						/* illegal */
	L1 | L4 | L7 | AREQ | BREQ | NOWM,		/* set word mark */
	L1 | L4 | L7 | AREQ | BREQ | MDV,		/* divide */
	0,						/* illegal */
	0,						/* illegal */
	0,						/* illegal */
	0,						/* 40: illegal */
	0,						/* illegal */
	L2 | L5,					/* select stacker */
	L1 | L4 | L7 | L8 | BREQ | MLS | IO,		/* load */
	L1 | L4 | L7 | L8 | BREQ | MLS | IO,		/* move */
	HNOP | L1 | L2 | L4 | L5 | L7 | L8,		/* nop */
	0,						/* illegal */
	L1 | L4 | L7 | AREQ | BREQ | MR,		/* move to record */
	L1 | L4 | AREQ | MLS,				/* 50: store A addr */
	0,						/* illegal */
	L1 | L4 | L7 | AREQ | BREQ,			/* zero and subtract */
	0,						/* illegal */
	0,						/* illegal */
	0,						/* illegal */
	0,						/* illegal */
	0,						/* illegal */
	0,						/* 60: illegal */
	L1 | L4 | L7 | AREQ | BREQ,			/* add */
	L1 | L4 | L5 | L8,				/* branch */
	L1 | L4 | L7 | AREQ | BREQ,			/* compare */
	L1 | L4 | L7 | AREQ | BREQ,			/* move numeric */
	L1 | L4 | L7 | AREQ | BREQ,			/* move char edit */
	L2 | L5,					/* carriage control */
	0,						/* illegal */
	L1 | L4 | L7 | AREQ | MLS,			/* 70: store B addr */
	0,						/* illegal */
	L1 | L4 | L7 | AREQ | BREQ,			/* zero and add */
	HNOP | L1 | L2 | L4 | L5 | L7 | L8,		/* halt */
	L1 | L4 | L7 | AREQ | BREQ,			/* clear word mark */
	0,						/* illegal */
	0,						/* illegal */
	0 };						/* illegal */

const int32 len_table[9] = { 0, L1, L2, 0, L4, L5, 0, L7, L8 };

/* Address character conversion tables.  Illegal characters are marked by
   the flag BA but also contain the post-adder value for indexing  */

const int32 hun_table[64] = {
	BA+000, 100, 200, 300, 400, 500, 600, 700,
	800, 900, 000, BA+300, BA+400, BA+500, BA+600, BA+700,
	BA+1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700,
	1800, 1900, 1000, BA+1300, BA+1400, BA+1500, BA+1600, BA+1700,
	BA+2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700,
	2800, 2900, 2000, BA+2300, BA+2400, BA+2500, BA+2600, BA+2700,
	BA+3000, 3100, 3200, 3300, 3400, 3500, 3600, 3700,
	3800, 3900, 3000, BA+3300, BA+3400, BA+3500, BA+3600, BA+3700 };

const int32 ten_table[64] = {
	BA+00, 10, 20, 30, 40, 50, 60, 70,
	80, 90, 00, BA+30, BA+40, BA+50, BA+60, BA+70,
	X1+00, X1+10, X1+20, X1+30, X1+40, X1+50, X1+60, X1+70,
	X1+80, X1+90, X1+00, X1+30, X1+40, X1+50, X1+60, X1+70,
	X2+00, X2+10, X2+20, X2+30, X2+40, X2+50, X2+60, X2+70,
	X2+80, X2+90, X2+00, X2+30, X2+40, X2+50, X2+60, X2+70,
	X3+00, X3+10, X3+20, X3+30, X3+40, X3+50, X3+60, X3+70,
	X3+80, X3+90, X3+00, X3+30, X3+40, X3+50, X3+60, X3+70 };

const int32 one_table[64] = {
	BA+0, 1, 2, 3, 4, 5, 6, 7,
	8, 9, 0, BA+3, BA+4, BA+5, BA+6, BA+7,
	BA+4000, 4001, 4002, 4003, 4004, 4005, 4006, 4007,
	4008, 4009, 4000, BA+4003, BA+4004, BA+4005, BA+4006, BA+4007,
	BA+8000, 8001, 8002, 8003, 8004, 8005, 8006, 8007,
	8008, 8009, 8000, BA+8003, BA+8004, BA+8005, BA+8006, BA+8007,
	BA+12000, 12001, 12002, 12003, 12004, 12005, 12006, 12007,
	12008, 12009, 12000, BA+12003, BA+12004, BA+12005, BA+12006, BA+12007 };

const int32 bin_to_bcd[16] = {
	10, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 };

const int32 bcd_to_bin[16] = {
	0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 3, 4, 5, 6, 7 };

/* Indicator resets - a 1 marks an indicator that resets when tested */

static const int32 ind_table[64] = {
	0, 0, 0, 0, 0, 0, 0, 0,				/* 00 - 07 */
	0, 0, 0, 0, 0, 0, 0, 0,				/* 10 - 17 */
	0, 0, 0, 0, 0, 0, 0, 0,				/* 20 - 27 */
	0, 1, 1, 0, 1, 0, 0, 0,				/* 30 - 37 */
	0, 0, 1, 0, 0, 0, 0, 0,				/* 40 - 47 */
	0, 0, 1, 0, 1, 0, 0, 0,				/* 50 - 57 */
	0, 0, 0, 0, 0, 0, 0, 0,				/* 60 - 67 */
	0, 0, 1, 0, 0, 0, 0, 0 };			/* 70 - 77 */

/* Character collation table for compare with HLE option */

static const int32 col_table[64] = {
	000, 067, 070, 071, 072, 073, 074, 075,
	076, 077, 066, 024, 025, 026, 027, 030,
	023, 015, 056, 057, 060, 061, 062, 063,
	064, 065, 055, 016, 017, 020, 021, 022,
	014, 044, 045, 046, 047, 050, 051, 052,
	053, 054, 043, 007, 010, 011, 012, 013,
	006, 032, 033, 034, 035, 036, 037, 040,
	041, 042, 031, 001, 002, 003, 004, 005 };

/* Summing table for two decimal digits, converted back to BCD
   Also used for multiplying two decimal digits, converted back to BCD,
   with carry forward
*/
	
static const int32 sum_table[100] = {
	BCD_ZERO, BCD_ONE, BCD_TWO, BCD_THREE, BCD_FOUR,
	BCD_FIVE, BCD_SIX, BCD_SEVEN, BCD_EIGHT, BCD_NINE,
	BCD_ZERO, BCD_ONE, BCD_TWO, BCD_THREE, BCD_FOUR,
	BCD_FIVE, BCD_SIX, BCD_SEVEN, BCD_EIGHT, BCD_NINE,
	BCD_ZERO, BCD_ONE, BCD_TWO, BCD_THREE, BCD_FOUR,
	BCD_FIVE, BCD_SIX, BCD_SEVEN, BCD_EIGHT, BCD_NINE,
	BCD_ZERO, BCD_ONE, BCD_TWO, BCD_THREE, BCD_FOUR,
	BCD_FIVE, BCD_SIX, BCD_SEVEN, BCD_EIGHT, BCD_NINE,
	BCD_ZERO, BCD_ONE, BCD_TWO, BCD_THREE, BCD_FOUR,
	BCD_FIVE, BCD_SIX, BCD_SEVEN, BCD_EIGHT, BCD_NINE,
	BCD_ZERO, BCD_ONE, BCD_TWO, BCD_THREE, BCD_FOUR,
	BCD_FIVE, BCD_SIX, BCD_SEVEN, BCD_EIGHT, BCD_NINE,
	BCD_ZERO, BCD_ONE, BCD_TWO, BCD_THREE, BCD_FOUR,
	BCD_FIVE, BCD_SIX, BCD_SEVEN, BCD_EIGHT, BCD_NINE,
	BCD_ZERO, BCD_ONE, BCD_TWO, BCD_THREE, BCD_FOUR,
	BCD_FIVE, BCD_SIX, BCD_SEVEN, BCD_EIGHT, BCD_NINE,
	BCD_ZERO, BCD_ONE, BCD_TWO, BCD_THREE, BCD_FOUR,
	BCD_FIVE, BCD_SIX, BCD_SEVEN, BCD_EIGHT, BCD_NINE,
	BCD_ZERO, BCD_ONE, BCD_TWO, BCD_THREE, BCD_FOUR,
	BCD_FIVE, BCD_SIX, BCD_SEVEN, BCD_EIGHT, BCD_NINE };

static const int32 cry_table[100] = {
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
	2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
	3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
	4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
	5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
	6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
	7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
	8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
	9, 9, 9, 9, 9, 9, 9, 9, 9, 9 };

/* Legal modifier tables */

static const int32 w_mod[] = { BCD_S, BCD_SQUARE, -1 };
static const int32 ss_mod[] = { 1, 2, 4, 8, -1 };
static const int32 mtf_mod[] = { BCD_B, BCD_E, BCD_M, BCD_R, BCD_U, -1 };

t_stat sim_instr (void)
{
extern int32 sim_interval;
int32 IS, ilnt, flags;
int32 op, xa, t, wm, ioind, dev, unit;
int32 a, b, i, asave, bsave;
int32 carry, lowprd, sign, ps;
int32 quo, ahigh, qs;
int32 qzero, qawm, qbody, qsign, qdollar, qaster, qdecimal;
t_stat reason, r1, r2;

/* Restore saved state */


IS = saved_IS;
if (as_err) AS = AS | BA;				/* flag bad addresses */
if (bs_err) BS = BS | BA;
as_err = bs_err = 0;					/* reset error flags */
reason = 0;

/* Main instruction fetch/decode loop */

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

if (hb_pend) {						/* halt br pending? */
	hb_pend = 0;					/* clear flag */
	BRANCH;  }					/* execute branch */

saved_IS = IS;						/* commit prev instr */
if (sim_interval <= 0) {				/* check clock queue */
	if (reason = sim_process_event ()) break;  }

if (sim_brk_summ && sim_brk_test (IS, SWMASK ('E'))) {	/* breakpoint? */
	reason = STOP_IBKPT;				/* stop simulation */
	break;  }

sim_interval = sim_interval - 1;

/* Instruction fetch - 1401 fetch works as follows:

   - Each character fetched enters the B register.  This register is not
     visible; the variable t represents the B register.
   - Except for the first and last cycles, each character fetched enters
     the A register.  This register is not visible; the variable D represents
     the A register, because this is the instruction modifier for 2, 5, and 8
     character instructions.
   - At the start of the second cycle (first address character), the A-address
     register and, for most instructions, the B-address register, are cleared
     to blanks.  The simulator represents addresses in binary and creates the
     effect of blanks (address is bad) if less than three A-address characters
     are found.  Further, the simulator accumulates only the A-address, and
     replicates it to the B-address at the appropriate point.
   - At the start of the fifth cycle (fourth address character), the B-address
     register is cleared to blanks.  Again, the simulator creates the effect of
     blanks (address is bad) if less than three B-address characters are found.

   The 1401 does explicitly check for valid instruction lengths.  Most 2,3,5,6
   character instructions will be invalid because the A-address or B-address
   (or both) are invalid.
*/

if ((M[IS] & WM) == 0) {				/* I-Op: WM under op? */
	reason = STOP_NOWM;				/* no, error */
	break;  }
op = M[IS] & CHAR;					/* get opcode */
flags = op_table[op];					/* get op flags */
if ((flags == 0) || (flags & ALLOPT & ~cpu_unit.flags)) {
	reason = STOP_NXI;				/* illegal inst? */
	break;  }
if (op == OP_SAR) BS = AS;				/* SAR? save ASTAR */
PP (IS);

if ((t = M[IS]) & WM) goto CHECK_LENGTH;		/* I-1: WM? 1 char inst */
D = ioind = t;						/* could be D char, % */
AS = hun_table[t];					/* could be A addr */
PP (IS);						/* if %xy, BA is set */

if ((t = M[IS]) & WM) {					/* I-2: WM? 2 char inst */
	AS = AS | BA;					/* ASTAR bad */
	if (!(flags & MLS)) BS = AS;
	goto CHECK_LENGTH;  }
D = dev = t;						/* could be D char, dev */
AS = AS + ten_table[t];					/* build A addr */
PP (IS);

if ((t = M[IS]) & WM) {					/* I-3: WM? 3 char inst */
	AS = AS | BA;					/* ASTAR bad */
	if (!(flags & MLS)) BS = AS;
	goto CHECK_LENGTH;  }
D = unit = t;						/* could be D char, unit */
if (unit == BCD_ZERO) unit = 0;				/* convert unit to binary */
AS = AS + one_table[t];					/* finish A addr */
xa = (AS >> V_INDEX) & M_INDEX;				/* get index reg */
if (xa && (ioind != BCD_PERCNT) && (cpu_unit.flags & XSA)) {	/* indexed? */
	AS = AS + hun_table[M[xa] & CHAR] + ten_table[M[xa + 1] & CHAR] +
	    one_table[M[xa + 2] & CHAR];
	AS = (AS & INDEXMASK) % MAXMEMSIZE;  }
if (!(flags & MLS)) BS = AS;				/* not MLS? B = A */
PP (IS);

if ((t = M[IS]) & WM) goto CHECK_LENGTH;		/* I-4: WM? 4 char inst */
if ((op == OP_B) && (t == BCD_BLANK)) goto CHECK_LENGTH; /* BR + space? */
D = t;							/* could be D char */
BS = hun_table[t];					/* could be B addr */
PP (IS);

if ((t = M[IS]) & WM) {					/* I-5: WM? 5 char inst */
	BS = BS | BA;					/* BSTAR bad */
	goto CHECK_LENGTH;  }
D = t;							/* could be D char */
BS = BS + ten_table[t];					/* build B addr */
PP (IS);

if ((t = M[IS]) & WM) {					/* I-6: WM? 6 char inst */
	BS = BS | BA;					/* BSTAR bad */
	goto CHECK_LENGTH;  }
D = t;							/* could be D char */
BS = BS + one_table[t];					/* finish B addr */
xa = (BS >> V_INDEX) & M_INDEX;				/* get index reg */
if (xa && (cpu_unit.flags & XSA)) {			/* indexed? */
	BS = BS + hun_table[M[xa] & CHAR] + ten_table[M[xa + 1] & CHAR] +
	    one_table[M[xa + 2] & CHAR];
	BS = (BS & INDEXMASK) % MAXMEMSIZE;  }
PP (IS);

if (flags & NOWM) goto CHECK_LENGTH;			/* I-7: SWM? done */
if ((t = M[IS]) & WM) goto CHECK_LENGTH;			/* WM? 7 char inst */
D = t;							/* last char is D */
for (;;) {						/* I-8: repeats until WM */
	if ((t = M[IS]) & WM) break;			/* WM? done */
	D = t;						/* last char is D */
	PP (IS);  }

CHECK_LENGTH:
ilnt = IS - saved_IS;					/* get lnt */
//if (((flags & len_table [(ilnt <= 8)? ilnt: 8]) == 0) &&	/* valid lnt? */
//	((flags & HNOP) == 0)) reason = STOP_INVL;
if ((flags & BREQ) && ADDR_ERR (BS)) reason = STOP_INVB;	/* valid A? */
if ((flags & AREQ) && ADDR_ERR (AS)) reason = STOP_INVA;	/* valid B? */
if (reason) break;					/* error in fetch? */
switch (op) {						/* case on opcode */	

/* Move/load character instructions			A check	B check

   MCW		copy A to B, preserving B WM,		here	fetch
		until either A or B WM
   LCA		copy A to B, overwriting B WM,		here	fetch
		until A WM

   Instruction lengths:

   1		chained A and B
   2,3		invalid A-address
   4		chained B address
   5,6		invalid B-address
   7		normal
   8+		normal + modifier
*/

case OP_MCW:						/* move char */
	if ((ilnt >= 4) && (ioind == BCD_PERCNT)) {	/* I/O form? */
	    reason = iodisp (dev, unit, MD_NORM, D);	/* dispatch I/O */
	    break;  }
	if (ADDR_ERR (AS)) {				/* check A addr */
	    reason = STOP_INVA;
	    break;  }
	do {
	    wm = M[AS] | M[BS];
	    M[BS] = (M[BS] & WM) | (M[AS] & CHAR);	/* move char */
	    MM (AS); MM (BS);  }			/* decr pointers */
	while ((wm & WM) == 0);				/* stop on A,B WM */
	break;

case OP_LCA:						/* load char */
	if ((ilnt >= 4) && (ioind == BCD_PERCNT)) {	/* I/O form? */
	    reason = iodisp (dev, unit, MD_WM, D);
	    break;  }
	if (ADDR_ERR (AS)) {				/* check A addr */
	    reason = STOP_INVA;
	    break;  }
	do {
	    wm = M[BS] = M[AS];				/* move char + wmark */
	    MM (AS); MM (BS);  }			/* decr pointers */
	while ((wm & WM) == 0);				/* stop on A WM */
	break;

/* Other move instructions				A check	B check

   MCM		copy A to B, preserving B WM,		fetch	fetch
		until record or group mark
   MCS		copy A to B, clearing B WM, until A WM;	fetch	fetch
		reverse scan and suppress leading zeroes
   MN		copy A char digit to B char digit,	fetch	fetch
		preserving B zone and WM
   MZ		copy A char zone to B char zone,	fetch	fetch
		preserving B digit and WM

   Instruction lengths:

   1		chained
   2,3		invalid A-address
   4		self (B-address = A-address)
   5,6		invalid B-address
   7		normal
   8+		normal + ignored modifier
*/

case OP_MCM:						/* move to rec/group */
	do {
	    t = M[AS];
	    M[BS] = (M[BS] & WM) | (M[AS] & CHAR);	/* move char */
	    PP (AS); PP (BS);  }			/* incr pointers */
	while (((t & CHAR) != BCD_RECMRK) && (t != (BCD_GRPMRK + WM)));
	break;

case OP_MCS:						/* move suppress zero */
	bsave = BS;					/* save B start */
	qzero = 1;					/* set suppress */
	do {
	    wm = M[AS];
	    M[BS] = M[AS] & ((BS != bsave)? CHAR: DIGIT);	/* copy char */
	    MM (AS); MM (BS);  }			/* decr pointers */
	while ((wm & WM) == 0);				/* stop on A WM */
	if (reason) break;				/* addr err? stop */
	do {
	    PP (BS);					/* adv B */
	    t = M[BS];					/* get B, cant be WM */
	    if ((t == BCD_ZERO) || (t == BCD_COMMA)) {
		if (qzero) M[BS] = 0;  }
	    else if ((t == BCD_BLANK) || (t == BCD_MINUS)) ;
	    else if (((t == BCD_DECIMAL) && (cpu_unit.flags & EPE)) ||
		 (t <= BCD_NINE)) qzero = 0;
	    else qzero = 1;  }
	while (BS < bsave);
	PP (BS);					/* BS end is B+1 */
	break;	

case OP_MN:						/* move numeric */
	M[BS] = (M[BS] & ~DIGIT) | (M[AS] & DIGIT);	/* move digit */
	MM (AS); MM (BS);				/* decr pointers */
	break;

case OP_MZ:						/* move zone */
	M[BS] = (M[BS] & ~ZONE) | (M[AS] & ZONE);	/* move high bits */
	MM (AS); MM (BS);				/* decr pointers */
	break;

/* Branch instruction					A check	    B check

   Instruction lengths:

   1		branch if B char equals d, chained	if branch   here
   2,3		invalid B-address			if branch   here
   4		unconditional branch			if branch
   5		branch if indicator[d] is set		if branch
   6		invalid B-address			if branch   here
   7		branch if B char equals d,		if branch   here
		d is last character of B-address
   8		branch if B char equals d		if branch   here
*/

case OP_B:						/* branch */
	if (ilnt == 4) { BRANCH; }			/* uncond branch? */
	else if (ilnt == 5) {				/* branch on ind? */
	    if (ind[D]) { BRANCH;  }			/* test indicator */
	    if (ind_table[D]) ind[D] = 0;  }		/* reset if needed */
	else {						/* branch char eq */
	    if (ADDR_ERR (BS)) {			/* validate B addr */
		reason = STOP_INVB;
		break;  }
	    if ((M[BS] & CHAR) == D) { BRANCH;  }	/* char equal? */
	    else { MM (BS);  }  }
	break;

/* Other branch instructions				A check     B check

   BWZ		branch if (d<0>: B char WM)		if branch   here
		(d<1>: B char zone = d zone)
   BBE		branch if B char & d non-zero		if branch   here

   Instruction lengths:
   1	chained
   2,3	invalid A-address and B-address
   4	self (B-address = A-address, d = last character of A-address)
   5,6	invalid B-address
   7	normal, d = last character of B-address
   8+	normal
*/

case OP_BWZ:						/* branch wm or zone */
	if (((D & 1) && (M[BS] & WM)) ||		/* d1? test wm */
	    ((D & 2) && ((M[BS] & ZONE) == (D & ZONE)))) /* d2? test zone */
		{ BRANCH;  }
	else { MM (BS);  }				/* decr pointer */
	break;

case OP_BBE:						/* branch if bit eq */
	if (M[BS] & D & CHAR) { BRANCH;  }		/* any bits set? */
	else { MM (BS);  }				/* decr pointer */
	break;

/* Arithmetic instructions				A check     B check

   ZA		move A to B, normalizing A sign,	fetch	    fetch
		preserving B WM, until B WM
   ZS		move A to B, complementing A sign,	fetch	    fetch
		preserving B WM, until B WM
   A		add A to B				fetch	    fetch
   S		subtract A from B			fetch	    fetch
   C		compare A to B				fetch	    fetch

   Instruction lengths:

   1		chained
   2,3		invalid A-address
   4		self (B-address = A-address)
   5,6		invalid B-address
   7		normal
   8+		normal + ignored modifier
*/

case OP_ZA: case OP_ZS:					/* zero and add/sub */
	a = i = 0;					/* clear flags */
	do {
	    if (a & WM) wm = M[BS] = (M[BS] & WM) | BCD_ZERO;
	    else {
	    	a = M[AS];				/* get A char */
		t = (a & CHAR)? bin_to_bcd[a & DIGIT]: 0;
		wm = M[BS] = (M[BS] & WM) | t;		/* move digit */
		MM (AS);  }
	    if (i == 0) i = M[BS] = M[BS] |
		((((a & ZONE) == BBIT) ^ (op == OP_ZS))? BBIT: ZONE);
	    MM (BS);  }
	while ((wm & WM) == 0);				/* stop on B WM */
	break;

case OP_A: case OP_S:					/* add/sub */
	bsave = BS;					/* save sign pos */
	a = M[AS];					/* get A digit/sign */
	b = M[BS];					/* get B digit/sign */
	MM (AS);
	qsign = ((a & ZONE) == BBIT) ^ ((b & ZONE) == BBIT) ^ (op == OP_S);
	t = bcd_to_bin[a & DIGIT];			/* get A binary */
	t = bcd_to_bin[b & DIGIT] + (qsign? 10 - t: t);	/* sum A + B */
	carry = (t >= 10);				/* get carry */
	b = (b & ~DIGIT) | sum_table[t];		/* get result */
	if (qsign && ((b & BBIT) == 0)) b = b | ZONE;	/* normalize sign */
	M[BS] = b;					/* store result */
	MM (BS);
	if (b & WM) {					/* b wm? done */
	    if (qsign && (carry == 0)) M[bsave] =	/* compl, no carry? */
		WM + ((b & ZONE) ^ ABIT) + sum_table[10 - t];
	    break;  }
	do {
	    if (a & WM) a = WM;				/* A WM? char = 0 */
	    else {
	    	a = M[AS];				/* else get A */
		MM (AS);  }
	    b = M[BS];					/* get B */
	    t = bcd_to_bin[a & DIGIT];			/* get A binary */
	    t = bcd_to_bin[b & DIGIT] + (qsign? 9 - t: t) + carry;
	    carry = (t >= 10);				/* get carry */
	    if ((b & WM) && (qsign == 0)) {		/* last, no recomp? */
		M[BS] = WM + sum_table[t] +		/* zone add */
		    (((a & ZONE) + b + (carry? ABIT: 0)) & ZONE);
		ind[IN_OVF] = carry;  }			/* ovflo if carry */
	    else M[BS] = (b & WM) + sum_table[t];	/* normal add */
	    MM (BS);  }
	while ((b & WM) == 0);				/* stop on B WM */
	if (reason) break;				/* address err? */
	if (qsign && (carry == 0)) {			/* recompl, no carry? */
	    M[bsave] = M[bsave] ^ ABIT;			/* XOR sign */
	    for (carry = 1; bsave != BS; --bsave) {	/* rescan */
		t = 9 - bcd_to_bin[M[bsave] & DIGIT] + carry;
		carry = (t >= 10);
		M[bsave] = (M[bsave] & ~DIGIT) | sum_table[t];  }  }
	break;

case OP_C:						/* compare */
	if (ilnt != 1) {				/* if not chained */
	    ind[IN_EQU] = 1;				/* clear indicators */
	    ind[IN_UNQ] = ind[IN_HGH] = ind[IN_LOW] = 0;  }
	do {
	    a = M[AS];					/* get characters */
	    b = M[BS];
	    wm = a | b;					/* get word marks */
	    if ((a & CHAR) != (b & CHAR)) {		/* unequal? */
		ind[IN_EQU] = 0;			/* set indicators */
		ind[IN_UNQ] = 1;
		ind[IN_HGH] = col_table[b & CHAR] > col_table [a & CHAR];
		ind[IN_LOW] = ind[IN_HGH] ^ 1;  }
	    MM (AS); MM (BS);  }			/* decr pointers */
	while ((wm & WM) == 0);				/* stop on A, B WM */
	if ((a & WM) && !(b & WM)) {			/* short A field? */
	    ind[IN_EQU] = ind[IN_LOW] = 0;
	    ind[IN_UNQ] = ind[IN_HGH] = 1;  }
	if (!(cpu_unit.flags & HLE))			/* no HLE? */
	    ind[IN_EQU] = ind[IN_LOW] = ind[IN_HGH] = 0;
	break;

/* I/O instructions					A check     B check

   R		read a card				if branch
   W		write to line printer			if branch
   WR		write and read				if branch
   P		punch a card				if branch
   RP		read and punch				if branch
   WP	:	write and punch				if branch
   WRP		write read and punch			if branch
   RF		read feed (nop)
   PF		punch feed (nop)
   SS		select stacker				if branch
   CC		carriage control			if branch

   Instruction lengths:

   1		normal
   2,3		normal, with modifier
   4		branch; modifier, if any, is last character of branch address
   5		branch + modifier
   6+		normal, with modifier
*/

case OP_R:						/* read */
	if (reason = iomod (ilnt, D, NULL)) break;	/* valid modifier? */
	reason = read_card (ilnt, D);			/* read card */
	BS = CDR_BUF + CDR_WIDTH;
	if ((ilnt == 4) || (ilnt == 5)) { BRANCH;  }	/* check for branch */
	break;

case OP_W:						/* write */
	if (reason = iomod (ilnt, D, w_mod)) break;	/* valid modifier? */
	reason = write_line (ilnt, D);			/* print line */
	BS = LPT_BUF + LPT_WIDTH;
	if ((ilnt == 4) || (ilnt == 5)) { BRANCH;  }	/* check for branch */
	break;

case OP_P:						/* punch */
	if (reason = iomod (ilnt, D, NULL)) break;	/* valid modifier? */
	reason = punch_card (ilnt, D);			/* punch card */
	BS = CDP_BUF + CDP_WIDTH;
	if ((ilnt == 4) || (ilnt == 5)) { BRANCH;  }	/* check for branch */
	break;

case OP_WR:						/* write and read */
	if (reason = iomod (ilnt, D, w_mod)) break;	/* valid modifier? */
	reason = write_line (ilnt, D);			/* print line */
	r1 = read_card (ilnt, D);			/* read card */
	BS = CDR_BUF + CDR_WIDTH;
	if ((ilnt == 4) || (ilnt == 5)) { BRANCH;  }	/* check for branch */
	if (reason == SCPE_OK) reason = r1;		/* merge errors */
	break;

case OP_WP:						/* write and punch */
	if (reason = iomod (ilnt, D, w_mod)) break;	/* valid modifier? */
	reason = write_line (ilnt, D);			/* print line */
	r1 = punch_card (ilnt, D);			/* punch card */
	BS = CDP_BUF + CDP_WIDTH;
	if ((ilnt == 4) || (ilnt == 5)) { BRANCH;  }	/* check for branch */
	if (reason == SCPE_OK) reason = r1;		/* merge errors */
	break;

case OP_RP:						/* read and punch */
	if (reason = iomod (ilnt, D, NULL)) break;	/* valid modifier? */
	reason = read_card (ilnt, D);			/* read card */
	r1 = punch_card (ilnt, D);			/* punch card */
	BS = CDP_BUF + CDP_WIDTH;  
	if ((ilnt == 4) || (ilnt == 5)) { BRANCH;  }	/* check for branch */
	if (reason == SCPE_OK) reason = r1;		/* merge errors */
	break;

case OP_WRP:						/* write, read, punch */
	if (reason = iomod (ilnt, D, w_mod)) break;	/* valid modifier? */
	reason = write_line (ilnt, D);			/* print line */
	r1 = read_card (ilnt, D);			/* read card */
	r2 = punch_card (ilnt, D);			/* punch card */
	BS = CDP_BUF + CDP_WIDTH;
	if ((ilnt == 4) || (ilnt == 5)) { BRANCH;  }	/* check for branch */
	if (reason == SCPE_OK) reason = (r1 == SCPE_OK)? r2: r1;
	break;

case OP_SS:						/* select stacker */
	if (reason = iomod (ilnt, D, ss_mod)) break;	/* valid modifier? */
	if (reason = select_stack (D)) break;		/* sel stack, error? */
	if ((ilnt == 4) || (ilnt == 5)) { BRANCH;  }	/* check for branch */
	break;

case OP_CC:						/* carriage control */
	if (reason = carriage_control (D)) break;	/* car ctrl, error? */
	if ((ilnt == 4) || (ilnt == 5)) { BRANCH;  }	/* check for branch */
	break;

/* MTF - magtape functions - must be at least 4 characters

   Instruction lengths:

   1-3		invalid I/O address
   4		normal, d-character is unit
   5		normal
   6+		normal, d-character is last character
*/

case OP_MTF:						/* magtape function */
	if (ilnt < 4) reason = STOP_INVL;		/* too short? */
	else if (ioind != BCD_PERCNT) reason = STOP_INVA;
	else if (reason = iomod (ilnt, D, mtf_mod)) break;	/* valid modifier? */
	reason = mt_func (unit, D);			/* mt func, error? */
	break;						/* can't branch */

case OP_RF: case OP_PF:					/* read, punch feed */
	break;						/* nop's */

/* Move character and edit

   Control flags
	qsign		sign of A field (0 = +, 1 = minus)
	qawm		A field WM seen and processed
	qzero		zero suppression enabled
	qbody		in body (copying A field characters)
	qdollar		EPE only; $ seen in body
	qaster		EPE only; * seen in body
	qdecimal	EPE only; . seen on first rescan

   MCE operates in one to three scans, the first of which has three phases

	1	right to left	qbody = 0, qawm = 0 => right status
				qbody = 1, qawm = 0 => body
				qbody = 0, qawm = 1 => left status
	2	left to right
	3	right to left, extended print end only

   The first A field character is masked to its digit part, all others
   are copied intact		

   Instruction lengths:

   1		chained
   2,3		invalid A-address
   4		self (B-address = A-address)
   5,6		invalid B-address
   7		normal
   8+		normal + ignored modifier
*/

case OP_MCE:						/* edit */
	a = M[AS];					/* get A char */
	b = M[BS];					/* get B char */
	if (a & WM) {					/* one char A field? */
	    reason = STOP_MCE1;
	    break;  }
	if (b & WM) {					/* one char B field? */
	    reason = STOP_MCE2;
	    break;  }
	t = a & DIGIT; MM (AS);				/* get A digit */
	qsign = ((a & ZONE) == BBIT);			/* get A field sign */
	qawm = qzero = qbody = 0;			/* clear other flags */
	qdollar = qaster = qdecimal = 0;		/* clear EPE flags */

/* Edit pass 1 - from right to left, under B field control

	*	in status or !epe, skip B; else, set qaster, repl with A
	$	in status or !epe, skip B; else, set qdollar, repl with A
	0	in right status or body, if !qzero, set A WM; set qzero, repl with A
		else, if !qzero, skip B; else, if (!B WM) set B WM 
	blank	in right status or body, repl with A; else, skip B
	C,R,-	in status, blank B; else, skip B
	,	in status, blank B, else, skip B
	&	blank B
*/

	do {
	    b = M[BS];					/* get B char */
	    M[BS] = M[BS] & ~WM;			/* clr WM */
	    switch (b & CHAR) {				/* case on B char */
	    case BCD_ASTER:				/* * */
		if (!qbody || qdollar || !(cpu_unit.flags & EPE)) break;
		qaster = 1;				/* flag */
		goto A_CYCLE;				/* take A cycle */
	    case BCD_DOLLAR:				/* $ */
		if (!qbody || qaster || !(cpu_unit.flags & EPE)) break;
		qdollar = 1;				/* flag */
		goto A_CYCLE;				/* take A cycle */
	    case BCD_ZERO:				/* 0 */
		if (qawm && !qzero && !(b & WM)) {
		    M[BS] = BCD_ZERO + WM;		/* mark with WM */
		    qzero = 1;				/* flag supress */
		    break;  }
		if (!qzero) t = t | WM;			/* first? set WM */
		qzero = 1;				/* flag supress */
							/* fall through */
	    case BCD_BLANK:				/* blank */
		if (qawm) break;			/* any A left? */
	    A_CYCLE:
		M[BS] = t;				/* copy char */
		if (a & WM) {				/* end of A field? */
		    qbody = 0;				/* end body */
		    qawm = 1;  }
		else {
		    qbody = 1;				/* in body */
		    a = M[AS]; MM (AS);			/* next A */
		    t = a & CHAR;  }
		break;
	    case BCD_C: case BCD_R: case BCD_MINUS:	/* C, R, - */
		if (!qsign && !qbody) M[BS] = BCD_BLANK;
		break;
	    case BCD_COMMA:				/* , */
		if (!qbody) M[BS] = BCD_BLANK;		/* bl if status */
		break;
	    case BCD_AMPER:				/* & */
		M[BS] = BCD_BLANK;			/* blank B field */
		break;  }				/* end switch */
	    MM (BS);  }					/* decr B pointer */
	while ((b & WM) == 0);				/* stop on B WM */

	if (reason) break;				/* address err? */
	if (!qawm || !qzero) {				/* rescan? */
	    if (qdollar) reason = STOP_MCE3;		/* error if $ */
	    break;  }

/* Edit pass 2 - from left to right, supressing zeroes */

	do {
	    b = M[++BS];				/* get B char */
	    switch (b & CHAR) {				/* case on B char */
	    case BCD_ONE: case BCD_TWO: case BCD_THREE:
	    case BCD_FOUR: case BCD_FIVE: case BCD_SIX:
	    case BCD_SEVEN: case BCD_EIGHT: case BCD_NINE:
		qzero = 0;				/* turn off supr */
		break;
	    case BCD_ZERO: case BCD_COMMA:		/* 0 or , */
		if (qzero && !qdecimal)			/* if supr, blank */
		    M[BS] = qaster? BCD_ASTER: BCD_BLANK;
		break;
	    case BCD_BLANK:				/* blank */
		if (qaster) M[BS] = BCD_ASTER;		/* if EPE *, repl */
		break;
	    case BCD_DECIMAL:				/* . */
	    	if (qzero && (cpu_unit.flags & EPE))
		    qdecimal = 1;			/* flag for EPE */
	    case BCD_PERCNT: case BCD_WM: case BCD_BS:
	    case BCD_TS: case BCD_MINUS:
		break;					/* ignore */
	    default:					/* other */
	    	qzero = 1;				/* restart supr */
		break;  }  }				/* end case, do */
	while ((b & WM) == 0);

	M[BS] = M[BS] & ~WM;				/* clear B WM */
	if (!qdollar && !(qdecimal && qzero)) {		/* rescan again? */
	    BS++;					/* BS = addr WM + 1 */
	    break;  }
	if (qdecimal && qzero) qdollar = 0;		/* no digits? clr $ */

/* Edit pass 3 (extended print only) - from right to left */

	for (;; ) {					/* until chars */
	    b = M[BS];					/* get B char */
	    if ((b == BCD_BLANK) && qdollar) {		/* blank & flt $? */
		M[BS] = BCD_DOLLAR;			/* insert $ */
		break;  }				/* exit for */
	    if (b == BCD_DECIMAL) {			/* decimal? */
		M[BS] = qaster? BCD_ASTER: BCD_BLANK;
		break;  }				/* exit for */
	    if ((b == BCD_ZERO) && !qdollar)		/* 0 & ~flt $ */
		M[BS] = qaster? BCD_ASTER: BCD_BLANK;
	    BS--;  }					/* end for */
	break;						/* done at last! */	

/* Multiply.  Comments from the PDP-10 based simulator by Len Fehskens.

   Multiply, with variable length operands, is necessarily done the same
   way you do it with paper and pencil, except that partial products are
   added into the incomplete final product as they are computed, rather
   than at the end.  The 1401 multiplier format allows the product to
   be developed in place, without scratch storage.

   The A field contains the multiplicand, length LD.  The B field must be
   LD + 1 + length of multiplier.  Locate the low order multiplier digit,
   and at the same time zero out the product field.  Then compute the sign
   of the result.

   Instruction lengths:

   1		chained
   2,3		invalid A-address
   4		self (B-address = A-address)
   5,6		invalid B-address
   7		normal
   8+		normal + ignored modifier
*/

case OP_MUL:
	asave = AS; bsave = lowprd = BS;		/* save AS, BS */
	do {
	    a = M[AS];					/* get mpcd char */
	    M[BS] = BCD_ZERO;				/* zero prod */
	    MM (AS); MM (BS);  }			/* decr pointers */
	while ((a & WM) == 0);				/* until A WM */
	if (reason) break;				/* address err? */
	M[BS] = BCD_ZERO;				/* zero hi prod */
	MM (BS);					/* addr low mpyr */
	sign = ((M[asave] & ZONE) == BBIT) ^ ((M[BS] & ZONE) == BBIT);

/* Outer loop on multiplier (BS) and product digits (ps),
   inner loop on multiplicand digits (AS).
   AS and ps cannot produce an address error.
*/

	do {
	    ps = bsave;					/* ptr to prod */
	    AS = asave;					/* ptr to mpcd */
	    carry = 0;					/* init carry */
	    b = M[BS];					/* get mpyr char */
	    do {
	    	a = M[AS];				/* get mpcd char */
		t = (bcd_to_bin[a & DIGIT] *		/* mpyr * mpcd */
		     bcd_to_bin[b & DIGIT]) +		/* + c + partial prod */
		     carry + bcd_to_bin[M[ps] & DIGIT];
		carry = cry_table[t];
		M[ps] = (M[ps] & WM) | sum_table[t];
		MM (AS); ps--;  }
	    while ((a & WM) == 0);			/* until mpcd done */
	    M[BS] = (M[BS] & WM) | BCD_ZERO;		/* zero mpyr just used */
	    t = bcd_to_bin[M[ps] & DIGIT] + carry;	/* add carry to prod */
	    M[ps] = (M[ps] & WM) | sum_table[t];	/* store */
	    bsave--;					/* adv prod ptr */
	    MM (BS);  }					/* adv mpyr ptr */
	while ((b & WM) == 0);				/* until mpyr done */
	M[lowprd] = M[lowprd] | ZONE;			/* assume + */
	if (sign) M[lowprd] = M[lowprd] & ~ABIT;	/* if minus, B only */
	break;	

/* Divide.  Comments from the PDP-10 based simulator by Len Fehskens.

   Divide is done, like multiply, pretty much the same way you do it with
   pencil and paper; successive subtraction of the divisor from a substring
   of the dividend while counting up the corresponding quotient digit.

   Let LS be the length of the divisor, LD the length of the dividend:
   - AS points to the low order divisor digit.
   - BS points to the high order dividend digit.
   - The low order dividend digit is identified by sign (zone) bits.
   - To the left of the dividend is a zero field of length LS + 1.
   The low quotient is at low dividend - LS - 1.  As BS points to the
   high dividend, the low dividend is at BS + LD - 1, so the low
   quotient is at BS + LD - LS - 2.  The longest possible quotient is
   LD - LS + 1, so the first possible non-zero quotient bit will be
   found as BS - 2.

   This pointer calculation assumes that the divisor has no leading zeroes.
   For each leading zero, the start of the quotient will be one position
   further left.

   Start by locating the high order non-zero digit of the divisor.  This
   also tests for a divide by zero.

   Instruction lengths:

   1		chained
   2,3		invalid A-address
   4		self (B-address = A-address)
   5,6		invalid B-address
   7		normal
   8+		normal + ignored modifier
*/

case OP_DIV:
	asave = AS; ahigh = -1;
	do {
	    a = M[AS];					/* get dvr char */
	    if ((a & CHAR) != BCD_ZERO) ahigh = AS;	/* mark non-zero */
	    MM (AS);  }
	while ((a & WM) == 0);
	if (reason) break;				/* address err? */
	if (ahigh < 0) {				/* div? by zero */
	    ind[IN_OVF] = 1;				/* set ovf indic */
	    qs = bsave = BS;				/* quo, dividend */
	    do {
	    	b = M[bsave];				/* find end divd */
		PP (bsave);  }				/* marked by zone */
	    while ((b & ZONE) == 0);
	    if (reason) break;				/* address err? */
	    if (ADDR_ERR (qs)) {			/* address err? */
		reason = STOP_WRAP;			/* address wrap? */
		break;  }
	    div_sign (M[asave], b, qs - 1, bsave - 1);	/* set signs */
	    BS = (BS - 2) - (asave - (AS + 1));		/* final bs */
	    break;	}
	bsave = BS + (asave - ahigh);			/* end subdivd */
	qs = (BS - 2) - (ahigh - (AS + 1));		/* quo start */

/* Divide loop - done with subroutines to keep the code clean.
   In the loop,
	
   asave =	low order divisor
   bsave =	low order subdividend
   qs	=	current quotient digit
*/

	do {
	    quo = 0;					/* clear quo digit */
	    if (ADDR_ERR (qs) || ADDR_ERR (bsave)) {
		reason = STOP_WRAP;			/* address wrap? */
		break;  }
	    b = M[bsave];				/* save low divd */
	    do {
	    	t = div_sub (asave, bsave, ahigh);	/* subtract */
		quo++;  }				/* incr quo digit */
	    while (t == 0);				/* until borrow */
	    div_add (asave, bsave, ahigh); quo--;	/* restore */
	    M[qs] = (M[qs] & WM) | sum_table[quo];	/* store quo digit */
	    bsave++; qs++;  }				/* adv divd, quo */
	while ((b & ZONE) == 0);			/* until B sign */
	if (reason) break;				/* address err? */

/* At this point,

   AS	=	high order divisor - 1
   asave =	unit position of divisor
   b	=	unit character of dividend
   bsave =	unit position of remainder + 1
   qs	=	unit position of quotient + 1
*/

	div_sign (M[asave], b, qs - 1, bsave - 1);	/* set signs */
	BS = qs - 2;					/* BS = quo 10's pos */
	break;

/* Word mark instructions				A check	   B check

   SWM		set WM on A char and B char		fetch	   fetch
   CWM		clear WM on A char and B char		fetch	   fetch

   Instruction lengths:

   1		chained
   2,3		invalid A-address
   4		one operand (B-address = A-address)
   5,6		invalid B-address
   7		two operands (SWM cannot be longer than 7)
   8+		two operands + ignored modifier
*/

case OP_SWM:						/* set word mark */
	M[BS] = M[BS] | WM;				/* set A field mark */
	M[AS] = M[AS] | WM;				/* set B field mark */
	MM (AS); MM (BS);				/* decr pointers */
	break;

case OP_CWM:						/* clear word mark */
	M[BS] = M[BS] & ~WM;				/* clear A field mark */
	M[AS] = M[AS] & ~WM;				/* clear B field mark */
	MM (AS); MM (BS);				/* decr pointers */
	break;

/* Clear storage instruction				A check    B check

   CS		clear from B down to nearest hundreds	if branch  fetch
		address

   Instruction lengths:

   1		chained
   2,3		invalid A-address and B-address
   4		one operand (B-address = A-address)
   5,6		invalid B-address
   7		branch
   8+		one operand, branch ignored
*/

case OP_CS:						/* clear storage */
	t = (BS / 100) * 100;				/* lower bound */
	while (BS >= t) M[BS--] = 0;			/* clear region */
	if (BS < 0) BS = BS + MEMSIZE;			/* wrap if needed */
	if (ilnt == 7) { BRANCH; }			/* branch variant? */
	break;

/* Modify address instruction				A check    B check

   MA		add A addr and B addr, store at B addr	fetch	   fetch

   Instruction lengths:
   1		chained
   2,3		invalid A-address and B-address
   4		self (B-address = A-address)
   5,6		invalid B-address
   7		normal
   8+		normal + ignored modifier
*/

case OP_MA:						/* modify address */
	a = one_table[M[AS] & CHAR]; MM (AS);		/* get A address */
	a = a + ten_table[M[AS] & CHAR]; MM (AS);
	a = a + hun_table[M[AS] & CHAR]; MM (AS);
	b = one_table[M[BS] & CHAR]; MM (BS);		/* get B address */
	b = b + ten_table[M[BS] & CHAR]; MM (BS);
	b = b + hun_table[M[BS] & CHAR]; MM (BS);
	t = ((a + b) & INDEXMASK) % MAXMEMSIZE;		/* compute sum */
	M[BS + 3] = (M[BS + 3] & WM) | store_addr_u (t);
	M[BS + 2] = (M[BS + 2] & (WM + ZONE)) | store_addr_t (t);
	M[BS + 1] = (M[BS + 1] & WM) | store_addr_h (t);
	if (((a % 4000) + (b % 4000)) >= 4000) BS = BS + 2;	/* carry? */
	break;

/* Store address instructions				A-check     B-check

   SAR		store A* at A addr			fetch
   SBR		store B* at A addr			fetch

   Instruction lengths:
   1		chained
   2,3		invalid A-address
   4		normal
   5+		B-address overwritten from instruction;
		invalid address ignored
*/

case OP_SAR: case OP_SBR:				/* store A, B reg */
	M[AS] = (M[AS] & WM) | store_addr_u (BS); MM (AS);
	M[AS] = (M[AS] & WM) | store_addr_t (BS); MM (AS);
	M[AS] = (M[AS] & WM) | store_addr_h (BS); MM (AS);
	break;

/* NOP - no validity checking, all instructions length ok */

case OP_NOP:						/* nop */
	break;

/* HALT - unless length = 4 (branch), no validity checking; all lengths ok */

case OP_H:						/* halt */
	if (ilnt == 4) hb_pend = 1;			/* set pending branch */
	reason = STOP_HALT;				/* stop simulator */
	saved_IS = IS;					/* commit instruction */
	break;

default:
	reason = STOP_NXI;				/* unimplemented */
	break;  }					/* end switch */
}							/* end while */

/* Simulation halted */

as_err = ADDR_ERR (AS);					/* get addr err flags */
bs_err = ADDR_ERR (BS);
AS = AS & ADDRMASK;					/* clean addresses */
BS = BS & ADDRMASK;
pcq_r->qptr = pcq_p;					/* update pc q ptr */
return reason;
}							/* end sim_instr */

/* store addr_x - convert address to BCD character in x position

   Inputs:
	addr	=	address to convert
   Outputs:
	char	=	converted address character
*/

int32 store_addr_h (int32 addr)
{
int32 thous;

thous = (addr / 1000) & 03;
return  bin_to_bcd[(addr % 1000) / 100] | (thous << V_ZONE);
}

int32 store_addr_t (int32 addr)
{
return bin_to_bcd[(addr % 100) / 10];
}

int32 store_addr_u (int32 addr)
{
int32 thous;

thous = (addr / 1000) & 014;
return bin_to_bcd[addr % 10] | (thous << (V_ZONE - 2));
}

/* div_add - add string for divide */

int32 div_add (int32 ap, int32 bp, int32 aend)
{
int32 a, b, c, r;

c = 0;							/* init carry */
do {	a = M[ap]; b = M[bp];				/* get operands */
	r = bcd_to_bin[b & DIGIT] +			/* sum digits + c */
	    bcd_to_bin[a & DIGIT] + c;
	c = (r >= 10);					/* set carry out */
	M[bp] = sum_table[r];				/* store result */
	ap--; bp--;  }
while (ap >= aend);
return c;
}

/* div_sub - substract string for divide */

int32 div_sub (int32 ap, int32 bp, int32 aend)
{
int32 a, b, c, r;

c = 0;							/* init borrow */
do {	a = M[ap]; b = M[bp];				/* get operands */
	r = bcd_to_bin[b & DIGIT] -			/* a - b - borrow */
	    bcd_to_bin[a & DIGIT] - c;
	c = (r < 0);					/* set borrow out */
	M[bp] = sum_table[r + 10];			/* store result */
	ap--; bp--;  }
while (ap >= aend);
b = M[bp] & CHAR;					/* borrow position */
if (b != BCD_ZERO) {					/* non-zero? */
	r = bcd_to_bin[b & DIGIT] - c;			/* subtract borrow */
	M[bp] = sum_table[r];				/* store result */
	return 0;  }					/* subtract worked */
return c;						/* return borrow */
}

/* div_sign - set signs for divide */

void div_sign (int32 dvrc, int32 dvdc, int32 qp, int32 rp)
{
int32 sign = dvrc & ZONE;				/* divisor sign */

M[rp] = M[rp] | ZONE;					/* assume rem pos */
if (sign == BBIT) M[rp] = M[rp] & ~ABIT;		/* if dvr -, rem - */
M[qp] = M[qp] | ZONE;					/* assume quo + */
if (((dvdc & ZONE) == BBIT) ^ (sign == BBIT))		/* dvr,dvd diff? */
	M[qp] = M[qp] & ~ABIT;				/* make quo - */
return;
}

/* iomod - check on I/O modifiers

   Inputs:
	ilnt	=	instruction length
	mod	=	modifier character
	tptr	=	pointer to table of modifiers, end is -1
   Output:
	status	=	SCPE_OK if ok, STOP_INVM if invalid
*/

t_stat iomod (int32 ilnt, int32 mod, const int32 *tptr)
{
if ((ilnt != 2) && (ilnt != 5) && (ilnt < 8)) return SCPE_OK;
if (tptr == NULL) return STOP_INVM;
do {	if (mod == *tptr++) return SCPE_OK;  }
while (*tptr >= 0);
return STOP_INVM;
}

/* iodisp - dispatch load or move to I/O routine

   Inputs:
	dev	=	device number
	unit	=	unit number
	flag	=	move (MD_NORM) vs load (MD_WM)
	mod	=	modifier
*/

t_stat iodisp (int32 dev, int32 unit, int32 flag, int32 mod)
{
if (dev == IO_INQ) return inq_io (flag, mod);		/* inq terminal? */
if (dev == IO_DP) return dp_io (unit, flag, mod);	/* disk pack? */
if (dev == IO_MT) return mt_io (unit, flag, mod);	/* magtape? */
if (dev == IO_MTB) {					/* binary? */
	if (flag == MD_WM) return STOP_INVM;		/* invalid */
	return mt_io (unit, MD_BIN, mod);  }
return STOP_NXD;					/* not implemented */
}

/* Reset routine */

t_stat cpu_reset (DEVICE *dptr)
{
int32 i;

for (i = 0; i < 64; i++) ind[i] = 0;			/* clr indicators */
ind[IN_UNC] = 1;					/* ind[0] always on */
AS = 0; as_err = 1;					/* clear AS */
BS = 0; bs_err = 1;					/* clear BS */
D = 0;							/* clear D */
hb_pend = 0;						/* no halt br */
pcq_r = find_reg ("ISQ", 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] & (WM + CHAR);
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 & (WM + CHAR);
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 % 1000) != 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;
if (MEMSIZE > 4000) cpu_unit.flags = cpu_unit.flags | MA;
else cpu_unit.flags = cpu_unit.flags & ~MA;
return SCPE_OK;
}