diff --git a/src/apr.c b/src/apr.c
index 8066ea2..f7e1087 100644
--- a/src/apr.c
+++ b/src/apr.c
@@ -1,20 +1,12 @@
 #include "pdp6.h"
 #include <unistd.h>
 
-word iobus0, iobus1;
-word iobus1_last, iobus1_pulse;
-int iodev;
-
 #define DBG_AR debug("AR: %012llo\n", apr->ar)
 #define DBG_MB debug("MB: %012llo\n", apr->mb)
 #define DBG_MQ debug("MQ: %012llo\n", apr->mq)
 #define DBG_MA debug("MA: %06o\n", apr->ma)
 #define DBG_IR debug("IR: %06o\n", apr->ir)
 
-Apr apr;
-
-int pulsestepping = 0;
-
 #define SWAPLTRT(a) (a = (a)<<18 & LT | (a)>>18 & RT)
 #define CONS(a, b) ((a)&LT | (b)&RT)
 
@@ -163,14 +155,14 @@ decodeir(Apr *apr)
 	apr->io_inst = apr->ir & 0700340;
 
 	// 5-7
-	iobus1 &= ~037777000000LL;
-	iobus1 |= apr->ir & H3 ? IOBUS_IOS3_1 : IOBUS_IOS3_0;
-	iobus1 |= apr->ir & H4 ? IOBUS_IOS4_1 : IOBUS_IOS4_0;
-	iobus1 |= apr->ir & H5 ? IOBUS_IOS5_1 : IOBUS_IOS5_0;
-	iobus1 |= apr->ir & H6 ? IOBUS_IOS6_1 : IOBUS_IOS6_0;
-	iobus1 |= apr->ir & H7 ? IOBUS_IOS7_1 : IOBUS_IOS7_0;
-	iobus1 |= apr->ir & H8 ? IOBUS_IOS8_1 : IOBUS_IOS8_0;
-	iobus1 |= apr->ir & H9 ? IOBUS_IOS9_1 : IOBUS_IOS9_0;
+	apr->iobus.c34 &= ~037777000000LL;
+	apr->iobus.c34 |= apr->ir & H3 ? IOBUS_IOS3_1 : IOBUS_IOS3_0;
+	apr->iobus.c34 |= apr->ir & H4 ? IOBUS_IOS4_1 : IOBUS_IOS4_0;
+	apr->iobus.c34 |= apr->ir & H5 ? IOBUS_IOS5_1 : IOBUS_IOS5_0;
+	apr->iobus.c34 |= apr->ir & H6 ? IOBUS_IOS6_1 : IOBUS_IOS6_0;
+	apr->iobus.c34 |= apr->ir & H7 ? IOBUS_IOS7_1 : IOBUS_IOS7_0;
+	apr->iobus.c34 |= apr->ir & H8 ? IOBUS_IOS8_1 : IOBUS_IOS8_0;
+	apr->iobus.c34 |= apr->ir & H9 ? IOBUS_IOS9_1 : IOBUS_IOS9_0;
 
 	apr->ir_fp = (apr->inst & 0740) == 0140;
 
@@ -325,14 +317,14 @@ set_pih(Apr *apr, int pih)
 
 /* Recalculate the PI requests on the IO bus from each device */
 void
-recalc_req(void)
+recalc_req(IOBus *bus)
 {
 	int i;
 	int req;
 	req = 0;
 	for(i = 0; i < 128; i++)
-		req |= iobus[i].pireq;
-	iobus1 = iobus1&~0177LL | req&0177;
+		req |= bus->dev[i].req;
+	bus->c34 = bus->c34&~0177LL | req&0177;
 }
 
 void
@@ -345,9 +337,9 @@ recalc_cpa_req(Apr *apr)
 	   apr->ar_pc_chg_flag && apr->cpa_pc_chg_enable ||
 	   apr->ar_ov_flag && apr->cpa_arov_enable)
 		req = apr->cpa_pia;
-	if(iobus[CPA].pireq != req){
-		iobus[CPA].pireq = req;
-		recalc_req();
+	if(apr->iobus.dev[CPA].req != req){
+		apr->iobus.dev[CPA].req = req;
+		recalc_req(&apr->iobus);
 	}
 }
 
@@ -615,15 +607,15 @@ pulse(ex_clr){
 }
 
 pulse(ex_set){
-	apr->pr  = iobus0>>28 & 0377;	// 7-4
-	apr->rlr = iobus0>>10 & 0377;	// 7-5
+	apr->pr  = apr->iobus.c12>>28 & 0377;	// 7-4
+	apr->rlr = apr->iobus.c12>>10 & 0377;	// 7-5
 }
 
 pulse(mr_start){
 	trace("MR START\n");
 
 	// 8-1
-	iobus1 |= IOBUS_IOB_RESET;
+	apr->iobus.c34 |= IOBUS_IOB_RESET;
 
 	// 8-5
 	apr->cpa_iot_user = 0;
@@ -635,7 +627,7 @@ pulse(mr_start){
 	apr->cpa_pdl_ov = 0;
 	apr->cpa_arov_enable = 0;
 	apr->cpa_pia = 0;
-	iobus[CPA].pireq = 0;
+	apr->iobus.dev[CPA].req = 0;
 
 	// PI
 	apr->pi_ov = 0;		// 8-4
@@ -665,40 +657,42 @@ void
 wake_cpa(void *dev)
 {
 	Apr *apr;
+	IOBus *bus;
 
 	apr = dev;
-	if(iodev != CPA)
+	bus = &apr->iobus;
+	if(apr->iobus.devcode != CPA)
 		return;
 	// 8-5
 	if(IOB_STATUS){
-		if(apr->cpa_pdl_ov)        iobus0 |= F19;
-		if(apr->cpa_iot_user)      iobus0 |= F20;
-		if(apr->ex_user)           iobus0 |= F21;
-		if(apr->cpa_illeg_op)      iobus0 |= F22;
-		if(apr->cpa_non_exist_mem) iobus0 |= F23;
-		if(apr->cpa_clock_enable)  iobus0 |= F25;
-		if(apr->cpa_clock_flag)    iobus0 |= F26;
-		if(apr->cpa_pc_chg_enable) iobus0 |= F28;
-		if(apr->ar_pc_chg_flag)    iobus0 |= F29;
-		if(apr->cpa_arov_enable)   iobus0 |= F31;
-		if(apr->ar_ov_flag)        iobus0 |= F32;
-		iobus0 |= apr->cpa_pia & 7;
+		if(apr->cpa_pdl_ov)        bus->c12 |= F19;
+		if(apr->cpa_iot_user)      bus->c12 |= F20;
+		if(apr->ex_user)           bus->c12 |= F21;
+		if(apr->cpa_illeg_op)      bus->c12 |= F22;
+		if(apr->cpa_non_exist_mem) bus->c12 |= F23;
+		if(apr->cpa_clock_enable)  bus->c12 |= F25;
+		if(apr->cpa_clock_flag)    bus->c12 |= F26;
+		if(apr->cpa_pc_chg_enable) bus->c12 |= F28;
+		if(apr->ar_pc_chg_flag)    bus->c12 |= F29;
+		if(apr->cpa_arov_enable)   bus->c12 |= F31;
+		if(apr->ar_ov_flag)        bus->c12 |= F32;
+		bus->c12 |= apr->cpa_pia & 7;
 	}
 	if(IOB_CONO_SET){
-		if(iobus0 & F18) apr->cpa_pdl_ov = 0;
-		if(iobus0 & F19) iobus1 |= IOBUS_IOB_RESET;	// 8-1
-		if(iobus0 & F22) apr->cpa_illeg_op = 0;
-		if(iobus0 & F23) apr->cpa_non_exist_mem = 0;
-		if(iobus0 & F24) apr->cpa_clock_enable = 0;
-		if(iobus0 & F25) apr->cpa_clock_enable = 1;
-		if(iobus0 & F26) apr->cpa_clock_flag = 0;
-		if(iobus0 & F27) apr->cpa_pc_chg_enable = 0;
-		if(iobus0 & F28) apr->cpa_pc_chg_enable = 1;
-		if(iobus0 & F29) apr->ar_pc_chg_flag = 0;	// 6-10
-		if(iobus0 & F30) apr->cpa_arov_enable = 0;
-		if(iobus0 & F31) apr->cpa_arov_enable = 1;
-		if(iobus0 & F32) apr->ar_ov_flag = 0;		// 6-10
-		apr->cpa_pia = iobus0 & 7;
+		if(bus->c12 & F18) apr->cpa_pdl_ov = 0;
+		if(bus->c12 & F19) bus->c34 |= IOBUS_IOB_RESET;	// 8-1
+		if(bus->c12 & F22) apr->cpa_illeg_op = 0;
+		if(bus->c12 & F23) apr->cpa_non_exist_mem = 0;
+		if(bus->c12 & F24) apr->cpa_clock_enable = 0;
+		if(bus->c12 & F25) apr->cpa_clock_enable = 1;
+		if(bus->c12 & F26) apr->cpa_clock_flag = 0;
+		if(bus->c12 & F27) apr->cpa_pc_chg_enable = 0;
+		if(bus->c12 & F28) apr->cpa_pc_chg_enable = 1;
+		if(bus->c12 & F29) apr->ar_pc_chg_flag = 0;	// 6-10
+		if(bus->c12 & F30) apr->cpa_arov_enable = 0;
+		if(bus->c12 & F31) apr->cpa_arov_enable = 1;
+		if(bus->c12 & F32) apr->ar_ov_flag = 0;		// 6-10
+		apr->cpa_pia = bus->c12 & 7;
 		recalc_cpa_req(apr);
 	}
 
@@ -716,35 +710,37 @@ void
 wake_pi(void *dev)
 {
 	Apr *apr;
+	IOBus *bus;
 
 	apr = dev;
-	if(iodev != PI)
+	bus = &apr->iobus;
+	if(bus->devcode != PI)
 		return;
 	// 8-4, 8-5
 	if(IOB_STATUS){
-		trace("PI STATUS %llo\n", iobus0);
-		if(apr->pi_active) iobus0 |= F28;
-		iobus0 |= apr->pio;
+		trace("PI STATUS %llo\n", bus->c12);
+		if(apr->pi_active) bus->c12 |= F28;
+		bus->c12 |= apr->pio;
 	}
 
 	// 8-4, 8-3
 	if(IOB_CONO_CLEAR){
-		trace("PI CONO CLEAR %llo\n", iobus0);
-		if(iobus0 & F23)
+		trace("PI CONO CLEAR %llo\n", bus->c12);
+		if(bus->c12 & F23)
 			// can call directly
 			pi_reset(apr);
 	}
 	if(IOB_CONO_SET){
-		trace("PI CONO SET %llo\n", iobus0);
-		if(iobus0 & F24)
-			set_pir(apr, apr->pir | iobus0&0177);
-		if(iobus0 & F25)
-			apr->pio |= iobus0&0177;
-		if(iobus0 & F26)
-			apr->pio &= ~(iobus0&0177);
-		if(iobus0 & F27)
+		trace("PI CONO SET %llo\n", bus->c12);
+		if(bus->c12 & F24)
+			set_pir(apr, apr->pir | bus->c12&0177);
+		if(bus->c12 & F25)
+			apr->pio |= bus->c12&0177;
+		if(bus->c12 & F26)
+			apr->pio &= ~(bus->c12&0177);
+		if(bus->c12 & F27)
 			apr->pi_active = 0;
-		if(iobus0 & F28)
+		if(bus->c12 & F28)
 			apr->pi_active = 1;
 	}
 }
@@ -757,9 +753,9 @@ wake_pi(void *dev)
 pulse(iot_t4){
 	trace("IOT T4\n");
 	/* Clear what was set in IOT T2 */
-	iobus1 &= ~(IOBUS_IOB_STATUS | IOBUS_IOB_DATAI);
+	apr->iobus.c34 &= ~(IOBUS_IOB_STATUS | IOBUS_IOB_DATAI);
 	/* and do something like IOB BUS RESET */
-	iobus0 = 0;
+	apr->iobus.c12 = 0;
 }
 
 pulse(iot_t3a){
@@ -774,10 +770,10 @@ pulse(iot_t3){
 	// 8-1
 	/* Pulses, cleared in the main loop. */
 	if(IOT_DATAO)
-		iobus1 |= IOBUS_DATAO_SET;
+		apr->iobus.c34 |= IOBUS_DATAO_SET;
 	if(IOT_CONO)
-		iobus1 |= IOBUS_CONO_SET;
-	apr->ar |= iobus0;		// 6-8
+		apr->iobus.c34 |= IOBUS_CONO_SET;
+	apr->ar |= apr->iobus.c12;	// 6-8
 	nextpulse(apr, et5);		// 5-5
 }
 
@@ -788,16 +784,16 @@ pulse(iot_t2){
 	/* These are asserted during INIT SETUP, IOT T2 and FINAL SETUP.
 	 * We clear them in IOT T4 which happens after FINAL SETUP */
 	if(IOT_OUTGOING)
-		iobus0 |= apr->ar;
+		apr->iobus.c12 |= apr->ar;
 	if(IOT_STATUS)
-		iobus1 |= IOBUS_IOB_STATUS;
+		apr->iobus.c34 |= IOBUS_IOB_STATUS;
 	if(IOT_DATAI)
-		iobus1 |= IOBUS_IOB_DATAI;
+		apr->iobus.c34 |= IOBUS_IOB_DATAI;
 	/* Pulses, cleared in the main loop. */
 	if(IOT_DATAO)
-		iobus1 |= IOBUS_DATAO_CLEAR;
+		apr->iobus.c34 |= IOBUS_DATAO_CLEAR;
 	if(IOT_CONO)
-		iobus1 |= IOBUS_CONO_CLEAR;
+		apr->iobus.c34 |= IOBUS_CONO_CLEAR;
 	nextpulse(apr, iot_t3);
 	nextpulse(apr, iot_t3a);
 }
@@ -2066,7 +2062,7 @@ pulse(xct_t0){
 
 pulse(pir_stb){
 	trace("PIR STB\n");
-	set_pir(apr, apr->pir | apr->pio & iobus1);	// 8-3
+	set_pir(apr, apr->pir | apr->pio & apr->iobus.c34);	// 8-3
 }
 
 pulse(pi_sync){
@@ -3029,8 +3025,18 @@ nextpulse(Apr *apr, Pulse *p)
 void
 initapr(Apr *apr)
 {
-	iobus[CPA] = (Busdev){ apr, wake_cpa, 0 };
-	iobus[PI] = (Busdev){ apr, wake_pi, 0 };
+	apr->iobus.dev[CPA] = (Busdev){ apr, wake_cpa, 0 };
+	apr->iobus.dev[PI] = (Busdev){ apr, wake_pi, 0 };
+}
+
+/* find out which bits were turned on */
+void
+updatebus(void *bus)
+{
+	IOBus *b;
+	b = bus;
+	b->c12_pulse = (b->c12_prev ^ b->c12) & b->c12;
+	b->c34_pulse = (b->c34_prev ^ b->c34) & b->c34;
 }
 
 void*
@@ -3039,7 +3045,7 @@ aprmain(void *p)
 	Apr *apr;
 	Busdev *dev;
 	Pulse **tmp;
-	int i;
+	int i, devcode;
 
 	apr = (Apr*)p;
 	apr->clist = apr->pulses1;
@@ -3048,8 +3054,8 @@ aprmain(void *p)
 	apr->nnextpulses = 0;
 	apr->ia_inh = 0;
 
-	iobus0 = 0;
-	iobus1 = 0;
+	apr->iobus.c12 = 0;
+	apr->iobus.c34 = 0;
 
 	nextpulse(apr, mr_pwr_clr);
 	while(apr->sw_power){
@@ -3057,22 +3063,21 @@ aprmain(void *p)
 		apr->nnextpulses = 0;
 		tmp = apr->clist; apr->clist = apr->nlist; apr->nlist = tmp;
 
-		if(pulsestepping){
+		if(apr->pulsestepping){
 			int c;
 			while(c = getchar(), c != EOF && c != '\n')
 				if(c == 'x')
-					pulsestepping = 0;
+					apr->pulsestepping = 0;
 		}
 
-		iobus1_last = iobus1;
+		apr->iobus.c12_prev = apr->iobus.c12;
+		apr->iobus.c34_prev = apr->iobus.c34;
 		membus0_last = membus0;
 
 		for(i = 0; i < apr->ncurpulses; i++)
 			apr->clist[i](apr);
 
-		/* find out which bits were turned on */
-		iobus1_pulse = (iobus1_last ^ iobus1) & iobus1;
-		iobus1_pulse &= ~037777000177LL;
+		updatebus(&apr->iobus);
 		membus0_pulse = (membus0_last ^ membus0) & membus0;
 
 		/* This is simplified, we have no IOT RESET,
@@ -3096,29 +3101,30 @@ aprmain(void *p)
 
 
 		/* Pulses and signals through IO bus */
-		iodev = -1;
-		if(iobus1_pulse & (IOBUS_PULSES | IOBUS_IOB_STATUS | IOBUS_IOB_DATAI)){
-			iodev = 0;
-			if(iobus1 & IOBUS_IOS3_1) iodev |= 0100;
-			if(iobus1 & IOBUS_IOS4_1) iodev |= 0040;
-			if(iobus1 & IOBUS_IOS5_1) iodev |= 0020;
-			if(iobus1 & IOBUS_IOS6_1) iodev |= 0010;
-			if(iobus1 & IOBUS_IOS7_1) iodev |= 0004;
-			if(iobus1 & IOBUS_IOS8_1) iodev |= 0002;
-			if(iobus1 & IOBUS_IOS9_1) iodev |= 0001;
-			dev = &iobus[iodev];
+		apr->iobus.devcode = -1;
+		if(apr->iobus.c34_pulse & (IOBUS_PULSES | IOBUS_IOB_STATUS | IOBUS_IOB_DATAI)){
+			devcode = 0;
+			if(apr->iobus.c34 & IOBUS_IOS3_1) devcode |= 0100;
+			if(apr->iobus.c34 & IOBUS_IOS4_1) devcode |= 0040;
+			if(apr->iobus.c34 & IOBUS_IOS5_1) devcode |= 0020;
+			if(apr->iobus.c34 & IOBUS_IOS6_1) devcode |= 0010;
+			if(apr->iobus.c34 & IOBUS_IOS7_1) devcode |= 0004;
+			if(apr->iobus.c34 & IOBUS_IOS8_1) devcode |= 0002;
+			if(apr->iobus.c34 & IOBUS_IOS9_1) devcode |= 0001;
+			apr->iobus.devcode = devcode;
+			dev = &apr->iobus.dev[devcode];
 			if(dev->wake)
 				dev->wake(dev->dev);
 		}
-		if(iobus1_pulse & IOBUS_IOB_RESET){
+		if(apr->iobus.c34_pulse & IOBUS_IOB_RESET){
 			int d;
-			for(d = 0; d < nelem(iobus); d++){
-				dev = &iobus[d];
+			for(d = 0; d < 128; d++){
+				dev = &apr->iobus.dev[d];
 				if(dev->wake)
 					dev->wake(dev->dev);
 			}
 		}
-		iobus1 &= ~(IOBUS_PULSES | IOBUS_IOB_RESET);
+		apr->iobus.c34 &= ~(IOBUS_PULSES | IOBUS_IOB_RESET);
 
 
 		/* Pulses to memory */
diff --git a/src/main.c b/src/main.c
index bd62777..25a0334 100644
--- a/src/main.c
+++ b/src/main.c
@@ -5,8 +5,6 @@
 #include <pthread.h>
 #include "args.h"
 
-Busdev iobus[128];
-
 typedef struct Point Point;
 struct Point
 {
@@ -338,7 +336,7 @@ updateapr(Apr *apr, Ptr *ptr)
 	setlights(apr->pr, pr_l, 8);
 	setlights(apr->rlr, rlr_l, 8);
 	setlights(apr->rla, rla_l, 8);
-	setlights(iobus0, iobus_l, 36);
+	setlights(apr->iobus.c12, iobus_l, 36);
 }
 
 void
@@ -663,9 +661,9 @@ main(int argc, char *argv[])
 
 	initapr(&apr);
 	initmem();
-	inittty(&tty);
-	initptr(&ptr);
-	initptp(&ptp);
+	inittty(&tty, &apr.iobus);
+	initptr(&ptr, &apr.iobus);
+	initptp(&ptp, &apr.iobus);
 
 	for(;;){
 		start = SDL_GetTicks();
diff --git a/src/pdp6.h b/src/pdp6.h
index eda3d92..a699189 100644
--- a/src/pdp6.h
+++ b/src/pdp6.h
@@ -21,6 +21,10 @@ extern int dotrace;
 void trace(char *fmt, ...);
 void debug(char *fmt, ...);
 
+enum {
+	MAXPULSE = 5
+};
+
 enum Mask {
 	FW   = 0777777777777,
 	RT   = 0000000777777,
@@ -102,16 +106,131 @@ enum Opcode {
 
 };
 
+/*
+ * Memory
+ */
+
+void initmem(void);
+void dumpmem(void);
+void wakemem(void);
+// 7-2, 7-10
 enum {
-	MAXPULSE = 5
+	MEMBUS_MA21         = 0000000000001,
+	MEMBUS_WR_RQ        = 0000000000004,
+	MEMBUS_RD_RQ        = 0000000000010,
+	MEMBUS_MA_FMC_SEL0  = 0000001000000,
+	MEMBUS_MA_FMC_SEL1  = 0000002000000,
+	MEMBUS_MA35_0       = 0000004000000,
+	MEMBUS_MA35_1       = 0000010000000,
+	MEMBUS_MA21_0       = 0000020000000,
+	MEMBUS_MA21_1       = 0000040000000,
+	MEMBUS_MA20_0       = 0000100000000,
+	MEMBUS_MA20_1       = 0000200000000,
+	MEMBUS_MA19_0       = 0000400000000,
+	MEMBUS_MA19_1       = 0001000000000,
+	MEMBUS_MA18_0       = 0002000000000,
+	MEMBUS_MA18_1       = 0004000000000,
+	MEMBUS_RQ_CYC       = 0020000000000,
+	MEMBUS_WR_RS        = 0100000000000,
+	MEMBUS_MAI_RD_RS    = 0200000000000,
+	MEMBUS_MAI_ADDR_ACK = 0400000000000,
+};
+/* 0 is cable 1 & 2 (above bits); 1 is cable 3 & 4 (data) */
+extern word membus0, membus1;
+/* record the state of membus0 of the last pulse step
+ * to recognize pulses or edges */
+extern word membus0_last, membus0_pulse;
+
+
+/*
+ * IO bus
+ */
+
+// 7-10
+enum {
+	IOBUS_PI_REQ_7    = F35,
+	IOBUS_PI_REQ_6    = F34,
+	IOBUS_PI_REQ_5    = F33,
+	IOBUS_PI_REQ_4    = F32,
+	IOBUS_PI_REQ_3    = F31,
+	IOBUS_PI_REQ_2    = F30,
+	IOBUS_PI_REQ_1    = F29,
+	IOBUS_IOB_STATUS  = F23,
+	IOBUS_IOB_DATAI   = F22,
+	IOBUS_CONO_SET    = F21,
+	IOBUS_CONO_CLEAR  = F20,
+	IOBUS_DATAO_SET   = F19,
+	IOBUS_DATAO_CLEAR = F18,
+	IOBUS_IOS9_1      = F17,
+	IOBUS_IOS9_0      = F16,
+	IOBUS_IOS8_1      = F15,
+	IOBUS_IOS8_0      = F14,
+	IOBUS_IOS7_1      = F13,
+	IOBUS_IOS7_0      = F12,
+	IOBUS_IOS6_1      = F11,
+	IOBUS_IOS6_0      = F10,
+	IOBUS_IOS5_1      = F9,
+	IOBUS_IOS5_0      = F8,
+	IOBUS_IOS4_1      = F7,
+	IOBUS_IOS4_0      = F6,
+	IOBUS_IOS3_1      = F5,
+	IOBUS_IOS3_0      = F4,
+	IOBUS_MC_DR_SPLIT = F3,
+	IOBUS_POWER_ON    = F1,
+	IOBUS_IOB_RESET   = F0,
+	IOBUS_PULSES = IOBUS_CONO_SET | IOBUS_CONO_CLEAR |
+	               IOBUS_DATAO_SET | IOBUS_DATAO_CLEAR
 };
 
+#define IOB_RESET       (bus->c34 & IOBUS_IOB_RESET)
+#define IOB_DATAO_CLEAR (bus->c34 & IOBUS_DATAO_CLEAR)
+#define IOB_DATAO_SET   (bus->c34 & IOBUS_DATAO_SET)
+#define IOB_CONO_CLEAR  (bus->c34 & IOBUS_CONO_CLEAR)
+#define IOB_CONO_SET    (bus->c34 & IOBUS_CONO_SET)
+#define IOB_STATUS      (bus->c34 & IOBUS_IOB_STATUS)
+#define IOB_DATAI       (bus->c34 & IOBUS_IOB_DATAI)
+
+/* A peripheral device connected to an IO bus */
+typedef struct Busdev Busdev;
+struct Busdev
+{
+	void *dev;
+	void (*wake)(void *dev);
+	u8 req;
+};
+
+typedef struct IOBus IOBus;
+struct IOBus
+{
+	/* c12 are cables 1 and 2, c34 3 and 4.
+	 * c??_prev records the previous state of the bus
+	 * and c??_pulse is used to detect leading edges. */
+	word c12, c12_prev, c12_pulse;
+	word c34, c34_prev, c34_pulse;
+	/* device code decoded from the cables */
+	int devcode;
+	/* All IO devices connected to this bus */
+	Busdev dev[128];
+};
+void recalc_req(IOBus *bus);
+
+
+/*
+ * Devices
+ */
+
+/* Arithmetic Processor 166 */
+#define CPA (0000>>2)
+#define PI (0004>>2)
 typedef struct Apr Apr;
 
 typedef void Pulse(Apr *apr);
 #define pulse(p) static void p(Apr *apr)
 
-struct Apr {
+struct Apr
+{
+	IOBus iobus;
+
 	hword ir;
 	word mi;
 	word data;
@@ -210,121 +329,23 @@ struct Apr {
 	/* needed for the emulation */
 	int extpulse;
 	bool ia_inh;	// this is asserted for some time
+	int pulsestepping;
 
 	Pulse *pulses1[MAXPULSE], *pulses2[MAXPULSE];
 	Pulse **clist, **nlist;
 	int ncurpulses, nnextpulses;
 };
+void initapr(Apr *apr);
 void nextpulse(Apr *apr, Pulse *p);
 void *aprmain(void *p);
 
-void initmem(void);
-void dumpmem(void);
-void wakemem(void);
-// 7-2, 7-10
-enum {
-	MEMBUS_MA21         = 0000000000001,
-	MEMBUS_WR_RQ        = 0000000000004,
-	MEMBUS_RD_RQ        = 0000000000010,
-	MEMBUS_MA_FMC_SEL0  = 0000001000000,
-	MEMBUS_MA_FMC_SEL1  = 0000002000000,
-	MEMBUS_MA35_0       = 0000004000000,
-	MEMBUS_MA35_1       = 0000010000000,
-	MEMBUS_MA21_0       = 0000020000000,
-	MEMBUS_MA21_1       = 0000040000000,
-	MEMBUS_MA20_0       = 0000100000000,
-	MEMBUS_MA20_1       = 0000200000000,
-	MEMBUS_MA19_0       = 0000400000000,
-	MEMBUS_MA19_1       = 0001000000000,
-	MEMBUS_MA18_0       = 0002000000000,
-	MEMBUS_MA18_1       = 0004000000000,
-	MEMBUS_RQ_CYC       = 0020000000000,
-	MEMBUS_WR_RS        = 0100000000000,
-	MEMBUS_MAI_RD_RS    = 0200000000000,
-	MEMBUS_MAI_ADDR_ACK = 0400000000000,
-};
-/* 0 is cable 1 & 2 (above bits); 1 is cable 3 & 4 (data) */
-extern word membus0, membus1;
-/* record the state of membus0 of the last pulse step
- * to recognize pulses or edges */
-extern word membus0_last, membus0_pulse;
-
-// 7-10
-enum {
-	IOBUS_PI_REQ_7    = F35,
-	IOBUS_PI_REQ_6    = F34,
-	IOBUS_PI_REQ_5    = F33,
-	IOBUS_PI_REQ_4    = F32,
-	IOBUS_PI_REQ_3    = F31,
-	IOBUS_PI_REQ_2    = F30,
-	IOBUS_PI_REQ_1    = F29,
-	IOBUS_IOB_STATUS  = F23,
-	IOBUS_IOB_DATAI   = F22,
-	IOBUS_CONO_SET    = F21,
-	IOBUS_CONO_CLEAR  = F20,
-	IOBUS_DATAO_SET   = F19,
-	IOBUS_DATAO_CLEAR = F18,
-	IOBUS_IOS9_1      = F17,
-	IOBUS_IOS9_0      = F16,
-	IOBUS_IOS8_1      = F15,
-	IOBUS_IOS8_0      = F14,
-	IOBUS_IOS7_1      = F13,
-	IOBUS_IOS7_0      = F12,
-	IOBUS_IOS6_1      = F11,
-	IOBUS_IOS6_0      = F10,
-	IOBUS_IOS5_1      = F9,
-	IOBUS_IOS5_0      = F8,
-	IOBUS_IOS4_1      = F7,
-	IOBUS_IOS4_0      = F6,
-	IOBUS_IOS3_1      = F5,
-	IOBUS_IOS3_0      = F4,
-	IOBUS_MC_DR_SPLIT = F3,
-	IOBUS_POWER_ON    = F1,
-	IOBUS_IOB_RESET   = F0,
-	IOBUS_PULSES = IOBUS_CONO_SET | IOBUS_CONO_CLEAR |
-	               IOBUS_DATAO_SET | IOBUS_DATAO_CLEAR
-};
-/* 0 is cable 1 & 2 (data); 1 is cable 3 & 4 (above bits) */
-extern word iobus0, iobus1;
-/* record the state of iobus1 of the last pulse step
- * to recognize pulses or edges */
-extern word iobus1_last, iobus1_pulse;
-extern int iodev;
-
-#define IOB_RESET       (iobus1 & IOBUS_IOB_RESET)
-#define IOB_DATAO_CLEAR (iobus1 & IOBUS_DATAO_CLEAR)
-#define IOB_DATAO_SET   (iobus1 & IOBUS_DATAO_SET)
-#define IOB_CONO_CLEAR  (iobus1 & IOBUS_CONO_CLEAR)
-#define IOB_CONO_SET    (iobus1 & IOBUS_CONO_SET)
-#define IOB_STATUS      (iobus1 & IOBUS_IOB_STATUS)
-#define IOB_DATAI       (iobus1 & IOBUS_IOB_DATAI)
-
-typedef struct Busdev Busdev;
-struct Busdev
-{
-	void *dev;
-	void (*wake)(void *dev);
-	u8 pireq;
-};
-
-extern Busdev iobus[128];
-void recalc_req(void);
-
-/*
- * Devices
- */
-
-/* Arithmetic Processor 166 */
-
-#define CPA (0000>>2)
-#define PI (0004>>2)
-void initapr(Apr *apr);
 
 /* Paper tape punch 761 */
 #define PTP (0100>>2)
 typedef struct Ptp Ptp;
 struct Ptp
 {
+	IOBus *bus;
 	uchar ptp;
 	bool busy, flag, b;
 	int pia;
@@ -332,13 +353,15 @@ struct Ptp
 	FILE *fp;
 	int waitdatao;
 };
-void initptp(Ptp *ptp);
+void initptp(Ptp *ptp, IOBus *bus);
+
 
 /* Paper tape reader 760 */
 #define PTR (0104>>2)
 typedef struct Ptr Ptr;
 struct Ptr
 {
+	IOBus *bus;
 	int motor_on;
 	word sr;
 	word ptr;
@@ -347,21 +370,23 @@ struct Ptr
 
 	FILE *fp;
 };
-void initptr(Ptr *ptr);
+void initptr(Ptr *ptr, IOBus *bus);
 void ptr_setmotor(Ptr *ptr, int m);
 
+
 /* Teletype 626 */
 #define TTY (0120>>2)
 typedef struct Tty Tty;
 struct Tty
 {
+	IOBus *bus;
 	uchar tto, tti;
 	bool tto_busy, tto_flag;
 	bool tti_busy, tti_flag;
 	int pia;
 	int fd;
 };
-void inittty(Tty *tty);
+void inittty(Tty *tty, IOBus *bus);
 
 
 
diff --git a/src/pt.c b/src/pt.c
index 5a930aa..0c3f186 100644
--- a/src/pt.c
+++ b/src/pt.c
@@ -9,32 +9,36 @@
 
 /* TODO? implement motor delays */
 
-void
+static void
 recalc_ptp_req(Ptp *ptp)
 {
 	u8 req;
+	IOBus *bus;
+	bus = ptp->bus;
 	req = ptp->flag ? ptp->pia : 0;
-	if(req != iobus[PTP].pireq){
-		iobus[PTP].pireq = req;
-		recalc_req();
+	if(req != bus->dev[PTP].req){
+		bus->dev[PTP].req = req;
+		recalc_req(bus);
 	}
 }
 
-void
+static void
 recalc_ptr_req(Ptr *ptr)
 {
 	u8 req;
+	IOBus *bus;
+	bus = ptr->bus;
 	req = ptr->flag ? ptr->pia : 0;
-	if(req != iobus[PTR].pireq){
-		iobus[PTR].pireq = req;
-		recalc_req();
+	if(req != bus->dev[PTR].req){
+		bus->dev[PTR].req = req;
+		recalc_req(bus);
 	}
 }
 
 /* We have to punch after DATAO SET has happened. But BUSY is set by
  * DATAO CLEAR. So we use waitdatao to record when SET has happened */
 
-void*
+static void*
 ptpthread(void *dev)
 {
 	Ptp *ptp;
@@ -57,7 +61,7 @@ ptpthread(void *dev)
 	return nil;
 }
 
-void*
+static void*
 ptrthread(void *dev)
 {
 	Ptr *ptr;
@@ -100,20 +104,22 @@ static void
 wake_ptp(void *dev)
 {
 	Ptp *ptp;
+	IOBus *bus;
 
 	ptp = dev;
+	bus = ptp->bus;
 	if(IOB_RESET){
 		ptp->pia = 0;
 		ptp->busy = 0;
 		ptp->flag = 0;
 		ptp->b = 0;
 	}
-	if(iodev == PTP){
+	if(bus->devcode == PTP){
 		if(IOB_STATUS){
-			if(ptp->b) iobus0 |= F30;
-			if(ptp->busy) iobus0 |= F31;
-			if(ptp->flag) iobus0 |= F32;
-			iobus0 |= ptp->pia & 7;
+			if(ptp->b) bus->c12 |= F30;
+			if(ptp->busy) bus->c12 |= F31;
+			if(ptp->flag) bus->c12 |= F32;
+			bus->c12 |= ptp->pia & 7;
 		}
 		if(IOB_CONO_CLEAR){
 			ptp->pia = 0;
@@ -122,10 +128,10 @@ wake_ptp(void *dev)
 			ptp->b = 0;
 		}
 		if(IOB_CONO_SET){
-			if(iobus0 & F30) ptp->b = 1;
-			if(iobus0 & F31) ptp->busy = 1;
-			if(iobus0 & F32) ptp->flag = 1;
-			ptp->pia |= iobus0 & 7;
+			if(bus->c12 & F30) ptp->b = 1;
+			if(bus->c12 & F31) ptp->busy = 1;
+			if(bus->c12 & F32) ptp->flag = 1;
+			ptp->pia |= bus->c12 & 7;
 		}
 		if(IOB_DATAO_CLEAR){
 			ptp->ptp = 0;
@@ -134,7 +140,7 @@ wake_ptp(void *dev)
 			ptp->waitdatao = 0;
 		}
 		if(IOB_DATAO_SET){
-			ptp->ptp = iobus0 & 0377;
+			ptp->ptp = bus->c12 & 0377;
 			ptp->waitdatao = 1;
 		}
 	}
@@ -145,8 +151,10 @@ static void
 wake_ptr(void *dev)
 {
 	Ptr *ptr;
+	IOBus *bus;
 
 	ptr = dev;
+	bus = ptr->bus;
 	if(IOB_RESET){
 		ptr->pia = 0;
 		ptr->busy = 0;
@@ -154,16 +162,16 @@ wake_ptr(void *dev)
 		ptr->b = 0;
 	}
 
-	if(iodev == PTR){
+	if(bus->devcode == PTR){
 		if(IOB_STATUS){
-			if(ptr->motor_on) iobus0 |= F27;
-			if(ptr->b) iobus0 |= F30;
-			if(ptr->busy) iobus0 |= F31;
-			if(ptr->flag) iobus0 |= F32;
-			iobus0 |= ptr->pia & 7;
+			if(ptr->motor_on) bus->c12 |= F27;
+			if(ptr->b) bus->c12 |= F30;
+			if(ptr->busy) bus->c12 |= F31;
+			if(ptr->flag) bus->c12 |= F32;
+			bus->c12 |= ptr->pia & 7;
 		}
 		if(IOB_DATAI){
-			iobus0 |= ptr->ptr;
+			bus->c12 |= ptr->ptr;
 			ptr->flag = 0;
 			// actually when DATAI is negated again
 			ptr->busy = 1;
@@ -175,10 +183,10 @@ wake_ptr(void *dev)
 			ptr->b = 0;
 		}
 		if(IOB_CONO_SET){
-			if(iobus0 & F30) ptr->b = 1;
-			if(iobus0 & F31) ptr->busy = 1;
-			if(iobus0 & F32) ptr->flag = 1;
-			ptr->pia |= iobus0 & 7;
+			if(bus->c12 & F30) ptr->b = 1;
+			if(bus->c12 & F31) ptr->busy = 1;
+			if(bus->c12 & F32) ptr->flag = 1;
+			ptr->pia |= bus->c12 & 7;
 		}
 	}
 	recalc_ptr_req(ptr);
@@ -197,20 +205,22 @@ ptr_setmotor(Ptr *ptr, int m)
 }
 
 void
-initptp(Ptp *ptp)
+initptp(Ptp *ptp, IOBus *bus)
 {
 	pthread_t thread_id;
-	iobus[PTP] = (Busdev){ ptp, wake_ptp, 0 };
+	ptp->bus = bus;
+	bus->dev[PTP] = (Busdev){ ptp, wake_ptp, 0 };
 	pthread_create(&thread_id, nil, ptpthread, ptp);
 
 	ptp->fp = fopen("../code/ptp.out", "wb");
 }
 
 void
-initptr(Ptr *ptr)
+initptr(Ptr *ptr, IOBus *bus)
 {
 	pthread_t thread_id;
-	iobus[PTR] = (Busdev){ ptr, wake_ptr, 0 };
+	ptr->bus = bus;
+	bus->dev[PTR] = (Busdev){ ptr, wake_ptr, 0 };
 	pthread_create(&thread_id, nil, ptrthread, ptr);
 
 	ptr->fp = fopen("../code/test.rim", "rb");
diff --git a/src/tty.c b/src/tty.c
index bbfbca1..5045b1a 100644
--- a/src/tty.c
+++ b/src/tty.c
@@ -7,18 +7,20 @@
 #include <pthread.h>    
 #include <poll.h>
 
-void
+static void
 recalc_tty_req(Tty *tty)
 {
 	u8 req;
+	IOBus *bus;
+	bus = tty->bus;
 	req = tty->tto_flag || tty->tti_flag ? tty->pia : 0;
-	if(req != iobus[TTY].pireq){
-		iobus[TTY].pireq = req;
-		recalc_req();
+	if(req != bus->dev[TTY].req){
+		bus->dev[TTY].req = req;
+		recalc_req(bus);
 	}
 }
 
-void*
+static void*
 ttythread(void *dev)
 {
 	int sockfd, newsockfd, portno, clilen;
@@ -70,8 +72,10 @@ static void
 wake_tty(void *dev)
 {
 	Tty *tty;
+	IOBus *bus;
 
 	tty = dev;
+	bus = tty->bus;
 	if(IOB_RESET){
 		tty->pia = 0;
 		tty->tto_busy = 0;
@@ -81,30 +85,30 @@ wake_tty(void *dev)
 		tty->tti_flag = 0;
 		tty->tti = 0;
 	}
-	if(iodev == TTY){
+	if(bus->devcode == TTY){
 		if(IOB_STATUS){
-			if(tty->tti_busy) iobus0 |= F29;
-			if(tty->tti_flag) iobus0 |= F30;
-			if(tty->tto_busy) iobus0 |= F31;
-			if(tty->tto_flag) iobus0 |= F32;
-			iobus0 |= tty->pia & 7;
+			if(tty->tti_busy) bus->c12 |= F29;
+			if(tty->tti_flag) bus->c12 |= F30;
+			if(tty->tto_busy) bus->c12 |= F31;
+			if(tty->tto_flag) bus->c12 |= F32;
+			bus->c12 |= tty->pia & 7;
 		}
 		if(IOB_DATAI){
-			iobus0 |= tty->tti;
+			bus->c12 |= tty->tti;
 			tty->tti_flag = 0;
 		}
 		if(IOB_CONO_CLEAR)
 			tty->pia = 0;
 		if(IOB_CONO_SET){
-			if(iobus0 & F25) tty->tti_busy = 0;
-			if(iobus0 & F26) tty->tti_flag = 0;
-			if(iobus0 & F27) tty->tto_busy = 0;
-			if(iobus0 & F28) tty->tto_flag = 0;
-			if(iobus0 & F29) tty->tti_busy = 1;
-			if(iobus0 & F30) tty->tti_flag = 1;
-			if(iobus0 & F31) tty->tto_busy = 1;
-			if(iobus0 & F32) tty->tto_flag = 1;
-			tty->pia |= iobus0 & 7;
+			if(bus->c12 & F25) tty->tti_busy = 0;
+			if(bus->c12 & F26) tty->tti_flag = 0;
+			if(bus->c12 & F27) tty->tto_busy = 0;
+			if(bus->c12 & F28) tty->tto_flag = 0;
+			if(bus->c12 & F29) tty->tti_busy = 1;
+			if(bus->c12 & F30) tty->tti_flag = 1;
+			if(bus->c12 & F31) tty->tto_busy = 1;
+			if(bus->c12 & F32) tty->tto_flag = 1;
+			tty->pia |= bus->c12 & 7;
 		}
 		if(IOB_DATAO_CLEAR){
 			tty->tto = 0;
@@ -112,7 +116,7 @@ wake_tty(void *dev)
 			tty->tto_flag = 0;
 		}
 		if(IOB_DATAO_SET){
-			tty->tto = iobus0 & 0377;
+			tty->tto = bus->c12 & 0377;
 			if(tty->fd >= 0){
 				tty->tto &= ~0200;
 				write(tty->fd, &tty->tto, 1);
@@ -126,10 +130,11 @@ wake_tty(void *dev)
 }
 
 void
-inittty(Tty *tty)
+inittty(Tty *tty, IOBus *bus)
 {
 	pthread_t thread_id;
 	tty->fd = -1;
-	iobus[TTY] = (Busdev){ tty, wake_tty, 0 };
+	tty->bus = bus;
+	bus->dev[TTY] = (Busdev){ tty, wake_tty, 0 };
 	pthread_create(&thread_id, nil, ttythread, tty);
 }