diff --git a/emu/Makefile b/emu/Makefile
index eac4f5c..4d32bdf 100644
--- a/emu/Makefile
+++ b/emu/Makefile
@@ -14,3 +14,6 @@ pdp6: $(SRC) $(H)
 
 test_dc: test_dc.c dc.c $(H)
 	$(CC) -o $@ $(CFLAGS) test_dc.c dc.c
+
+test_dt: test_dt.c dc.c dt.c $(H)
+	$(CC) -o $@ $(CFLAGS) test_dt.c dc.c dt.c
diff --git a/emu/apr.c b/emu/apr.c
index 9701938..b94eade 100644
--- a/emu/apr.c
+++ b/emu/apr.c
@@ -230,11 +230,8 @@ makeapr(int argc, char *argv[])
 
 	apr->dev.type = apr_ident;
 	apr->dev.name = "";
-	apr->dev.attach = nil;
-	apr->dev.ioconnect = nil;
 	apr->dev.examine = ex_apr;
 	apr->dev.deposit = dep_apr;
-	apr->dev.next = nil;
 
 	apr->iobus.dev[CPA] = (Busdev){ apr, wake_cpa, 0 };
 	apr->iobus.dev[PI] = (Busdev){ apr, wake_pi, 0 };
diff --git a/emu/cmd.c b/emu/cmd.c
index 1be104a..71992e3 100644
--- a/emu/cmd.c
+++ b/emu/cmd.c
@@ -260,6 +260,27 @@ c_attach(int argc, char *argv[])
 	dev->attach(dev, argv[2]);
 }
 
+static void
+c_detach(int argc, char *argv[])
+{
+	Device *dev;
+
+	if(argc < 2){
+		printf("Not enough arguments\n");
+		return;
+	}
+	dev = getdevice(argv[1]);
+	if(dev == nil){
+		printf("No device: %s\n", argv[1]);
+		return;
+	}
+	if(dev->detach == nil){
+		printf("No detach for device type %s\n", dev->type);
+		return;
+	}
+	dev->detach(dev);
+}
+
 /* ioconnect dev busdev */
 static void
 c_ioconnect(int argc, char *argv[])
@@ -615,6 +636,8 @@ struct {
 		"make a device: mkdev name type [args]" },
 	{ "attach", c_attach,
 		"attach a file to a devie: attach name filename" },
+	{ "detach", c_detach,
+		"detach a file from a devie: attach name" },
 	{ "connectio", c_ioconnect,
 		"connect device to IO bus: connectio devname procname" },
 	{ "connectmem", c_memconnect,
diff --git a/emu/dc.c b/emu/dc.c
index 4350fd7..84ddfdd 100644
--- a/emu/dc.c
+++ b/emu/dc.c
@@ -244,7 +244,6 @@ makedc(int argc, char *argv[])
 
 	dc->dev.type = dc_ident;
 	dc->dev.name = "";
-	dc->dev.attach = nil;
 	dc->dev.ioconnect = dcioconnect;
 
 	return &dc->dev;
diff --git a/emu/dt.c b/emu/dt.c
index 9df883c..c4bb4ca 100644
--- a/emu/dt.c
+++ b/emu/dt.c
@@ -10,11 +10,14 @@ char *dx_ident = DX_IDENT;
 
 enum
 {
-	DTSIZE = 922512
+//	DTSIZE = 922512
+	// 01102 blocks, 2700 word for start/end each + safe zone
+	DTSIZE = 987288 + 1000
 };
 
 /* Transport */
 
+#if 0
 static uchar
 dxread(Dx555 *dx)
 {
@@ -32,6 +35,25 @@ dxwrite(Dx555 *dx, uchar d)
 	else
 		*dx->cur = d;
 }
+#endif
+
+static uchar
+dxread_(Dx555 *dx)
+{
+	if(dx->dt->ut_rev)
+		return *dx->cur ^ 017;
+	else
+		return *dx->cur;
+}
+
+static void
+dxwrite_(Dx555 *dx, uchar d)
+{
+	if(dx->dt->ut_rev)
+		*dx->cur = d ^ 017;
+	else
+		*dx->cur = d;
+}
 
 static void
 dxmove(Dx555 *dx)
@@ -48,19 +70,31 @@ dxmove(Dx555 *dx)
 	}
 }
 
+static void
+dxdetach(Device *dev)
+{
+	Dx555 *dx;
+
+	dx = (Dx555*)dev;
+	if(dx->fd < 0)
+		return;
+
+	lseek(dx->fd, 0, SEEK_SET);
+	write(dx->fd, dx->start, dx->size);
+	close(dx->fd);
+	dx->fd = -1;
+	memset(dx->start, 0, dx->size);
+}
+
 static void
 dxattach(Device *dev, const char *path)
 {
 	Dx555 *dx;
-	int fd;
 
 	dx = (Dx555*)dev;
-	fd = dx->fd;
-	if(fd >= 0){
-		dx->fd = -1;
-		close(dx->fd);
-	}
-	memset(dx->start, 0, DTSIZE);
+	dxdetach(dev);
+	memset(dx->start, 0, dx->size);
+	// TODO: resize
 	dx->fd = open(path, O_RDWR | O_CREAT, 0666);
 	if(dx->fd < 0)
 		fprintf(stderr, "couldn't open file %s\n", path);
@@ -88,12 +122,14 @@ makedx(int argc, char *argv[])
 	dx->dev.type = dx_ident;
 	dx->dev.name = "";
 	dx->dev.attach = dxattach;
-	dx->dev.ioconnect = nil;
+	dx->dev.detach = dxdetach;
 
-	dx->start = malloc(DTSIZE);
+	dx->fd = -1;
+	dx->size = DTSIZE;
+	dx->start = malloc(dx->size);
 	dx->cur = dx->start;
-	dx->end = dx->start + DTSIZE;
-	memset(dx->start, 0, DTSIZE);
+	dx->end = dx->start + dx->size;
+	memset(dx->start, 0, dx->size);
 
 	return &dx->dev;
 }
@@ -105,8 +141,10 @@ enum
 	TapeEndF  = 022,
 	TapeEndR  = 055,
 
+	/* block mark high half */
 	BlockSpace = 025,
 
+	/* block mark low half */
 	BlockEndF = 026,
 	BlockEndR = 045,
 
@@ -126,19 +164,17 @@ enum
 
 #define DATA_SYNC (dt->tmk == (0600|DataSync))
 #define END_ZONE (dt->tmk == (0600|TapeEndF))
-#define BM_SPACE (dt->tmk == (0400|BlockSpace) ||\
-		dt->tmk == (0500|BlockSpace) ||\
-		dt->tmk == (0600|BlockSpace) ||\
-		dt->tmk == (0700|BlockSpace))
+#define BM_SPACE ((dt->tmk&0477) == (0400|BlockSpace))
 #define BM_END (dt->tmk == (0500|BlockEndF))
-#define DATA_SYNC (dt->tmk == (0600|DataSync))
 #define BM_SYNC (dt->tmk == (0700|BlockSync))
+#define REV_BM (dt->tmk == (0500|BlockEndR))
 #define FWD_DATA_END (dt->tmk == (0400|DataEndF) ||\
 		dt->tmk == (0700|DataEndF))
 #define REV_DATA_END (dt->tmk == (0400|DataEndR) ||\
 		dt->tmk == (0600|DataEndR))
 #define DATA (dt->tmk == (0400|Data))
 
+
 #define STOP (dt->tdata == 0 || dt->tdata == 1)
 #define SYNC (dt->tdata == 0200)
 #define REV_CKS (dt->tdata == 0100)
@@ -146,6 +182,7 @@ enum
 #define PRE_FINAL (dt->tdata == 004)
 #define FWD_CKS (dt->tdata == 002)
 
+
 #define UT_WRITE ((dt->ut_fcn & 04) == 04)
 #define UT_READ ((dt->ut_fcn & 04) == 0)
 #define UT_ALL ((dt->ut_fcn & 03) == 01)
@@ -154,26 +191,33 @@ enum
 #define UT_DN (dt->ut_fcn == 0)
 #define UT_WRTM (dt->ut_fcn == 04)
 
+
 #define DC_SELECT1 (dt->dc->devp[dt->dc->device] == dt)
-#define UTE_DC_DISCONNECT (!(DC_SELECT1 && !dt->dc->da_rq))
-#define UT_WRITE_PREVENT (dt->seldx->wrlock)
+#define UT_WRITE_PREVENT (UT_WRITE && dt->seldx->wrlock)
 #define UT_WREN_DATA (UT_WRITE && dt->ut_wren && !UT_WRITE_PREVENT)
-#define RW_ODD (!dt->tct && dt->rw_state == RW_ACTIVE)
+#define UT_WREN_BTM (UT_WREN_DATA && dt->ut_btm_switch && UT_WRTM)	// 3-14
+#define RW_ODD (!dt->tct && dt->rw_state == RW_ACTIVE)	// 3-6
+#define RW_EVEN (dt->tct && dt->rw_state == RW_ACTIVE)
 #define RW_BM_DONE (UT_BM && BM_END)
-#define RW_DATA_CONT (DC_SELECT1 && !dt->dc->da_rq && UT_DATA && dt->rw_state == RW_ACTIVE)
-#define RW_DATA_STOP (UTE_DC_DISCONNECT && UT_DATA && dt->rw_state == RW_ACTIVE)
+#define RW_DATA_CONT (DC_SELECT1 && !dt->dc->da_rq && UT_DATA && prev.rw_state == RW_ACTIVE)
+// TODO: what is RW_DATA_WR_CONT?
+// TODO: this is really RW_DATA_WR_STOP
+#define RW_DATA_STOP (UTE_DC_DISCONNECT && UT_DATA && prev.rw_state == RW_ACTIVE)
 
 #define TBM0 (dt->tbm == 10)
 #define TBM3 (dt->tbm == 1)
 
-// all but block space
+#define UTE_UNIT_OK (nunits == 1)
+#define UTE_DC_DISCONNECT (!(DC_SELECT1 && !dt->dc->da_rq))
+// all but block space and rev block mark
 #define UTE_MK (DATA_SYNC || END_ZONE || BM_END ||\
-	DATA_SYNC || BM_SYNC || FWD_DATA_END || REV_DATA_END || DATA)
-#define UTE_ERROR (dt->uteck && (dt->utek == 040) != UTE_MK)
-// not sure if BM SYNC || DATA SYNC is correct
-#define UTE_ACTIVE_ERROR (!UT_ALL && dt->rw_state == RW_ACTIVE && (BM_SYNC || DATA_SYNC))
-
+	BM_SYNC || FWD_DATA_END || REV_DATA_END || DATA)
+#define UTE_ERROR (dt->uteck && (prev.utek == 040) != UTE_MK)
+// TODO: not sure if BM SYNC || DATA SYNC is correct
+#define UTE_ACTIVE_ERROR (!UT_ALL && prev.rw_state == RW_ACTIVE && (BM_SYNC || DATA_SYNC))
 
+//#define debug(...) printf(__VA_ARGS__)
+#define debug(...) 0
 
 #define PRINT1 \
 	printf("%c%02o(%o,%02o,%d)", dt->rw_state == RW_ACTIVE ? '+' : dt->rw_state == RW_RQ ? '-' : ' ', \
@@ -194,21 +238,23 @@ enum
 
 #define PRINT2 \
 	if(END_ZONE) \
-		printf("TapeEndF "); \
+		debug("%c%d %02o TapeEndF ", state, dt->tct, dt->lb); \
 	else if(BM_SPACE) \
-		printf("BlockSpace %o ", dt->tbm); \
+		debug("%c%d %02o BlockSpace %o ", state, dt->tct, dt->lb, dt->tbm); \
 	else if(BM_END) \
-		printf("BlockEndF "); \
+		debug("%c%d %02o BlockEndF %o ", state, dt->tct, dt->lb, dt->tbm); \
+	else if(REV_BM) \
+		debug("%c%d %02o BlockEndR %o ", state, dt->tct, dt->lb, dt->tbm); \
 	else if(DATA_SYNC) \
-		printf("DataSync "); \
+		debug("%c%d %02o DataSync %o ", state, dt->tct, dt->lb, dt->tdata); \
 	else if(BM_SYNC) \
-		printf("BlockSync "); \
+		debug("%c%d %02o BlockSync %o ", state, dt->tct, dt->lb, dt->tbm); \
 	else if(FWD_DATA_END) \
-		printf("DataEndF "); \
+		debug("%c%d %02o DataEndF %o ", state, dt->tct, dt->lb, dt->tdata); \
 	else if(REV_DATA_END) \
-		printf("DataEndR "); \
+		debug("%c%d %02o DataEndR %o ", state, dt->tct, dt->lb, dt->tdata); \
 	else if(DATA) \
-		printf("Data ");
+		debug("%c%d %02o Data %o ", state, dt->tct, dt->lb, dt->tdata);
 
 static void
 recalc_dt_req(Dt551 *dt)
@@ -225,14 +271,278 @@ recalc_dt_req(Dt551 *dt)
 	setreq(dt->bus, UTC, req);
 }
 
+#define LDB(p, s, w) ((w)>>(p) & (1<<(s))-1)
+#define XLDB(ppss, w) LDB((ppss)>>6 & 077, (ppss)&077, w)
+#define MASK(p, s) ((1<<(s))-1 << (p))
+#define DPB(b, p, s, w) ((w)&~MASK(p,s) | (b)<<(p) & MASK(p,s))
+#define XDPB(b, ppss, w) DPB(b, (ppss)>>6 & 077, (ppss)&077, w)
+
+word dtbuf;
+char state = 'n';
+
+#define REV (dt->ut_rev && !UT_BM)
+
+static void
+dt_tp0(Dt551 *dt)
+{
+	dt->sense = dxread_(dt->seldx);
+
+	// 3-7
+	if(UT_WRITE && dt->rw_state == RW_ACTIVE)
+		dt->ut_wren = 1;
+
+	// 3-7
+	if(UT_WRITE){	// TODO
+		int c;
+		c = REV ? 7 : 0;
+		if(dt->tct)
+			dt->wb = dt->rwb&7 ^ c;
+		else
+			dt->wb = dt->rwb>>3&7 ^ c;
+		/* copy to mark track; 3-13/14 */
+		if(UT_WREN_BTM)
+			dt->wb |= dt->wb<<1 & 010;
+		else
+			dt->wb = dt->wb&7 | dt->sense&010;
+		if(UT_WRITE_PREVENT)
+			dt->ut_illegal_op = 1;		// 3-16
+		if(UT_WREN_DATA){
+			dxwrite_(dt->seldx, dt->wb);
+debug("writing %02o\n", dt->wb);
+		}
+	}
+}
+
+static void
+dt_tp1(Dt551 *dt)
+{
+	/* complement WB here, totally useless */
+	//dt->wb = ~dt->wb & 07;
+
+	/* advance mark track window */
+	if(!UT_WRTM)
+		dt->tmk = ((dt->tmk << 1) | LDB(3, 1, dt->sense)) & 0777 |
+			dt->tmk&0400;
+
+	/* Strobe sensors into RWB */
+	// 3-12
+	if(dt->rw_state == RW_ACTIVE && UT_READ){
+		int c = REV ? 7 : 0;
+		if(dt->tct)
+			dt->rwb |= (dt->sense^c)&07;
+		else
+			dt->rwb |= ((dt->sense^c)&07) << 3;
+	}
+}
+
+static void
+dt_tp2(Dt551 *dt)
+{
+	Dt551 prev;
+	int tbedge, tdedge;
+
+	prev = *dt;
+
+	if(END_ZONE){
+		dt->ut_go = 0;
+		dt->ut_tape_end_flag = 1;
+		/* TODO: is this right? */
+		dt->rw_state = RW_NULL;
+	}
+
+	/* Block mark timing.
+	 * Shift down starting at block sync,
+	 * shifts to 10 before rev block number,
+	 * shifts to 01 before fwd block number. */
+	// 3-5
+	if(BM_SYNC)
+		dt->tbm |= 020;
+	if(BM_SYNC || BM_SPACE)
+		dt->tbm >>= 1;
+	tbedge = ~prev.tbm & dt->tbm;
+
+	/* Data timing.
+	 * Shift down starting at data sync.
+	 * shifts to 100 before reverse check sum,
+	 * shifts to 001 after forward check sum */
+	// 3-8, 3-9
+	if(DATA_SYNC)
+		dt->tdata = 0400;
+	if(FWD_DATA_END || REV_DATA_END || DATA_SYNC)
+		dt->tdata >>= 1;
+	tdedge = ~prev.tdata & dt->tdata;
+
+
+	/* Send 6 bit byte to DC every two steps when reading */
+	if(UT_READ && RW_EVEN)
+		/* except checksums */
+		if((prev.tdata & 0102) == 0){
+			int rwb = dt->rwb;
+			if(REV)
+				rwb = rwb>>3 & 07 | rwb<<3 & 070;
+			dcgv(dt->dc, DEVNO, rwb, REV);
+			if(UTE_DC_DISCONNECT)
+				dt->ut_incomp_block = 1;	// 3-17
+		}
+
+	/* Clear LB before reading the reverse checksum */
+	if(tdedge & 0100)
+		dt->lb = 0;	// 3-9
+
+	/* Keep track of checksum */
+	if(dt->tct)
+		// Manual says XOR, but also talks of parity of zeros
+//		dt->lb ^= dt->rwb;
+		dt->lb ^= ~dt->rwb & 077;
+
+	if(prev.rw_state != RW_ACTIVE)
+		dt->tct = 0;
+	else
+		dt->tct ^= 1;
+
+	/* Ring count UTEK */
+	if(!BM_SPACE){
+		if(dt->utek == 1)
+			dt->utek = 0100;
+		dt->utek >>= 1;
+	}
+
+	/* Go into active state */
+	if(prev.rw_state == RW_RQ)
+		if(UT_BM && tbedge & 1 ||	// 3-6
+		   UT_DATA && tdedge & 0100 ||	// 3-9
+		   UT_ALL && tbedge & 010)	// 3-11
+			dt->rw_state = RW_ACTIVE;
+
+	/* What to do after block */
+	if(tdedge & 1){
+		if(RW_DATA_CONT)		// 3-10
+			dt->rw_state = RW_RQ;
+		if(RW_DATA_STOP)		// 3-11
+			dt->ut_jb_done_flag = 1;
+		/* test checksum. NB: manual says should be 0
+		 * TODO: not sure about ACTIVE */
+		if(prev.rw_state == RW_ACTIVE && dt->lb != 077)
+debug("CHECKSUM ERR\n"),
+			dt->ut_info_error = 1;	// 3-13
+	}
+
+	/* Job done after block mark */
+	if(prev.rw_state == RW_ACTIVE && RW_BM_DONE)	// 3-8
+		dt->ut_jb_done_flag = 1;
+
+	/* Catch wrongly set active state; 3-16 */
+	if(UTE_ACTIVE_ERROR){
+		dt->ut_info_error = 1;
+		dt->rw_state = RW_NULL;
+	}
+	/* Catch invalid mark code */
+	if(UTE_ERROR)
+debug("TRACK ERROR\n"),
+		dt->ut_info_error = 1;		// 3-15
+
+	/* Stop activity once job gets done */
+	if(!prev.ut_jb_done_flag && dt->ut_jb_done_flag)	// 3-8
+		dt->rw_state = RW_NULL;
+}
+
+static void
+dt_tp3(Dt551 *dt)
+{
+	// 3-6
+	if(RW_ODD)
+		dt->rwb = 0;
+
+	// 3-15
+	/* before fwd block mark */
+	if(dt->tbm & 1){
+		if(BM_SPACE)
+			dt->utek = 040;		// UTE PRESET 100000
+		else
+			dt->uteck = 1;
+	}
+	if(BM_SYNC)
+		dt->uteck = 0;
+}
+
+static void
+dt_tp4(Dt551 *dt)
+{
+	/* stop writing WB here */
+	if(dt->rw_state != RW_ACTIVE)
+		dt->ut_wren = 0;
+
+	/* get 6 bit byte from DC every two steps when writing */
+	if(UT_WRITE && RW_ODD){
+		/* except checksums */
+		if((dt->tdata & 0102) == 0){
+			dt->rwb |= dctk(dt->dc, DEVNO, REV);
+			if(UTE_DC_DISCONNECT)
+				dt->ut_incomp_block = 1;	// 3-17
+		}else
+debug("Getting LB\n"),
+			dt->rwb |= dt->lb;
+		// 3-6
+		if(REV)
+			dt->rwb = dt->rwb>>3 & 07 | dt->rwb<<3 & 070;
+	}
+}
+
+/* count dialled transports and select last */
+static int
+selectdx(Dt551 *dt)
+{
+	int i, n, nunits;
+	n = dt->ut_units == 0 ? 8 : dt->ut_units;
+	nunits = 0;
+	dt->seldx = nil;
+	if(dt->ut_units_select)
+		for(i = 0; i < 8; i++)
+			if(dt->dx[i] && dt->dx[i]->unit == n){
+				dt->seldx = dt->dx[i];
+				nunits++;
+			}
+	return nunits;
+}
+
+/* Read one line of bits and move the tape.
+ * This should occur once every 33.33μs */
 static void
 dtcycle(void *p)
 {
 	Dt551 *dt;
 	dt = (Dt551*)p;
-	uchar l, wb;
-	int data1edge, data7edge;
-	int bm0edge, bm3edge;
+
+	if(dt->delay){
+		if(--dt->delay == 0){
+			// UT_START goes to 0 here
+
+			int nunits;
+
+			dt->time_flag = 1;
+
+			/* check for legal selection */
+			nunits = selectdx(dt);
+
+			// UTE_SW_ERROR; 3-16
+			if(!UTE_UNIT_OK ||
+			   dt->ut_btm_switch != UT_WRTM)
+debug("ILL op %d %d %d\n", nunits, dt->ut_btm_switch, UT_WRTM),
+				dt->ut_illegal_op = 1;
+			if(!UTE_UNIT_OK){
+				dt->ut_go = 0;		// 3-3
+				return;
+			}
+
+			if(UT_WRTM)		// 3-14
+				dt->rw_state = RW_ACTIVE;
+			else if(!UT_DN)		// TODO: is this right?
+				dt->rw_state = RW_RQ;
+		}else
+			/* Don't move during delay.
+			 * TODO: is this right? */
+			return;
+	}
 
 	// TODO: maybe do something else here?
 	if(dt->seldx == nil)
@@ -241,166 +551,33 @@ dtcycle(void *p)
 	if(!dt->ut_go)
 		return;
 
+	// sense; start writing
+	dt_tp0(dt);
 
-	if(dt->rw_state != RW_ACTIVE)
-		dt->tct = 0;
-	l = dxread(dt->seldx);
+dtbuf = dtbuf<<3 & FW;
+dtbuf |= dt->sense & 07;
 
-/* TP0 */
+	// read strobe
+	dt_tp1(dt);
 
-	if(dt->rw_state == RW_ACTIVE)
-		dt->ut_wren = 1;
+	// interpret mark code
+	dt_tp2(dt);
 
-	if(UT_WRITE){
-		if(UT_WRITE_PREVENT)
-			dt->ut_illegal_op = 1;
-		if(dt->tct)
-			wb = l&010 | dt->rwb&07;
-		else
-			wb = l&010 | dt->rwb>>3 & 07;
-	}
-	if(UT_WREN_DATA)
-		dxwrite(dt->seldx, wb);
+	// clear rwb; mark track error sync
+	dt_tp3(dt);
 
-/* TP1 */
+	// get next rwb from DC to write
+	dt_tp4(dt);
 
-//**/				PRINT1
-//**/				PRINT2
-
-	dt->tmk = ((dt->tmk << 1) | !!(l&010)) & 0377 |
-		dt->tmk&0400 | (dt->tmk<<1)&0400;
-	if(UT_READ){
-		if(dt->tct != dt->ut_rev)
-			dt->rwb |= l&07;
-		else
-			dt->rwb |= (l&07) << 3;
-	}
-
-/* TP2 */
-///**/				printf("%2o%c%c%c  ", dt->utek, UTE_MK ? 'M' : ' ', UTE_ERROR ? 'E' : ' ', dt->uteck ?  'C' : ' ');
-
-	if(UTE_ERROR || UTE_ACTIVE_ERROR)
-printf("UT INFO ERROR\n"),
-		dt->ut_info_error = 1;
-
-	if(!BM_SPACE){
-		if(dt->utek == 1)
-			dt->utek = 040;
-		else
-			dt->utek >>= 1;
-	}
-
-	if(dt->tct)
-		dt->lb ^= ~dt->rwb & 077;
-
-//**/			if(dt->rw_state == RW_ACTIVE && dt->tct)
-//**/				printf("(%02o, %02o) ", dt->rwb, dt->lb);
-
-	if(UT_READ && dt->rw_state == RW_ACTIVE && dt->tct){
-		// before tct complement
-		if(!(UT_DATA && dt->tdata & 0102)){
-			dcgv(dt->dc, DEVNO, dt->rwb, dt->ut_rev);
-			if(UTE_DC_DISCONNECT)
-				dt->ut_incomp_block = 1;
-		}
-	}
-
-
-	if(END_ZONE){
-		dt->ut_tape_end_flag = 1;
-		dt->ut_go = 0;
-	}
-
-	bm0edge = 0;
-	bm3edge = 0;
-	if(BM_SYNC)
-		dt->tbm |= 020;	// shifted to 010 (TBM0) now
-	if(BM_SPACE || BM_SYNC){
-		dt->tbm >>= 1;
-		if(TBM0)
-			bm0edge = 1;
-		else if(TBM3)
-			bm3edge = 1;
-	}
-
-	data1edge = 0;	// goes up before rev checksum is expected
-	data7edge = 0;	// goes up when checksum is seen
-	if(DATA_SYNC)
-		dt->tdata |= 0400;	// shifted to 0200 now
-	if(FWD_DATA_END || REV_DATA_END || DATA_SYNC){
-		dt->tdata >>= 1;
-		if(dt->tdata == 0100)
-			data1edge = 1;
-		else if(dt->tdata == 1)
-			data7edge = 1;
-	}
-
-	if(data1edge)
-		dt->lb = 0;
-
-	// TODO: not sure about rw_active
-	if(data7edge && dt->rw_state == RW_ACTIVE)
-		if(dt->lb != 077)
-printf("UT INFO ERROR\n"),
-			dt->ut_info_error = 1;
-
-	// before active is set below
-	if(dt->rw_state == RW_ACTIVE)
-		dt->tct = !dt->tct;
-
-	// manual mentions tdata6(0) going to 0???
-	if(dt->rw_state == RW_ACTIVE){
-		if(data7edge && RW_DATA_STOP ||
-		   RW_BM_DONE){
-printf("FINISHING JOB\n");
-			dt->ut_jb_done_flag = 1;
-			dt->rw_state = RW_NULL;
-		}
-		if(UTE_ACTIVE_ERROR)
-			dt->rw_state = RW_NULL;
-		if(data7edge && RW_DATA_CONT)
-			dt->rw_state = RW_RQ;
-	}else if(dt->rw_state == RW_RQ){
-		if(UT_BM && bm3edge ||
-		   UT_DATA && data1edge ||
-		   UT_ALL && bm0edge)
-			dt->rw_state = RW_ACTIVE;
-	}
-
-/* TP3 */
-
-	if(RW_ODD)
-		dt->rwb = 0;
-
-	if(TBM3 && BM_SPACE)
-		dt->utek = 040;
-	if(BM_SYNC)
-		dt->uteck = 0;
-	if(TBM3 && !BM_SPACE)
-		dt->uteck = 1;
-
-/* TP4 */
-
-	if(UT_WRITE && RW_ODD){
-		/* Take checksum data from LB */
-		if(UT_DATA && dt->tdata & 0102)
-			dt->rwb = dt->lb;
-		else{
-			dt->rwb |= dctk(dt->dc, DEVNO, dt->ut_rev);
-			if(UTE_DC_DISCONNECT)
-				dt->ut_incomp_block = 1;
-		}
-		if(dt->ut_rev)
-			dt->rwb = dt->rwb>>3 & 07 | dt->rwb<<3 & 070;
-	}
-	if(dt->rw_state == RW_ACTIVE)
-		dt->ut_wren = 0;
-
-//**/				printf("\n");
+	state = dt->rw_state == RW_NULL ? 'n' :
+	        dt->rw_state == RW_RQ ? 'r' :
+	        dt->rw_state == RW_ACTIVE ? 'a' : 'x';
+	PRINT2
+	else
+		debug("%c%d -- ", state, dt->tct);
+	debug(" %012lo %02o %d.%02o\n", dtbuf, dt->rwb, dt->uteck, dt->utek);
 
 	dxmove(dt->seldx);
-
-	recalc_dt_req(dt);
 }
 
 static void
@@ -444,7 +621,7 @@ wake_dt(void *dev)
 	if(bus->devcode == UTS){
 		if(IOB_STATUS){
 //printf("UTS STATUS\n");
-			// we have no delays (yet?) F25
+			if(dt->delay) bus->c12 |= F25;
 			if(dt->rw_state == RW_RQ) bus->c12 |= F26;
 			if(dt->rw_state == RW_ACTIVE) bus->c12 |= F27;
 			if(dt->rw_state == RW_NULL) bus->c12 |= F28;
@@ -459,7 +636,7 @@ wake_dt(void *dev)
 	}
 	if(bus->devcode == UTC){
 		if(IOB_STATUS){
-//printf("UTC STATUS\n");
+printf("UTC STATUS\n");
 			if(dt->ut_units_select) bus->c12 |= F19;
 			if(dt->ut_tape_end_enable) bus->c12 |= F20;
 			if(dt->ut_jb_done_enable) bus->c12 |= F21;
@@ -481,12 +658,11 @@ wake_dt(void *dev)
 			dt->ut_tape_end_flag = 0;
 			dt->ut_jb_done_flag = 0;
 
+			// 3-5
 			dt->rw_state = RW_NULL;
 		}
 		if(IOB_CONO_SET){
 //printf("UTC CONO SET\n");
-			int i, n, nunits;
-
 			dt->ut_pia = bus->c12 & 07;
 			dt->ut_units = bus->c12>>3 & 07;
 			dt->ut_fcn = bus->c12>>6 & 07;
@@ -498,42 +674,37 @@ wake_dt(void *dev)
 			dt->ut_tape_end_enable = bus->c12>>15 & 1;
 			dt->ut_units_select = bus->c12>>16 & 1;
 
-			// UT CONO SET LONG
+			// UT CONO SET LONG, 1ms after UT CONO SET
 			//  triggers UT START level until end of delay
 
-			// find selected transport unit
-			n = dt->ut_units == 0 ? 8 : dt->ut_units;
-			dt->seldx = nil;
-			nunits = 0;
-			if(dt->ut_units_select)
-				for(i = 0; i < 8; i++)
-					if(dt->dx[i] && dt->dx[i]->unit == n){
-						dt->seldx = dt->dx[i];
-						nunits++;
-					}
+			/* update selection, also done after delay */
+			selectdx(dt);
+
+			dt->delay = 0;
 			if(dt->ut_time){
-				// wait delay
-				dt->time_flag = 1;
-				dt->uteck = 0;
-
-				if(nunits != 1){
-					dt->ut_go = 0;
-					dt->ut_illegal_op = 1;
-					goto ret;
+				switch(dt->ut_time){
+				case 1:	// 20ms
+					dt->delay = 600;
+					break;
+				case 2:	// 160ms
+					dt->delay = 4800;
+					break;
+				case 3: // 300ms
+					dt->delay = 9000;
+					break;
 				}
-				if(dt->ut_btm_switch != UT_WRTM)
-					dt->ut_illegal_op = 1;
-			}
+				// T CLEAR; 3-4
+				dt->tmk = 0;
+				dt->uteck = 0;	// 3-15
 
-			if(!UT_DN){
-				if(UT_WRTM)
-					dt->rw_state = RW_ACTIVE;
-				else
+				dt->time_flag = 0;
+				// now wait
+			}else
+				if(!UT_DN &&		// TODO: is this right?
+				   !UT_WRTM)		// 3-14
 					dt->rw_state = RW_RQ;
-			}
 		}
 	}
-ret:
 	recalc_dt_req(dt);
 }
 
@@ -555,6 +726,27 @@ dtconn(Dc136 *dc, Dt551 *dt)
 	dc->devp[DEVNO] = dt;
 }
 
+int
+dtdep(Device *dev, const char *reg, word data)
+{
+	Dt551 *dt;
+
+	dt = (Dt551*)dev;
+	if(strcmp(reg, "btm_wr") == 0) dt->ut_btm_switch = data&1;
+	else return 1;
+	return 0;
+}
+
+word
+dtex(Device *dev, const char *reg)
+{
+	Dt551 *dt;
+
+	dt = (Dt551*)dev;
+	if(strcmp(reg, "btm_wr") == 0) return dt->ut_btm_switch;
+	return ~0;
+}
+
 Device*
 makedt(int argc, char *argv[])
 {
@@ -566,11 +758,13 @@ makedt(int argc, char *argv[])
 
 	dt->dev.type = dt_ident;
 	dt->dev.name = "";
-	dt->dev.attach = nil;
 	dt->dev.ioconnect = dtioconnect;
+	dt->dev.deposit = dtdep;
+	dt->dev.examine = dtex;
 
-	// should have 30000 cycles per second
-	th = (Thread){ nil, dtcycle, dt, 1000, 0 };
+	// should have 30000 cycles per second, so one every 33μs
+	// APR at 1 has an approximate cycle time of 200-300ns
+	th = (Thread){ nil, dtcycle, dt, 150, 0 };
 	addthread(th);
 
 	return &dt->dev;
diff --git a/emu/init.ini b/emu/init.ini
index 243443d..609fdb7 100644
--- a/emu/init.ini
+++ b/emu/init.ini
@@ -12,10 +12,11 @@ mkdev cmem1 cmem161C mem_1
 mkdev cmem2 cmem161C mem_2
 mkdev cmem3 cmem161C mem_3
 #mkdev netmem netmem its.pdp10.se 10006
-mkdev netmem netmem localhost 10006
+#mkdev netmem netmem localhost 10006
+mkdev netmem netmem maya 10006
 
 connectdev dc dt0
-connectdev dt0 dx0 8
+connectdev dt0 dx0 1
 connectio tty apr
 connectio ptr apr
 connectio ptp apr
@@ -34,4 +35,3 @@ attach ptr ../code/test.rim
 attach ptp ../code/ptp.out
 #attach dx0 ../test/out.dt6
 attach dx0 ../test/test.dt6
-
diff --git a/emu/main.c b/emu/main.c
index 5c9b817..0a1eaab 100644
--- a/emu/main.c
+++ b/emu/main.c
@@ -621,7 +621,26 @@ dofile(const char *path)
 void
 quit(int code)
 {
-	//SDL_Quit();
+	int i;
+	Device *dev;
+	Membus *mbus;
+
+	/* Detach all files */
+	for(dev = devlist; dev; dev = dev->next)
+		if(dev->detach)
+			dev->detach(dev);
+
+	/* Sync memory to disk */
+	for(dev = devlist; dev; dev = dev->next){
+		if(strcmp(dev->type, apr_ident) != 0)
+			continue;
+
+		mbus = &((Apr*)dev)->membus;
+		for(i = 0; i < 16; i++)
+			if(mbus->cmem[i] && mbus->cmem[i]->sync)
+				mbus->cmem[i]->sync(mbus->cmem[i]);
+	}
+
 	putchar('\n');
 	exit(code);
 }
diff --git a/emu/mem.c b/emu/mem.c
index c108d22..e799a99 100644
--- a/emu/mem.c
+++ b/emu/mem.c
@@ -33,6 +33,36 @@ readmem(const char *file, word *mem, word size)
 	fclose(f);
 }
 
+void
+writemem(const char *file, word *mem, word size)
+{
+	FILE *f;
+	hword i, a;
+
+	if(f = fopen(file, "w"), f == nil)
+		return;
+
+	a = 0;
+	for(i = 0; i < size; i++)
+		if(mem[i] != 0){
+			if(a != i){
+				a = i;
+				fprintf(f, "%06o:\n", a);
+			}
+			fprintf(f, "%012lo\n", mem[a++]);
+		}
+
+	fclose(f);
+}
+
+static void
+synccore(Mem *mem)
+{
+	CMem *core;
+	core = mem->module;
+	writemem(core->filename, core->core, 040000);
+}
+
 /* Both functions below are very confusing. I'm sorry.
  * The schematics cannot be converted to C in a straightfoward way
  * but I tried my best. */
@@ -245,11 +275,9 @@ makecoremem(const char *file)
 	memset(core, 0, sizeof(CMem));
 	core->filename = strdup(file);
 	mem = malloc(sizeof(Mem));
+	memset(mem, 0, sizeof(Mem));
 	mem->dev.type = cmem_ident;
 	mem->dev.name = "";
-	mem->dev.attach = nil;
-	mem->dev.ioconnect = nil;
-	mem->dev.next = nil;
 
 	mem->module = core;
 	mem->bus[0] = &memterm;
@@ -258,6 +286,7 @@ makecoremem(const char *file)
 	mem->bus[3] = &memterm;
 	mem->wake = wakecore;
 	mem->poweron = powercore;
+	mem->sync = synccore;
 
 	return mem;
 }
@@ -274,11 +303,9 @@ makefastmem(int p)
 	ff->fmc_p_sel = p;
 
 	mem = malloc(sizeof(Mem));
+	memset(mem, 0, sizeof(Mem));
 	mem->dev.type = fmem_ident;
 	mem->dev.name = "";
-	mem->dev.attach = nil;
-	mem->dev.ioconnect = nil;
-	mem->dev.next = nil;
 
 	mem->module = ff;
 	mem->bus[0] = &memterm;
diff --git a/emu/mem_0 b/emu/mem_0
index 3083bc9..1910e3b 100644
--- a/emu/mem_0
+++ b/emu/mem_0
@@ -9,16 +9,14 @@
 720200000000
 254200000300
 777600000277
-0
-0
-0
-0
-0
+000020:
 710600000060
 710740000010
 254000000021
 710440000026
 710740000010
 254000000024
-0
+000027:
 254000000021
+000200:
+000000000567
diff --git a/emu/netmem.c b/emu/netmem.c
index c251fd2..9e8f2c9 100644
--- a/emu/netmem.c
+++ b/emu/netmem.c
@@ -32,9 +32,10 @@ netmemcycle(void *dev)
 	}
 	len = buf[0]<<8 | buf[1];
 	if(len > 9){
-		fprintf(stderr, "netmem botch, closing\n");
+		fprintf(stderr, "netmem botch(%d), closing\n", len);
 		close(nm->fd);
 		nm->fd = -1;
+		return;
 	}
 	memset(buf, 0, sizeof(buf));
 	readn(nm->fd, buf, len);
@@ -96,9 +97,6 @@ makenetmem(int argc, char *argv[])
 	memset(nm, 0, sizeof(Netmem));
 	nm->dev.type = netmem_ident;
 	nm->dev.name = "";
-	nm->dev.attach = nil;
-	nm->dev.ioconnect = nil;
-	nm->dev.next = nil;
 
 	// TODO: don't hardcode;
 	apr = getdevice("apr");
diff --git a/emu/pdp6.h b/emu/pdp6.h
index fb35632..a8ec46a 100644
--- a/emu/pdp6.h
+++ b/emu/pdp6.h
@@ -103,8 +103,10 @@ struct Device
 	char *name;
 	/* attach file to device */
 	void (*attach)(Device *dev, const char *path);
+	void (*detach)(Device *dev);
 	/* connect device to iobus */
 	void (*ioconnect)(Device *dev, IOBus *bus);
+	/* manipulate device registers */
 	word (*examine)(Device *dev, const char *reg);
 	int  (*deposit)(Device *dev, const char *reg, word data);
 };
@@ -146,10 +148,11 @@ enum {
 struct Mem
 {
 	Device dev;
-	void *module;
+	void *module;			/* CMem, FMem */
 	Membus *bus[4];
 	int (*wake)(Mem *mem, Membus *bus);
 	void (*poweron)(Mem *mem);
+	void (*sync)(Mem *mem);		/* sync file to disk */
 };
 
 struct Membus
@@ -554,6 +557,7 @@ struct Dx555
 	uchar *start;
 	uchar *cur;
 	uchar *end;
+	uint size;
 };
 #define DX_IDENT "dx555"
 extern char *dx_ident;
@@ -598,17 +602,21 @@ struct Dt551
 	int ut_info_error;
 	int ut_incomp_block;
 
-	int tmk;	/* 9 bits */
-	int rwb;	/* 6 bits */
-	int lb;		/* 6 bits */
-	int tbm;	/* 4 bits */
-	int tdata;	/* 8 bits */
-	int tct;	/* 1 bit  */
+	int tmk;	/* mark track window, 9b */
+	int rwb;	/* read write buffer, 6b */
+	int lb;		/* longitudinal buffer, computes check sum, 6b */
+	int tbm;	/* block mark timing, 4b */
+	int tdata;	/* data timing, 8b */
+	int tct;	/* selects half of 6 bit rwb, 1b */
+	/* error check */
 	int utek;	/* 6 bits */
 	int uteck;
 
 	int rw_state;	/* null, rq, active */
 
+	int delay;
+	int sense;
+	int wb;
 
 //	int req;
 };
@@ -616,11 +624,6 @@ struct Dt551
 extern char *dt_ident;
 Device *makedt(int argc, char *argv[]);
 void dtconn(Dc136 *dc, Dt551 *dt);
-/*
-void dtcono(Dt551 *dt, word iob);
-void dtcycle(Dt551 *dt);
-*/
-
 
 typedef struct Netmem Netmem;
 struct Netmem
diff --git a/emu/test_dt.c b/emu/test_dt.c
new file mode 100644
index 0000000..90c399d
--- /dev/null
+++ b/emu/test_dt.c
@@ -0,0 +1,345 @@
+#include "pdp6.h"
+
+Dc136 *dc;
+Dt551 *dt;
+Dx555 *dx;
+IOBus iobus;
+IOBus *bus = &iobus;
+int pireq;
+Thread dtthread;
+
+void
+setreq(IOBus *bus, int dev, u8 pia)
+{
+	if(dev == DC)
+		pireq = pia;
+}
+
+void
+addthread(Thread th)
+{
+	dtthread = th;
+}
+
+
+void
+wakedev(IOBus *bus, int d)
+{
+	Busdev *dev;
+	bus->devcode = d;
+	dev = &bus->dev[d];
+	if(dev->wake)
+		dev->wake(dev->dev);
+}
+
+void
+bus_reset(IOBus *bus)
+{
+	int d;
+	bus->c34 |= IOBUS_IOB_RESET;
+	for(d = 0; d < 128; d++)
+		wakedev(bus, d);
+	bus->c34 &= ~IOBUS_IOB_RESET;
+}
+
+void
+cono(IOBus *bus, int d, hword w)
+{
+	bus->c34 |= IOBUS_CONO_CLEAR;
+	wakedev(bus, d);
+	bus->c34 &= ~IOBUS_CONO_CLEAR;
+
+	bus->c34 |= IOBUS_CONO_SET;
+	bus->c12 = w;
+	wakedev(bus, d);
+	bus->c34 &= ~IOBUS_CONO_SET;
+	bus->c12 = 0;
+}
+
+void
+datao(IOBus *bus, int d, word w)
+{
+	bus->c34 |= IOBUS_DATAO_CLEAR;
+	wakedev(bus, d);
+	bus->c34 &= ~IOBUS_DATAO_CLEAR;
+
+	bus->c34 |= IOBUS_DATAO_SET;
+	bus->c12 = w;
+	wakedev(bus, d);
+	bus->c34 &= ~IOBUS_DATAO_SET;
+	bus->c12 = 0;
+}
+
+word
+coni(IOBus *bus, int d)
+{
+	word w;
+	bus->c34 |= IOBUS_IOB_STATUS;
+	wakedev(bus, d);
+	bus->c34 &= ~IOBUS_IOB_STATUS;
+	w = bus->c12;
+	bus->c12 = 0;
+	return w;
+}
+
+word
+datai(IOBus *bus, int d)
+{
+	word w;
+	bus->c34 |= IOBUS_IOB_DATAI;
+	wakedev(bus, d);
+	bus->c34 &= ~IOBUS_IOB_DATAI;
+	w = bus->c12;
+	bus->c12 = 0;
+	return w;
+}
+
+void
+printdc(Dc136 *dc)
+{
+	printf("DB/%012lo DA/%012lo CCT/%o SCT/%o CHMOD/%o\n",
+		dc->db, dc->da, dc->cct, dc->sct, dc->ch_mode);
+	printf(" CLBD/%o DBDAMOVE/%o DARQ/%o DBRQ/%o INOUT/%o DEV/%o PIA/%o\n",
+		dc->data_clbd, dc->dbda_move, dc->da_rq, dc->db_rq,
+		dc->inout, dc->device, dc->pia);
+}
+
+// DC controls
+enum {
+	CLBD = 010000,
+
+	DBDAMOVE = 04000,
+	DARQ = 02000,
+	DBRQ = 01000,
+
+	IN = 0000,
+	OUT = 0400,
+
+	CHMOD_6 = 0000,
+	CHMOD_36 = 0100,
+	CHMOD_12 = 0200,
+	CHMOD_18 = 0300,
+
+	DEV1 = 010,
+	DEV2 = 020,
+	DEV3 = 030,
+	DEV4 = 040,
+	DEV5 = 050,
+	DEV6 = 060,
+};
+
+enum {
+	SEL    = 0200000,
+	GO     = 0020000,
+	FWD    = 0000000,
+	REV    = 0010000,
+	DLY0   = 0000000,
+	DLY1   = 0001000,
+	DLY2   = 0002000,
+	DLY3   = 0003000,
+	FN_DN  = 0000000,
+	FN_RA  = 0000100,
+	FN_RBN = 0000200,
+	FN_RD  = 0000300,
+	FN_WTM = 0000400,
+	FN_WA  = 0000500,
+	FN_WBN = 0000600,
+	FN_WD  = 0000700
+};
+
+word writebuf[133*01102 + 6000];
+word *bufp;
+int bufsz;
+
+void
+fillbuf(int n)
+{
+	bufp = writebuf;
+	bufsz = n;
+	while(n--)
+		bufp[n] = n | 0112200000000;
+}
+
+void
+fillbuf_fmt(void)
+{
+	int i, j;
+	bufp = writebuf;
+
+	for(i = 0; i < 6; i++)
+		*bufp++ = 0707707707707;	// 55 55
+
+	for(i = 0; i < 01102; i++){
+		*bufp++ = 0070707070770;	// 25 26
+		*bufp++ = 0077070007000;	// 32 10
+		*bufp++ = 0007000007000;	// 10 10
+		*bufp++ = 0007000777000;	// 10 70
+
+		for(j = 0; j < 125; j++)
+			*bufp++ = 0777000777000;	// 70 70
+
+		*bufp++ = 0777000777077;	// 70 73
+		*bufp++ = 0777077777077;	// 73 73
+		*bufp++ = 0777077707007;	// 73 51
+		*bufp++ = 0700707070707;	// 45 25
+	}
+
+	for(i = 0; i < 6; i++)
+		*bufp++ = 0070070070070;	// 22 22
+
+	bufsz = bufp - writebuf;
+	bufp = writebuf;
+}
+
+void
+fwdtest_w(void)
+{
+	word w;
+	printf("forward test:\n\n");
+	dx->cur = dx->start;
+
+	fillbuf(128);
+
+	cono(bus, DC, DARQ|DBRQ|OUT|CHMOD_6|DEV1|7);
+	cono(bus, UTC, SEL|GO|FWD|DLY3|FN_WBN|DEV1|0);
+	while(dt->ut_go){
+		dtthread.f(dt);
+		if(pireq == 7){
+			if(bufsz > 0){
+				w = bufp[--bufsz];
+				datao(bus, DC, w);
+				printf("DC send: %012lo\n", w);
+			}
+//			break;
+		}
+	}
+//	printdc(dc);
+}
+
+void
+revtest_w(void)
+{
+	word w;
+	printf("reverse test:\n\n");
+	dx->cur = dx->end-1;
+
+	fillbuf(128);
+
+	cono(bus, DC, DARQ|DBRQ|OUT|CHMOD_6|DEV1|7);
+	cono(bus, UTC, SEL|GO|REV|DLY3|FN_WD|DEV1|0);
+	while(dt->ut_go){
+		dtthread.f(dt);
+		if(pireq == 7){
+			if(bufsz > 0){
+				w = bufp[--bufsz];
+				datao(bus, DC, w);
+				printf("DC send: %012lo\n", w);
+			}
+//			break;
+		}
+	}
+//	printdc(dc);
+}
+
+void
+fwdtest_r(void)
+{
+	word w;
+	printf("forward test:\n\n");
+	dx->cur = dx->start;
+
+	cono(bus, DC, DBDAMOVE|IN|CHMOD_6|DEV1|7);
+	cono(bus, UTC, SEL|GO|FWD|DLY3|FN_RD|DEV1|0);
+	while(dt->ut_go){
+		dtthread.f(dt);
+		if(pireq == 7){
+			w = datai(bus, DC);
+			printf("DC got: %012lo\n", w);
+//			break;
+		}
+	}
+//	printdc(dc);
+}
+
+void
+revtest_r(void)
+{
+	word w;
+	printf("reverse test:\n\n");
+	dx->cur = dx->end-1;
+
+	cono(bus, DC, DBDAMOVE|IN|CHMOD_6|DEV1|7);
+	cono(bus, UTC, SEL|GO|REV|DLY3|FN_RD|DEV1|0);
+	while(dt->ut_go){
+		dtthread.f(dt);
+		if(pireq == 7){
+			w = datai(bus, DC);
+			printf("DC got: %012lo\n", w);
+//			break;
+		}
+	}
+//	printdc(dc);
+}
+
+void
+fmttest(void)
+{
+	word w;
+	printf("format test:\n\n");
+	dx->cur = dx->start;
+
+	fillbuf_fmt();
+
+	cono(bus, DC, DARQ|DBRQ|OUT|CHMOD_6|DEV1|7);
+	cono(bus, UTC, SEL|GO|FWD|DLY3|FN_WTM|DEV1|0);
+	while(dt->ut_go){
+		dtthread.f(dt);
+		if(pireq == 7){
+			if(bufsz > 0){
+				--bufsz;
+				w = *bufp++;
+				datao(bus, DC, w);
+//				printf("DC send: %012lo\n", w);
+			}else
+				cono(bus, UTC, SEL|DLY2|DEV1|0);
+//			break;
+		}
+	}
+//	printdc(dc);
+}
+
+void
+dxstore(Dx555 *dx, const char *file)
+{
+	FILE *f;
+	f = fopen(file, "wb");
+	if(f == nil)
+		return;
+	fwrite(dx->start, 1, dx->size, f);
+	fclose(f);
+}
+
+int
+main()
+{
+	dc = (Dc136*)makedc(0, nil);
+	dc->dev.ioconnect((Device*)dc, bus);
+	dt = (Dt551*)makedt(0, nil);
+	dt->dev.ioconnect((Device*)dt, bus);
+	dtconn(dc, dt);
+	dx = (Dx555*)makedx(0, nil);
+	dxconn(dt, dx, 1);
+	dx->dev.attach((Device*)dx, "../test/out.dt6");
+
+	bus_reset(bus);
+	assert(coni(bus, DC) == 0);
+
+	printdc(dc);
+
+//	fmttest();
+//	fwdtest_r();
+
+	dxstore(dx, "foo.dt6");
+
+	return 0;
+}
diff --git a/emu/util.c b/emu/util.c
index 0bda9cb..b6ef53f 100644
--- a/emu/util.c
+++ b/emu/util.c
@@ -44,7 +44,7 @@ readn(int fd, void *data, int n)
 
 	while(n > 0){
 		m = read(fd, data, n);
-		if(m == -1)
+		if(m <= 0)
 			return -1;
 		data += m;
 		n -= m;