diff --git a/build.sh b/build.sh
index 6928d61..06612d5 100755
--- a/build.sh
+++ b/build.sh
@@ -21,5 +21,8 @@ cc -std=c89 -DNO_SDL -DDEBUG -Wall -Wno-unknown-pragmas -Wpedantic -Wshadow -Wex
 # cc uxn.c emulator.c -std=c89 -Os -DNDEBUG -g0 -s -Wall -Wno-unknown-pragmas -L/usr/local/lib -lSDL2 -o bin/emulator
 
 # run
+if [ "${#}" -gt 0 ]; then
+	exec ./run.sh "${@}"
+fi
 ./bin/assembler projects/software/noodle.usm bin/boot.rom
 ./bin/emulator bin/boot.rom
diff --git a/emulator.c b/emulator.c
index 25868fc..92d4d9a 100644
--- a/emulator.c
+++ b/emulator.c
@@ -412,6 +412,131 @@ file_poke(Uxn *u, Uint16 ptr, Uint8 b0, Uint8 b1)
 	return b1;
 }
 
+static void
+stack_diff(Stack *old, Stack *new, char *title)
+{
+	size_t i;
+	printf("%6s: ", title);
+	for (i = 0;; ++i) {
+		if (i < old->ptr) {
+			if (i < new->ptr) {
+				if (old->dat[i] == new->dat[i]) {
+					printf(" \033[0m%02x", new->dat[i]);
+				}
+				else {
+					printf(" \033[0;31m%02x\033[33;1m%02x", old->dat[i], new->dat[i]);
+				}
+			}
+			else { /* only in old stack */
+				printf(" \033[0;31m%02x", old->dat[i]);
+			}
+		}
+		else {
+			if (i < new->ptr) { /* only in new stack */
+				printf(" \033[33;1m%02x", new->dat[i]);
+			}
+			else { /* in neither stack, end of loop */
+				break;
+			}
+		}
+	}
+	printf("\033[0m\n");
+}
+
+static void
+memory_diff(Uint8 *old, Uint8 *new, size_t start, size_t end)
+{
+	size_t i, j;
+	for (i = start; i < end; i += 0x10) {
+		int changes = 0;
+		for (j = i; j < i + 0x10; ++j ) {
+			if (old[j] != new[j]) {
+				changes = 1;
+				break;
+			}
+		}
+		if (!changes) continue;
+		printf("0x%04lx:  ", i);
+		for (j = i; j < i + 0x10; ++j) {
+			printf("\033[%sm%02x", old[j] == new[j] ? "0" : "33;1", new[j]);
+			if (j % 2) putchar(' ');
+		}
+		printf("  ");
+		for (j = i; j < i + 0x10; ++j) {
+			printf("\033[%sm%c", old[j] == new[j] ? "0" : "33;1",
+				(new[j] < ' ' || new[j] > '~') ? '.' : new[j]);
+		}
+		printf("\033[0m\n");
+	}
+}
+
+Uint8
+debug_poke(Uxn *u, Uint16 ptr, Uint8 b0, Uint8 b1)
+{
+	size_t i;
+	(void)ptr;
+	switch (b0) {
+		case 0x08: /* stack */
+			printf("pc %04x working stack:", u->ram.ptr);
+			for (i = 0; i < u->wst.ptr; ++i) {
+				printf(" %02x", u->wst.dat[i]);
+			}
+			printf(", return stack: ");
+			for (i = 0; i < u->rst.ptr; ++i) {
+				printf(" %02x", u->rst.dat[i]);
+			}
+			printf("\n");
+			if (b1 && b1 != u->wst.ptr) {
+				printf("length %d failed to match %d!\n", b1, u->wst.ptr);
+				exit(1);
+			}
+			break;
+		case 0x09: /* snapshot */
+			if (u->snapshot != NULL) {
+				if (!(b1 & 0x01)) {
+					stack_diff(&u->snapshot->wst, &u->wst, "work");
+				}
+				if (!(b1 & 0x02)) {
+					stack_diff(&u->snapshot->rst, &u->rst, "return");
+				}
+				if (!(b1 & 0x04)) {
+					memory_diff(u->snapshot->ram.dat, u->ram.dat, 0, PAGE_DEVICE);
+					memory_diff(u->snapshot->ram.dat, u->ram.dat, PAGE_DEVICE + 0x0100, 0x10000);
+				}
+			}
+			{
+				int want_snapshot = !(b1 & 0x80);
+				if (want_snapshot) {
+					if (u->snapshot == NULL) {
+						u->snapshot = malloc(sizeof(*u));
+					}
+					for (i = 0; i < sizeof(*u); ++i) {
+						((char *) u->snapshot)[i] = ((char *) u)[i];
+					}
+				}
+				printf("pc 0x%04x snapshot%s taken\n", u->counter, want_snapshot ? "" : " not");
+			}
+			break;
+		case 0x0a: /* exit */
+			printf("Exited after 0x%04x cycles.\n", u->counter);
+			exit(b1);
+			break;
+		case 0x0f: /* test mode */
+			u->test_mode = b1;
+			printf("Test mode is now 0x%02x: ", u->test_mode);
+			if (b1 & 0x01) {
+				printf("BRK resets stacks to zero length");
+			}
+			else {
+				printf("all test mode features disabled");
+			}
+			printf("\n");
+			break;
+	}
+	fflush(stdout);
+	return b1;
+}
+
 Uint8
 system_poke(Uxn *u, Uint16 ptr, Uint8 b0, Uint8 b1)
 {
@@ -468,6 +593,10 @@ start(Uxn *u)
 			}
 		}
 #endif
+		if (u->test_mode & 0x01) {
+			u->wst.ptr = 0;
+			u->rst.ptr = 0;
+		}
 		evaluxn(u, u->vframe);
 #ifndef NO_SDL
 		if(screen.reqdraw)
@@ -504,7 +633,7 @@ main(int argc, char **argv)
 	portuxn(&u, "empty", ppnil);
 	portuxn(&u, "empty", ppnil);
 	portuxn(&u, "empty", ppnil);
-	portuxn(&u, "empty", ppnil);
+	portuxn(&u, "debug", debug_poke);
 	portuxn(&u, "system", system_poke);
 
 	/* Write screen size to dev/screen */
diff --git a/projects/tests/opcodes.usm b/projects/tests/opcodes.usm
index d7877d6..d6dedae 100644
--- a/projects/tests/opcodes.usm
+++ b/projects/tests/opcodes.usm
@@ -5,6 +5,7 @@
 |0140 ;Keys { key 1 }
 |0150 ;Mouse { x 2 y 2 state 1 chord 1 }
 |0160 ;File { pad 8 name 2 length 2 load 2 save 2 }
+|01E0 ;Debug { pad 8 stack 1 snapshot 1 exit 1 pad 4 test_mode 1 }
 |01F0 .RESET .FRAME .ERROR ( vectors )
 |01F8 [ f07c f0e2 f0c2 ]   ( palette )
 
@@ -22,10 +23,10 @@
 	TEST #01 #02 ADD #03 EQU PASS? [ add-result ]
 	TEST #01 #02 ADD #ff EQU PASS? [ this-test-fails ]
 
-	( infinite loop: change to HCF when implemented )
-	TEST #fd JMP
+	TEST #00 =Debug.exit
 
 @RESET
+	#01 =Debug.test_mode
 	,tests #0001 SUB2 =current-test
 	BRK
 	
diff --git a/uxn.h b/uxn.h
index fcd5c47..e61e80d 100644
--- a/uxn.h
+++ b/uxn.h
@@ -40,11 +40,12 @@ typedef struct Device {
 } Device;
 
 typedef struct Uxn {
-	Uint8 literal, status, devices;
+	Uint8 literal, status, devices, test_mode;
 	Uint16 counter, vreset, vframe, verror;
 	Stack wst, rst, *src, *dst;
 	Memory ram;
 	Device dev[16];
+	struct Uxn *snapshot;
 } Uxn;
 
 void setflag(Uint8 *status, char flag, int b);