diff --git a/Makefile b/Makefile
index 7bab229..836dcee 100644
--- a/Makefile
+++ b/Makefile
@@ -24,10 +24,11 @@ EMCC_ARGS += --js-library xterm-pty/emscripten-pty.js
 EMCC_ARGS += --preload-file plexus-sanitized.img
 EMCC_ARGS += --preload-file U15-MERGED.BIN
 EMCC_ARGS += --preload-file U17-MERGED.BIN
+EMCC_ARGS += -lidbfs.js
 EMCC_ARGS += -O2 -gsource-map --source-map-base=./
 
 plexem.mjs: $(SRC)
-	emcc -o $@ $(EMCC_ARGS) $^ -lm 
+	emcc -o $@ $(EMCC_ARGS) $^ emscripten_env.c -lm 
 
 clean:
 	rm -f $(SRC:.c=.o) 
diff --git a/emscripten_env.c b/emscripten_env.c
new file mode 100644
index 0000000..5ee65c8
--- /dev/null
+++ b/emscripten_env.c
@@ -0,0 +1,20 @@
+#include "emscripten_env.h"
+#include "emscripten.h"
+
+void emscripten_init() {
+	EM_ASM(
+		FS.mkdir("/persist");
+		FS.mount(IDBFS, {}, "/persist");
+		FS.syncfs(true, function (err) {
+			assert(!err);
+		});
+	);
+}
+
+void emscripten_syncfs() {
+	EM_ASM(
+		FS.syncfs(false, function (err) {
+			assert(!err);
+		});
+	);
+}
diff --git a/emscripten_env.h b/emscripten_env.h
new file mode 100644
index 0000000..735a718
--- /dev/null
+++ b/emscripten_env.h
@@ -0,0 +1,3 @@
+
+void emscripten_init();
+void emscripten_syncfs();
diff --git a/main.c b/main.c
index b8f6b46..1457327 100644
--- a/main.c
+++ b/main.c
@@ -4,6 +4,7 @@
 #include <string.h>
 #include "emu.h"
 #include "log.h"
+#include "emscripten_env.h"
 
 const char *log_str[]={
 	[LOG_SRC_UART]="uart",
@@ -63,14 +64,19 @@ int main(int argc, char **argv) {
 #ifdef __EMSCRIPTEN__
 		.u15_rom="U15-MERGED.BIN",
 		.u17_rom="U17-MERGED.BIN",
+		.cow_dir="persist/cow",
+		.rtcram="persist/rtcram.bin",
 		.realtime=1,
 #else
 		.u15_rom="../plexus-p20/ROMs/U15-MERGED.BIN",
 		.u17_rom="../plexus-p20/ROMs/U17-MERGED.BIN",
+		.rtcram="rtcram.bin",
 #endif
 		.hd0img="plexus-sanitized.img",
-		.rtcram="rtcram.bin"
 	};
+#ifdef __EMSCRIPTEN__
+	emscripten_init();
+#endif
 	int error=0;
 	for (int i=1; i<argc; i++) {
 		if (strcmp(argv[i], "-u15")==0 && i+1<argc) {
diff --git a/scsi.c b/scsi.c
index b7b8964..78129cc 100644
--- a/scsi.c
+++ b/scsi.c
@@ -29,7 +29,7 @@ struct scsi_t {
 	uint8_t cmd[10];
 	int selected;
 	int op_timeout_us;
-	uint8_t databuf[256*512]
+	uint8_t databuf[256*512];
 };
 
 /*
diff --git a/scsi_dev_hd.c b/scsi_dev_hd.c
index 3e1aa9e..55f567a 100644
--- a/scsi_dev_hd.c
+++ b/scsi_dev_hd.c
@@ -7,6 +7,11 @@
 #include "scsi.h"
 #include "emu.h"
 #include "log.h"
+#include "emscripten_env.h"
+
+//Might need to change if e.g. the backing file changes for the web version.
+#define COW_VERSION_MAJOR 0
+#define COW_VERSION_MINOR 0
 
 typedef struct {
 	scsi_dev_t dev;
@@ -42,6 +47,8 @@ static void write_block(scsi_hd_t *hd, int lba, uint8_t *data) {
 			perror("opening cow file for write");
 			exit(1);
 		}
+		uint8_t ver[2]={COW_VERSION_MAJOR, COW_VERSION_MINOR};
+		fwrite(ver, 2, 1, f);
 		fwrite(data, 512, 1, f);
 		fclose(f);
 	} else {
@@ -54,9 +61,16 @@ static void read_block(scsi_hd_t *hd, int lba, uint8_t *data) {
 	if (hd->cow_dir) {
 		FILE *f=open_cow_file(hd, lba, "rb");
 		if (f) {
-			fread(data, 512, 1, f);
-			fclose(f);
-			return;
+			uint8_t ver[2];
+			fread(ver, 2, 1, f);
+			if (ver[0]==COW_VERSION_MAJOR && ver[1]==COW_VERSION_MINOR) {
+				fread(data, 512, 1, f);
+				fclose(f);
+				return;
+			} else {
+				//not the same version; ignore
+				fclose(f);
+			}
 		}
 	}
 	fseek(hd->hdfile, lba*512, SEEK_SET);
@@ -128,6 +142,9 @@ static void hd_handle_data_out(scsi_dev_t *dev, uint8_t *msg, int len) {
 		for (int i=0; i<blen/512; i++) {
 			write_block(hd, lba+i, &msg[i*512]);
 		}
+#ifdef __EMSCRIPTEN__
+		emscripten_syncfs();
+#endif
 	}
 }