diff --git a/cen64.c b/cen64.c
index 5d7bf14..51014a4 100644
--- a/cen64.c
+++ b/cen64.c
@@ -196,17 +196,13 @@ int cen64_main(int argc, const char **argv) {
       memset(flashram.ptr, 0xFF, FLASHRAM_SIZE);
   }
 
-  if (options.is_viewer_present) {
-    if (!is_viewer_init(&is)) {
-      cen64_alloc_cleanup();
-      return EXIT_FAILURE;
-    } else {
-      is_in = &is;
-    }
+  if (!is_viewer_init(&is, options.is_viewer_output)) {
+    cen64_alloc_cleanup();
+    return EXIT_FAILURE;
+  } else {
+    is_in = &is;
   }
 
-
-
   // Allocate memory for and create the device.
   if (cen64_alloc(&cen64_device_mem, sizeof(*device), false) == NULL) {
     printf("Failed to allocate enough memory for a device.\n");
diff --git a/device/options.c b/device/options.c
index b088cea..19d4a7b 100644
--- a/device/options.c
+++ b/device/options.c
@@ -28,7 +28,7 @@ const struct cen64_options default_cen64_options = {
   NULL, // sram_path
   0,    // sram_size
   NULL, // flashram_path
-  0,    // is_viewer_present
+  0,    // is_viewer_output
   NULL, // controller
   false, // enable_debugger
   false, // enable_profiling
@@ -159,7 +159,7 @@ int parse_options(struct cen64_options *options, int argc, const char *argv[]) {
     }
 
     else if (!strcmp(argv[i], "-is-viewer"))
-      options->is_viewer_present = 1;
+      options->is_viewer_output = 1;
 
     else if (!strcmp(argv[i], "-controller")) {
       int num;
@@ -289,7 +289,7 @@ void print_command_line_usage(const char *invokation_string) {
       "  -headless                  : Run emulator without user-interface components.\n"
       "  -noaudio                   : Run emulator without audio.\n"
       "  -novideo                   : Run emulator without video.\n"
-      "  -is-viewer                 : IS Viewer 64 present.\n"
+      "  -is-viewer                 : Show IS Viewer 64 output.\n"
       "\n"
       "Controller Options:\n"
       "  -controller num=<1-4>      : Controller with no pak.\n"
diff --git a/device/options.h b/device/options.h
index b8a1919..acc8e24 100644
--- a/device/options.h
+++ b/device/options.h
@@ -24,7 +24,7 @@ struct cen64_options {
   const char *sram_path;
   size_t sram_size;
   const char *flashram_path;
-  int is_viewer_present;
+  int is_viewer_output;
 
   struct controller *controller;
 
diff --git a/pi/is_viewer.c b/pi/is_viewer.c
index dbe6608..cc88595 100644
--- a/pi/is_viewer.c
+++ b/pi/is_viewer.c
@@ -7,7 +7,7 @@
 // arbitrarily chosen
 #define IS_BUFFER_SIZE 0x200
 
-int is_viewer_init(struct is_viewer *is) {
+int is_viewer_init(struct is_viewer *is, int is_viewer_output) {
   memset(is, 0, sizeof(*is));
 
   // TODO support other addresses
@@ -17,6 +17,7 @@ int is_viewer_init(struct is_viewer *is) {
   is->buffer = calloc(IS_BUFFER_SIZE, 1);
   is->output_buffer = calloc(IS_BUFFER_SIZE, 1);
   is->output_buffer_conv = calloc(IS_BUFFER_SIZE * 3, 1);
+  is->show_output = is_viewer_output;
 
   is->cd = iconv_open("UTF-8", "EUC-JP");
 
@@ -61,7 +62,12 @@ int write_is_viewer(struct is_viewer *is, uint32_t address, uint32_t word, uint3
         memset(is->output_buffer_conv, 0, IS_BUFFER_SIZE * 3);
         iconv(is->cd, &inptr, &len, &outptr, &outlen);
 
-        printf("%s", is->output_buffer_conv);
+        if (is->show_output)
+          printf("%s", is->output_buffer_conv);
+        else if (!is->output_warning) {
+          printf("ISViewer debugging output detected and suppressed.\nRun cen64 with option -is-viewer to display it\n");
+          is->output_warning = 1;
+        }
 
         memset(is->output_buffer, 0, is->output_buffer_pos);
         is->output_buffer_pos = 0;
diff --git a/pi/is_viewer.h b/pi/is_viewer.h
index 4ad79d8..c30efee 100644
--- a/pi/is_viewer.h
+++ b/pi/is_viewer.h
@@ -15,11 +15,13 @@ struct is_viewer {
   uint8_t *output_buffer;
   size_t output_buffer_pos;
   uint8_t *output_buffer_conv;
+  int show_output;
+  int output_warning;
 
   iconv_t cd;
 };
 
-int is_viewer_init(struct is_viewer *is);
+int is_viewer_init(struct is_viewer *is, int show_output);
 int is_viewer_map(struct is_viewer *is, uint32_t address);
 int read_is_viewer(struct is_viewer *is, uint32_t address, uint32_t *word);
 int write_is_viewer(struct is_viewer *is, uint32_t address, uint32_t word, uint32_t dqm);