diff --git a/src/disk.c b/src/disk.c
index 42d4cb50..956672eb 100644
--- a/src/disk.c
+++ b/src/disk.c
@@ -21,6 +21,11 @@
 #define NIB_SIZE 232960
 #define DSK_SIZE 143360
 
+#ifdef TESTING
+static FILE *test_read_fp = NULL;
+static FILE *test_write_fp = NULL;
+#endif
+
 extern uint8_t slot6_rom[256];
 extern bool slot6_rom_loaded;
 
@@ -115,6 +120,30 @@ void c_init_6()
 #endif
 }
 
+#ifdef TESTING
+void c_begin_test_6(const char *read_file, const char *write_file) {
+    if (read_file) {
+        test_read_fp = fopen(read_file, "w");
+    }
+    if (write_file) {
+        test_write_fp = fopen(write_file, "w");
+    }
+}
+
+void c_end_test_6() {
+    if (test_read_fp) {
+        fflush(test_read_fp);
+        fclose(test_read_fp);
+        test_read_fp = NULL;
+    }
+    if (test_write_fp) {
+        fflush(test_write_fp);
+        fclose(test_write_fp);
+        test_write_fp = NULL;
+    }
+}
+#endif
+
 /* -------------------------------------------------------------------------
     c_eject_6() - ejects/gzips image file
    ------------------------------------------------------------------------- */
@@ -257,6 +286,12 @@ unsigned char c_read_nibblized_6_6()
         fseek(disk6.disk[disk6.drive].fp, -2 * PHASE_BYTES, SEEK_CUR);
     }
 
+#ifdef TESTING
+    if (test_read_fp) {
+        fputc(ch, test_read_fp);
+    }
+#endif
+
     return ch;
 }
 
@@ -427,6 +462,12 @@ unsigned char c_read_normal_6()
         disk6.disk[disk6.drive].run_byte = 0;
     }
 
+#ifdef TESTING
+    if (test_read_fp) {
+        fputc(value, test_read_fp);
+    }
+#endif
+
     disk6.disk_byte = value;
     return value;
 }
@@ -448,6 +489,13 @@ void c_write_nibblized_6_6()
     }
 
     fputc(disk6.disk_byte, disk6.disk[disk6.drive].fp);
+
+#ifdef TESTING
+    if (test_write_fp) {
+        fputc(disk6.disk_byte, test_write_fp);
+    }
+#endif
+
     /* track revolves... */
     if (ftell(disk6.disk[disk6.drive].fp) == (PHASE_BYTES * (disk6.disk[disk6.drive].phase + 2)))
     {
@@ -554,6 +602,11 @@ void c_write_normal_6()
 
         /* Write sector */
         fwrite(disk6.disk_data, 1, 256, disk6.disk[disk6.drive].fp);
+#ifdef TESTING
+        if (test_write_fp) {
+            fwrite(disk6.disk_data, 1, 256, test_write_fp);
+        }
+#endif
         fflush( disk6.disk[disk6.drive].fp );
         /* Increment sector number (and wrap if necessary) */
         disk6.disk[disk6.drive].sector++;
diff --git a/src/test/testvm.c b/src/test/testvm.c
index 4117c845..c67db741 100644
--- a/src/test/testvm.c
+++ b/src/test/testvm.c
@@ -50,7 +50,7 @@ static void sha1_to_str(const uint8_t * const md, char *buf) {
 // ----------------------------------------------------------------------------
 // VM TESTS ...
 
-TEST test_boot_disk() {
+TEST setup_boot_disk(void) {
     char *disk = NULL;
 #ifdef __APPLE__
     CFBundleRef mainBundle = CFBundleGetMainBundle();
@@ -74,7 +74,55 @@ TEST test_boot_disk() {
         disk[len-3] = '\0';
         ASSERT(!c_new_diskette_6(0, disk, 0));
     }
-    free(disk);
+    FREE(disk);
+
+    PASS();
+}
+
+TEST test_boot_disk_bytes() {
+    setup_boot_disk();
+
+    char *homedir = getenv("HOME");
+    char *disk = NULL;
+    asprintf(&disk, "%s/a2_read_disk_test.raw", homedir);
+    if (disk) {
+        unlink(disk);
+        c_begin_test_6(disk, NULL);
+    }
+
+    BOOT_TO_DOS();
+
+    c_end_test_6();
+    c_eject_6(0);
+
+    do {
+        uint8_t md[SHA_DIGEST_LENGTH];
+        char mdstr[(SHA_DIGEST_LENGTH*2)+1];
+
+        FILE *fp = fopen(disk, "r");
+        fseek(fp, 0L, SEEK_END);
+        long nbytes = ftell(fp);
+        fseek(fp, 0L, SEEK_SET);
+        char *buf = malloc(nbytes);
+        if (fread(buf, 1, nbytes, fp) != nbytes) {
+            ASSERT(false);
+        }
+        fclose(fp); fp = NULL;
+        SHA1(buf, nbytes, md);
+        FREE(buf);
+
+        sha1_to_str(md, mdstr);
+        ASSERT(strcmp(mdstr, "D21CC686571ADE868A909B5A7044A973DE70DFFB") == 0);
+    } while(0);
+
+    unlink(disk);
+    FREE(disk);
+
+    PASS();
+}
+
+TEST test_boot_disk() {
+    setup_boot_disk();
 
     BOOT_TO_DOS();
 
@@ -3244,6 +3292,8 @@ GREATEST_SUITE(test_suite_vm) {
 
     // TESTS --------------------------
 
+    RUN_TESTp(test_boot_disk_bytes);
+
     RUN_TESTp(test_boot_disk);
 
     RUN_TESTp(test_read_keyboard);