diff --git a/libstring/src.mk b/libstring/src.mk
index a4a67c9..98fb67a 100644
--- a/libstring/src.mk
+++ b/libstring/src.mk
@@ -1,3 +1,5 @@
-c += string.c
+c += string.c \
+		 strconv.c
 
-o += string.o
+o += string.o \
+		 strconv.o
diff --git a/libstring/strconv.c b/libstring/strconv.c
new file mode 100644
index 0000000..26f85f7
--- /dev/null
+++ b/libstring/strconv.c
@@ -0,0 +1,15 @@
+#include <stdint.h>
+
+uint64_t str_to_uint64 (const char* s) {
+  uint64_t r = 0;
+
+  if (!s)
+    return 0;
+
+  while (*s >= '0' && *s <= '9') {
+    r = r * 10 + (*s - '0');
+    s++;
+  }
+
+  return r;
+}
diff --git a/libstring/strconv.h b/libstring/strconv.h
new file mode 100644
index 0000000..fb9c911
--- /dev/null
+++ b/libstring/strconv.h
@@ -0,0 +1,12 @@
+#ifndef _LIBSTRING_STRCONV_H
+#define _LIBSTRING_STRCONV_H
+
+#include <stdint.h>
+
+#define str_to_uint32(s) ((uint32_t)str_to_uint64 ((s)))
+#define str_to_uint16(s) ((uint16_t)str_to_uint64 ((s)))
+#define str_to_uint8(s)  ((uint8_t)str_to_uint64 ((s)))
+
+uint64_t str_to_uint64 (const char* s);
+
+#endif // _LIBSTRING_STRCONV_H
diff --git a/sdutil/sdutil.c b/sdutil/sdutil.c
index 49e61ec..434926d 100644
--- a/sdutil/sdutil.c
+++ b/sdutil/sdutil.c
@@ -1,16 +1,99 @@
+#include <devices.h>
 #include <in_input.h>
 #include <mprintf.h>
 #include <process_self.h>
 #include <status.h>
+#include <str_status.h>
+#include <strconv.h>
 #include <string.h>
 #include <system.h>
 
 #define COMMAND_MAX 32
 
+struct dos_pte {
+  uint8_t drive_attrs;
+  uint8_t chs_start_addr[3];
+  uint8_t part_type;
+  uint8_t chs_last_sect_addr[3];
+  uint32_t start_lba;
+  uint32_t sector_count;
+} __attribute__ ((packed));
+
+struct dos_mbr {
+  uint8_t boot_code[440];
+  uint8_t signature[4];
+  uint8_t resv[2];
+  struct dos_pte ptes[4];
+  uint8_t valid_sign[2];
+} __attribute__ ((packed));
+
 static void start_part_dos (void) {
   char dev_name[64];
+  struct dos_mbr mbr;
+  memset (&mbr, 0, sizeof (mbr));
+  bool ok = false, write = false;
+  char confirm_buf[4];
 
   in_stream_read_line ("Device name: ", dev_name, sizeof (dev_name));
+
+#define prompt_partition(N)                                                                        \
+  char p##N##_start_lba_str[12], p##N##_sector_cout_str[12];                                       \
+  uint32_t p##N##_start_lba, p##N##_sector_count;                                                  \
+  mprintf ("\nConfigure parition %d:\n", N);                                                       \
+  in_stream_read_line ("Start LBA: ", p##N##_start_lba_str, sizeof (p##N##_start_lba_str));        \
+  in_stream_read_line ("Sector count: ", p##N##_sector_cout_str, sizeof (p##N##_sector_cout_str)); \
+  p##N##_start_lba = str_to_uint32 (p##N##_start_lba_str);                                         \
+  p##N##_sector_count = str_to_uint32 (p##N##_sector_cout_str);
+
+  prompt_partition (0);
+  prompt_partition (1);
+  prompt_partition (2);
+  prompt_partition (3);
+
+  mbr.ptes[0].start_lba = p0_start_lba;
+  mbr.ptes[0].sector_count = p0_sector_count;
+  mbr.ptes[1].start_lba = p1_start_lba;
+  mbr.ptes[1].sector_count = p1_sector_count;
+  mbr.ptes[2].start_lba = p2_start_lba;
+  mbr.ptes[2].sector_count = p2_sector_count;
+  mbr.ptes[3].start_lba = p3_start_lba;
+  mbr.ptes[3].sector_count = p3_sector_count;
+
+  mbr.valid_sign[0] = 0x55;
+  mbr.valid_sign[1] = 0xAA;
+
+  /* clang-format off */
+  mprintf ("\n\n-------------------------------------------------------\n");
+  mprintf ("Current configuration (%s):\n", dev_name);
+  mprintf ("Partition 0: start LBA = %u, sector count = %zu\n",mbr.ptes[0].start_lba, mbr.ptes[0].sector_count);
+  mprintf ("Partition 1: start LBA = %u, sector count = %zu\n",mbr.ptes[1].start_lba, mbr.ptes[1].sector_count);
+  mprintf ("Partition 2: start LBA = %u, sector count = %zu\n",mbr.ptes[2].start_lba, mbr.ptes[2].sector_count);
+  mprintf ("Partition 3: start LBA = %u, sector count = %zu\n",mbr.ptes[3].start_lba, mbr.ptes[3].sector_count);
+  /* clang-format on */
+
+  in_stream_read_line ("\nDoes this look plausible? [yes/no]: ", confirm_buf, sizeof (confirm_buf));
+
+  if (strcmp (confirm_buf, "yes") == 0) {
+    in_stream_read_line ("\nWrite to device? There's no going back! [yes/no]: ", confirm_buf,
+                         sizeof (confirm_buf));
+
+    ok = true;
+
+    if (strcmp (confirm_buf, "yes") == 0) {
+      write = true;
+    } else {
+      mprintf ("OK. Canceling\n");
+    }
+  } else {
+    mprintf ("OK. Canceling\n");
+  }
+
+  if (ok && write) {
+    size_t sector = 0;
+    size_t sector_count = 1;
+    int r = device_do (dev_name, XDRV_WRITE, &sector, &sector_count, &mbr, NULL);
+    mprintf ("Finished writing. Result: %s (%d)\n", str_status[r < 0 ? -r : r], r);
+  }
 }
 
 void app_main (void) {