Index: uspace/app/mkexfat/mkexfat.c
===================================================================
--- uspace/app/mkexfat/mkexfat.c	(revision b0fecad6d896010a48a55ba72f369a908bf1996e)
+++ uspace/app/mkexfat/mkexfat.c	(revision 6aeab59c23d9411fb2d0d41b6b21366e2dc8b04d)
@@ -81,4 +81,6 @@
 	unsigned long rootdir_cluster;
 	unsigned long total_clusters;
+	unsigned long allocated_clusters;
+	size_t   bitmap_size;
 	size_t   sector_size;
 	size_t   cluster_size;
@@ -97,4 +99,7 @@
 ebs_write(service_id_t service_id, exfat_cfg_t *cfg,
     int base, uint32_t *chksum);
+
+static int
+bitmap_write(service_id_t service_id, exfat_cfg_t *cfg);
 
 static void usage(void)
@@ -139,4 +144,15 @@
 	    cfg->fat_sector_count, cfg->cluster_size / cfg->sector_size);
 
+	/* Compute the bitmap size */
+	cfg->bitmap_size = n_req_clusters / 8;
+
+	cfg->allocated_clusters = div_round_up(cfg->bitmap_size,
+	    cfg->cluster_size);
+
+	/* This account for the root directory */
+	cfg->allocated_clusters++;
+	/* FIXME: add upcase table clusters to allocated_clusters */
+
+	/* FIXME: set the real rootdir cluster */
 	cfg->rootdir_cluster = 0;
 
@@ -312,5 +328,5 @@
 	int rc;
 
-	pfat = calloc(cfg->fat_sector_count, cfg->sector_size);
+	pfat = calloc(cfg->sector_size, 1);
 	if (!pfat)
 		return ENOMEM;
@@ -332,5 +348,5 @@
 	memset(pfat, 0, 5 * sizeof(uint32_t));
 
-	for (i = 1; i < cfg->fat_sector_count + 1; ++i) {
+	for (i = 1; i < cfg->fat_sector_count; ++i) {
 		rc = block_write_direct(service_id,
 		    FAT_SECTOR_START + i, 1, pfat);
@@ -341,4 +357,52 @@
 error:
 	free(pfat);
+	return rc;
+}
+
+/** Initialize the allocation bitmap.
+ *
+ * @param service_id   The service id.
+ * @param cfg  Pointer to the exfat configuration structure.
+ * @return  EOK on success or a negative error code.
+ */
+static int
+bitmap_write(service_id_t service_id, exfat_cfg_t *cfg)
+{
+	unsigned long i, sec;
+	unsigned long allocated_cls;
+	int rc = EOK;
+
+	/* Bitmap size in sectors */
+	size_t const bss = div_round_up(cfg->bitmap_size, cfg->sector_size);
+
+	uint8_t *bitmap = malloc(cfg->sector_size);
+	if (!bitmap)
+		return ENOMEM;
+
+	allocated_cls = cfg->allocated_clusters;
+
+	for (sec = 0; sec < bss; ++sec) {
+		memset(bitmap, 0, cfg->sector_size);
+		if (allocated_cls > 0) {
+			for (i = 0; i < allocated_cls; ++i) {
+				unsigned byte_idx = i / 8;
+				unsigned bit_idx = i % 8;
+
+				if (byte_idx == cfg->sector_size)
+					break;
+				bitmap[byte_idx] |= 1 << bit_idx;
+			}
+
+			allocated_cls -= i * 8;
+		}
+
+		rc = block_write_direct(service_id,
+		    cfg->data_start_sector + sec, 1, bitmap);
+		if (rc != EOK)
+			goto exit;
+	}
+
+exit:
+	free(bitmap);
 	return rc;
 }
@@ -459,4 +523,11 @@
 	}
 
+	rc = bitmap_write(service_id, &cfg);
+	if (rc != EOK) {
+		printf(NAME ": Error, failed to write the allocation" \
+		    " bitmap to disk.\n");
+		return 2;
+	}
+
 	return 0;
 }