diff --git a/boot/bootutil/src/bootutil_priv.h b/boot/bootutil/src/bootutil_priv.h
index 163d5e9..bc9c61f 100644
--- a/boot/bootutil/src/bootutil_priv.h
+++ b/boot/bootutil/src/bootutil_priv.h
@@ -164,7 +164,11 @@
         size_t num_sectors;
     } imgs[BOOT_NUM_SLOTS];
 
-    const struct flash_area *scratch_area;
+    struct {
+        const struct flash_area *area;
+        boot_sector_t *sectors;
+        size_t num_sectors;
+    } scratch;
 
     uint8_t write_sz;
 };
@@ -198,7 +202,7 @@
 
 /* These are macros so they can be used as lvalues. */
 #define BOOT_IMG_AREA(state, slot) ((state)->imgs[(slot)].area)
-#define BOOT_SCRATCH_AREA(state) ((state)->scratch_area)
+#define BOOT_SCRATCH_AREA(state) ((state)->scratch.area)
 #define BOOT_WRITE_SZ(state) ((state)->write_sz)
 
 static inline struct image_header*
@@ -213,6 +217,12 @@
     return state->imgs[slot].num_sectors;
 }
 
+static inline size_t
+boot_scratch_num_sectors(struct boot_loader_state *state)
+{
+    return state->scratch.num_sectors;
+}
+
 /*
  * Offset of the slot from the beginning of the flash device.
  */
@@ -224,7 +234,7 @@
 
 static inline size_t boot_scratch_area_size(struct boot_loader_state *state)
 {
-    return state->scratch_area->fa_size;
+    return BOOT_SCRATCH_AREA(state)->fa_size;
 }
 
 #ifndef MCUBOOT_USE_FLASH_AREA_GET_SECTORS
@@ -252,27 +262,26 @@
 boot_initialize_area(struct boot_loader_state *state, int flash_area)
 {
     int num_sectors = BOOT_MAX_IMG_SECTORS;
-    size_t slot;
     int rc;
 
     switch (flash_area) {
     case FLASH_AREA_IMAGE_0:
-        slot = 0;
+        rc = flash_area_to_sectors(flash_area, &num_sectors, state->imgs[0].sectors);
+        state->imgs[0].num_sectors = (size_t)num_sectors;
         break;
     case FLASH_AREA_IMAGE_1:
-        slot = 1;
+        rc = flash_area_to_sectors(flash_area, &num_sectors, state->imgs[1].sectors);
+        state->imgs[1].num_sectors = (size_t)num_sectors;
+        break;
+    case FLASH_AREA_IMAGE_SCRATCH:
+        rc = flash_area_to_sectors(flash_area, &num_sectors, state->scratch.sectors);
+        state->scratch.num_sectors = (size_t)num_sectors;
         break;
     default:
         return BOOT_EFLASH;
     }
 
-    rc = flash_area_to_sectors(flash_area, &num_sectors,
-                               state->imgs[slot].sectors);
-    if (rc != 0) {
-        return rc;
-    }
-    state->imgs[slot].num_sectors = (size_t)num_sectors;
-    return 0;
+    return rc;
 }
 
 #else  /* defined(MCUBOOT_USE_FLASH_AREA_GET_SECTORS) */
@@ -311,6 +320,11 @@
         out_sectors = state->imgs[1].sectors;
         out_num_sectors = &state->imgs[1].num_sectors;
         break;
+    case FLASH_AREA_IMAGE_SCRATCH:
+        num_sectors = BOOT_MAX_IMG_SECTORS;
+        out_sectors = state->scratch.sectors;
+        out_num_sectors = &state->scratch.num_sectors;
+        break;
     default:
         return -1;
     }
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index 3b21b79..ffcea73 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -316,7 +316,7 @@
      * We need to use the bigger of those 2 values.
      */
     elem_sz = flash_area_align(boot_data.imgs[0].area);
-    align = flash_area_align(boot_data.scratch_area);
+    align = flash_area_align(boot_data.scratch.area);
     if (align > elem_sz) {
         elem_sz = align;
     }
@@ -324,32 +324,84 @@
     return elem_sz;
 }
 
+/*
+ * Slots are compatible when all sectors that store upto to size of the image
+ * round up to sector size, in both slot's are able to fit in the scratch
+ * area, and have sizes that are a multiple of each other (powers of two
+ * presumably!).
+ */
 static int
 boot_slots_compatible(void)
 {
-    size_t num_sectors_0 = boot_img_num_sectors(&boot_data, 0);
-    size_t num_sectors_1 = boot_img_num_sectors(&boot_data, 1);
-    size_t size_0, size_1;
-    size_t i;
+    size_t num_sectors_0;
+    size_t num_sectors_1;
+    size_t sz0, sz1;
+    size_t slot0_sz, slot1_sz;
+    size_t scratch_sz;
+    size_t i, j;
+    int8_t smaller;
 
+    num_sectors_0 = boot_img_num_sectors(&boot_data, 0);
+    num_sectors_1 = boot_img_num_sectors(&boot_data, 1);
     if (num_sectors_0 > BOOT_MAX_IMG_SECTORS || num_sectors_1 > BOOT_MAX_IMG_SECTORS) {
         BOOT_LOG_WRN("Cannot upgrade: more sectors than allowed");
         return 0;
     }
 
-    /* Ensure both image slots have identical sector layouts. */
-    if (num_sectors_0 != num_sectors_1) {
-        BOOT_LOG_WRN("Cannot upgrade: number of sectors differ between slots");
-        return 0;
+    scratch_sz = boot_scratch_area_size(&boot_data);
+
+    /*
+     * The following loop scans all sectors in a linear fashion, assuring that
+     * for each possible sector in each slot, it is able to fit in the other
+     * slot's sector or sectors. Slot's should be compatible as long as any
+     * number of a slot's sectors are able to fit into another, which only
+     * excludes cases where sector sizes are not a multiple of each other.
+     */
+    i = sz0 = slot0_sz = 0;
+    j = sz1 = slot1_sz = 0;
+    smaller = 0;
+    while (i < num_sectors_0 || j < num_sectors_1) {
+        if (sz0 == sz1) {
+            sz0 += boot_img_sector_size(&boot_data, 0, i);
+            sz1 += boot_img_sector_size(&boot_data, 1, j);
+            i++;
+            j++;
+        } else if (sz0 < sz1) {
+            sz0 += boot_img_sector_size(&boot_data, 0, i);
+            /* guarantee that multiple sectors of slot1 fit into slot0 */
+            if (smaller == 2) {
+                BOOT_LOG_WRN("Cannot upgrade: slots have non-compatible sectors");
+                return 0;
+            }
+            smaller = 1;
+            i++;
+        } else {
+            sz1 += boot_img_sector_size(&boot_data, 1, j);
+            /* guarantee that multiple sectors of slot0 fit into slot1 */
+            if (smaller == 1) {
+                BOOT_LOG_WRN("Cannot upgrade: slots have non-compatible sectors");
+                return 0;
+            }
+            smaller = 2;
+            j++;
+        }
+        if (sz0 == sz1) {
+            slot0_sz += sz0;
+            slot1_sz += sz1;
+            /* scratch has to fit each swap operation to the size of the larger
+             * sector among slot0 and slot1
+             */
+            if (sz0 > scratch_sz || sz1 > scratch_sz) {
+                BOOT_LOG_WRN("Cannot upgrade: not all sectors fit inside scratch");
+                return 0;
+            }
+            smaller = sz0 = sz1 = 0;
+        }
     }
 
-    for (i = 0; i < num_sectors_0; i++) {
-        size_0 = boot_img_sector_size(&boot_data, 0, i);
-        size_1 = boot_img_sector_size(&boot_data, 1, i);
-        if (size_0 != size_1) {
-            BOOT_LOG_WRN("Cannot upgrade: an incompatible sector was found");
-            return 0;
-        }
+    if (i != num_sectors_0 || j != num_sectors_1 || slot0_sz != slot1_sz) {
+        BOOT_LOG_WRN("Cannot upgrade: slots are not compatible");
+        return 0;
     }
 
     return 1;
@@ -376,6 +428,11 @@
         return BOOT_EFLASH;
     }
 
+    rc = boot_initialize_area(&boot_data, FLASH_AREA_IMAGE_SCRATCH);
+    if (rc != 0) {
+        return BOOT_EFLASH;
+    }
+
     BOOT_WRITE_SZ(&boot_data) = boot_write_sz();
 
     return 0;
@@ -735,6 +792,11 @@
     scratch_sz = boot_scratch_area_size(&boot_data);
     for (i = last_sector_idx; i >= 0; i--) {
         new_sz = sz + boot_img_sector_size(&boot_data, 0, i);
+        /*
+         * slot1 is not being checked here, because `boot_slots_compatible`
+         * already provides assurance that the copy size will be compatible
+         * with slot0 and scratch.
+         */
         if (new_sz > scratch_sz) {
             break;
         }
@@ -951,8 +1013,8 @@
     copy_sz = sz;
     trailer_sz = boot_slots_trailer_sz(BOOT_WRITE_SZ(&boot_data));
 
-    /* sz in this function is always is always sized on a multiple of the
-     * sector size. The check against the start offset of the last sector
+    /* sz in this function is always sized on a multiple of the sector size.
+     * The check against the start offset of the last sector
      * is to determine if we're swapping the last sector. The last sector
      * needs special handling because it's where the trailer lives. If we're
      * copying it, we need to use scratch to write the trailer temporarily.
@@ -1193,6 +1255,7 @@
     uint32_t sz;
     int first_sector_idx;
     int last_sector_idx;
+    int last_idx_slot1;
     uint32_t swap_idx;
     struct image_header *hdr;
 #ifdef MCUBOOT_ENC_IMAGES
@@ -1202,6 +1265,8 @@
 #endif
     uint32_t size;
     uint32_t copy_size;
+    uint32_t slot0_size;
+    uint32_t slot1_size;
     int rc;
 
     /* FIXME: just do this if asked by user? */
@@ -1294,14 +1359,31 @@
 #endif
     }
 
-    size = 0;
+    slot0_size = 0;
+    slot1_size = 0;
     last_sector_idx = 0;
+    last_idx_slot1 = 0;
+
+    /*
+     * Knowing the size of the largest image between both slots, here we
+     * find what is the last sector in slot0 that needs swapping. Since we
+     * already know that both slots are compatible, slot1's last sector is
+     * not really required after this check is finished.
+     */
     while (1) {
-        size += boot_img_sector_size(&boot_data, 0, last_sector_idx);
-        if (size >= copy_size) {
+        if (slot0_size < copy_size || slot0_size < slot1_size) {
+            slot0_size += boot_img_sector_size(&boot_data, 0, last_sector_idx);
+        }
+        if (slot1_size < copy_size || slot1_size < slot0_size) {
+            slot1_size += boot_img_sector_size(&boot_data, 1, last_idx_slot1);
+        }
+        if (slot0_size >= copy_size &&
+                slot1_size >= copy_size &&
+                slot0_size == slot1_size) {
             break;
         }
         last_sector_idx++;
+        last_idx_slot1++;
     }
 
     swap_idx = 0;
@@ -1463,8 +1545,10 @@
      */
     static boot_sector_t slot0_sectors[BOOT_MAX_IMG_SECTORS];
     static boot_sector_t slot1_sectors[BOOT_MAX_IMG_SECTORS];
+    static boot_sector_t scratch_sectors[BOOT_MAX_IMG_SECTORS];
     boot_data.imgs[0].sectors = slot0_sectors;
     boot_data.imgs[1].sectors = slot1_sectors;
+    boot_data.scratch.sectors = scratch_sectors;
 
 #ifdef MCUBOOT_ENC_IMAGES
     /* FIXME: remove this after RAM is cleared by sim */