diff --git c/lib/packing.c i/lib/packing.c
index 80c95dacbfaa..c7aab00b106c 100644
--- c/lib/packing.c
+++ i/lib/packing.c
@@ -14,14 +14,16 @@ static void adjust_for_msb_right_quirk(u64 *to_write, size_t *box_start_bit,
 {
 	size_t box_bit_width = *box_start_bit - *box_end_bit + 1;
 	size_t new_box_start_bit, new_box_end_bit;
+	u8 new_box_mask;
 
-	*to_write >>= *box_end_bit;
-	*to_write = bitrev8(*to_write) >> (8 - box_bit_width);
-	*to_write <<= *box_end_bit;
 
-	new_box_end_bit   = box_bit_width - *box_start_bit - 1;
-	new_box_start_bit = box_bit_width - *box_end_bit - 1;
-	*box_mask = GENMASK_ULL(new_box_start_bit, new_box_end_bit);
+	*to_write = bitrev8(*to_write);
+
+	new_box_mask = bitrev8(*box_mask);
+	new_box_end_bit = ffs(new_box_mask) - 1;
+	new_box_start_bit = fls(new_box_mask) - 1;
+
+	*box_mask = new_box_mask;
 	*box_start_bit = new_box_start_bit;
 	*box_end_bit   = new_box_end_bit;
 }
@@ -170,13 +172,13 @@ int pack(void *pbuf, u64 uval, size_t startbit, size_t endbit, size_t pbuflen,
 		/* Write to pbuf, read from uval */
 		pval = uval & proj_mask;
 		pval >>= proj_end_bit;
+		pval <<= box_end_bit;
 		if (quirks & QUIRK_MSB_ON_THE_RIGHT)
 			adjust_for_msb_right_quirk(&pval,
 						   &box_start_bit,
 						   &box_end_bit,
 						   &box_mask);
 
-		pval <<= box_end_bit;
 		((u8 *)pbuf)[box_addr] &= ~box_mask;
 		((u8 *)pbuf)[box_addr] |= pval;
 	}