diff --git a/pyocd/probe/stlink_probe.py b/pyocd/probe/stlink_probe.py
index 4eb36e2cd..c603add1c 100644
--- a/pyocd/probe/stlink_probe.py
+++ b/pyocd/probe/stlink_probe.py
@@ -329,8 +329,8 @@ def read_memory_block8(self, addr: int, size: int, **attrs: Any) -> Sequence[int
         res = []
 
         # read leading unaligned bytes
-        unaligned_count = addr & 3
-        if (size > 0) and (unaligned_count > 0):
+        unaligned_count = 3 & (4 - (addr & 3))
+        if (size > unaligned_count > 0):
             res += self._link.read_mem8(addr, unaligned_count, self._apsel, csw)
             size -= unaligned_count
             addr += unaligned_count
@@ -355,8 +355,8 @@ def write_memory_block8(self, addr: int, data: Sequence[int], **attrs: Any) -> N
         idx = 0
 
         # write leading unaligned bytes
-        unaligned_count = addr & 3
-        if (size > 0) and (unaligned_count > 0):
+        unaligned_count = 3 & (4 - (addr & 3))
+        if (size > unaligned_count > 0):
             self._link.write_mem8(addr, data[:unaligned_count], self._apsel, csw)
             size -= unaligned_count
             addr += unaligned_count