diff --git a/std/mem.zig b/std/mem.zig
index 7ecd483020..ef001d5dab 100644
--- a/std/mem.zig
+++ b/std/mem.zig
@@ -1481,6 +1481,7 @@ test "subArrayPtr" {
 }
 
 /// Round an address up to the nearest aligned address
+/// The alignment must be a power of 2 and greater than 0.
 pub fn alignForward(addr: usize, alignment: usize) usize {
     return alignBackward(addr + (alignment - 1), alignment);
 }
@@ -1500,13 +1501,18 @@ test "alignForward" {
     testing.expect(alignForward(17, 8) == 24);
 }
 
+/// Round an address up to the previous aligned address
+/// The alignment must be a power of 2 and greater than 0.
 pub fn alignBackward(addr: usize, alignment: usize) usize {
+    assert(@popCount(usize, alignment) == 1);
     // 000010000 // example addr
     // 000001111 // subtract 1
     // 111110000 // binary not
     return addr & ~(alignment - 1);
 }
 
+/// Given an address and an alignment, return true if the address is a multiple of the alignment
+/// The alignment must be a power of 2 and greater than 0.
 pub fn isAligned(addr: usize, alignment: usize) bool {
     return alignBackward(addr, alignment) == addr;
 }