diff --git a/include/util/arena_alloc.nasm b/include/util/arena_alloc.nasm
index a77df0b..1b0a1dd 100644
--- a/include/util/arena_alloc.nasm
+++ b/include/util/arena_alloc.nasm
@@ -38,10 +38,28 @@ arena_init:
     ret
 
 ; size_t align_up(size_t x, size_t a)
+; eax, ebx. ecx are all clobbered
 arena_align_up:
     __CDECL16_ENTRY
 .func:
-    ; align the mark to the nearest specified alignment
+    ; align x up to the nearest specified alignment (a)
+    ; (x + (a-1)) & ~(a-1)
+    mov eax, [bp + 4]   ; x
+
+    mov ebx, [bp + 6]   ; a - 1
+    sub ebx, 1
+
+    mov ecx, eax
+    add ecx, ebx        ; x + (a+1)
+
+    not ebx
+    and ecx, ebx        ; ecx contains the result
+
+    xor eax, eax
+    xor ebx, ebx
+    mov eax, ecx        ; return result in eax
+    xor ecx, ecx
+
 .endp:
     __CDECL16_EXIT
     ret