diff --git a/zsnes/src/cpu/dspproc.asm b/zsnes/src/cpu/dspproc.asm
index d716f249..a2f3d742 100644
--- a/zsnes/src/cpu/dspproc.asm
+++ b/zsnes/src/cpu/dspproc.asm
@@ -1749,7 +1749,6 @@ section .text
     add edx,eax
 
     mov eax,[prev0]
-    mov [prev0],edx
     mov [prev1],eax
     cmp edx,-32768
     jnl %%notless
@@ -1761,6 +1760,8 @@ section .text
     mov edx,32767
     mov byte[filteron],1
 %%notgreater
+    movsx edx,dx
+    mov [prev0],edx
 %endmacro
 
 EXTSYM fir_downsample