diff --git a/src/hence/emulator.cr b/src/hence/emulator.cr
index 003797c..5fa5a4e 100644
--- a/src/hence/emulator.cr
+++ b/src/hence/emulator.cr
@@ -184,13 +184,13 @@ module Hence
         when 0x08 # shr
           @reg_tmp = @reg_a >> @reg_b
         when 0x09 # add
-          @reg_tmp = @reg_a &+ @reg_b
+          @reg_tmp = @reg_a &+ @reg_b &+ (@reg_s & 0b00000001_u8)
           @reg_s &= 0b11111110_u8
           if @reg_tmp < @reg_a
             @reg_s |= 0b00000001_u8
           end
         when 0x0a # sub
-          @reg_tmp = @reg_a &- @reg_b
+          @reg_tmp = @reg_a &- @reg_b &- (@reg_s & 0b00000001_u8)
           @reg_s &= 0b11111110_u8
           if @reg_tmp > @reg_a
             @reg_s |= 0b00000001_u8