6809: ashrsi3: 8 bit shift optimization

diff --git a/Kernel/lowlevel-6809.s b/Kernel/lowlevel-6809.s
index a72b0bf..f3a73b8 100644 (file)
--- a/Kernel/lowlevel-6809.s
+++ b/Kernel/lowlevel-6809.s
@@ -584,10 +584,22 @@ ___ashrsi3:
        ldu 6,s
        stu 2,x
        ldb 9,s
-       ;; FIXME: insert 16 and 8 optimization here
-       ;; remember to propigate top bit for signage
+       ;; FIXME: insert 16 optimization here
+       ;; remember to propagate top bit for signage
+try_8@ cmpb    #8
+       blo     try_rest@
+       subb    #8
+       ldu     1,x             ; shift what we can down by 1 byte
+       stu     2,x
+       lda     ,x
+       sta     1,x
+       clr     ,x              ; default top byte to positive
+       tst     1,x             ; test old msb for sign
+       bpl     try_8@          ; go try another 8 shifts
+       dec     ,x              ; dec to make top byte negative
+       bra     try_8@          ; go try another 8 shifts
 try_rest@
-       tstb                    ; redunant until optimizations are added
+       tstb            
        beq     done@
 do_rest@
        ; Shift by 1