## Uses the built-in unsigned multiplier to do signed 32-bit multiplication:
##
##   (x+(2**32)x[31])(y+(2**32)y[31]) =
##
##       xy + (2**32)(x[31]y + y[31]x) + (2**64)x[31]y[31]
##
## Anything above bit 64 is ignored, so we just need to subtract
##
##   x[31]y + y[31]x
##
## from the MSW (bits 32 to 63) of the unsigned result. The LSW is fine as-is.
## The interface is the same as the normal 'mult' instruction.

# ( x y -- lsw msw )
signed_mult:
   jump .ypos rel ge
   over
   jump .xtest rel
.ypos:
   immed 0
.xtest:
   push
   swap
   jump .xpos rel ge
   over
   pop
   add
   push
.xpos:
   mult
   pop
   sub return