Initial commit of files related to NED architecture.

[ned1] / software / assembly_fragments / multiply.asm

# Operands
WORD_174
WORD_186

# Multiplication routine
# Restrictions:
#   Positive numbers only (31 shifts max since operands are twos-complement)
#   No overflow checking

multiply
   WORD_2147483648     # For use in setting sign for left shifts.
   IM_0                # For magnitude of left/right shifts.
   LDSP+0              # Sets up stack location to build/store result.
                       #-----
testbit                 #
   LDSP+3              #
   LDSP+2              #
   SHIFT               # Check Nth bit of second operand.
   IM_1                #
   AND                 #
   TEST                #
                       #-----
   BRZ>skipadd         #
   LDSP+4              #
   LDSP+3              #
   LDSP+3              #
   OR                  # Shift and add first operand to result if indicated.
   SHIFT               #
   ADD                 #
skipadd                 #
                       #-----
   LDSP+1              #
   IM_1                # Increment counter for shift magnitude
   ADD                 #
   STSP+1              #
                       #-----
   IM_30               #
   NOT                 #
   IM_1                #
   ADD                 # Test for completion of multiplication subroutine (31 shifts).
   LDSP+2              #
   ADD                 #
   TEST                #
   BRZ>cleanup         # If finished, cleanup and return from subroutine with result on TOS.
                       #-----
   IM_0                #
   TEST                # If not finished, repeat this process on the next bit.
   BRZ>testbit         #
                       #-----
cleanup                 #
   STSP+0              #
   STSP+0              #
   STSP+0              # Remove all subroutine temporaries from stack and return result on TOS.
   STSP+0              #
   HALT                #