The BBC and Master Computer Public Domain Library

Submitted by Steve Fewell

Routine: imult
Name: Integer Multiplication
Starting Address: &9F64
Entry criteria: IWA contains the first Integer number. Y contains the Most Significant Byte of this Integer number (?&2D). ?&4 and ?&5 point to the second Integer variable. ?&27 = 4, X = &4.
Notes: If the IWA > &FFFF, it is truncated to 16 bits.
Exit: IWA contains the result of [Integer variable * IWA]

Description:
Exclusive-OR the Most significant bytes of both integers together, this determines the sign of the result (if only one of the MSBs is negative then the result is negative, otherwise it is positive).

Make both integers positive [using ipos] (so that we only have to do positive multiplication).

Store the multiplier in zero-page locations (&39 to &3C) and put the multiplicand into the IWA [popi].

Initialise the result to zero (The result is stored in Y [&3D], X [&3E], &3F and &40).

Keep repeating the following loop until the multiplier is zero [only the least significant two bytes of the multiplier are used]:

(1) Divide the multiplier by 2 (and make the result integer)

(2) If an integer result was not obtained, (i.e. Multiplier was an odd number before division) then add the multiplicand to the result. [Dividing the multiplier lost 0.5 of its value, so we add on the multiplicand (which at this stage is worth 0.5 * the actual multiplicand) before it is multiplied to perform the multiplication of the lost 0.5 of the multiplier].

(3) Multiply the multiplicand by 2

Until multiplier is zero ((&39 OR &3A) = zero).

Retrieve the result from the zero page locations (&3D to &40) and make it negative if the sign [at the beginning] was determined to be negative.

Jump to &9FCA to test the value of X [the next operator] when the routine was entered. If X contains the operator code for '*', '/', 'MOD' or 'DIV' then send the result to the appropriate routine, for further calculation, otherwise exit the multiplication routine.

Disassembly for the integer multiplication routine