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Mathematics In Assembly #3
by Krill/Plush


Hello dear sceners, this is the third chapter of my tutorial on maths in assembly, first published in "GO64!" magazine. The first two chapters of this tutorial were re-published in "Attitude #4" and "Domination #17". Have a nice read!


After increasing the precision of our numbers we're going to have a look at a slightly more difficult topic, namely the multiplication. But take it easy, this a bit more complex mathematic operation can be done in many different ways, depending on the memory and execution time requirements. The simplest implementation of the multiplication would of course be a simple loop which just increases a zero-initialised counter by the first multiplication factor as often as the absolute value of factor 2 demands. This extremely inelegant brute force method is naturally very inefficient and slow, that's why it's just mentioned in passing here and not going to be discussed in any more detail. Let's take a look at some more elegant approaches:


A kind of multiplication relatively often needed is to multiply by constants, as it just takes quite a small and fast algorithm. As an example constant we just take 11 ($0B, %00001011). Using our knowledge about binary arithmetic, we can easily reduce this number to powers of two:

11 = 8 + 2 + 1 = 2^3 + 2^1 + 2^0

If we wanted to multiply this number with let's say 25 ($19, %00011001), it would look like this if written on paper:

11 * 25 = 2^3 * 25 + 2^1 * 25 + 2^0 * 25 = 8 * 25 + 2 * 25 + 1 * 25

What's all this needed for, you might ask. Very simple: the separate summands of the created expression are all shifted 25's. So it's as easy as this: the accu contains an arbitrary number to be multiplied by the given constant. The number is shifted by the needed positions and buffered. Afterwards, all summands are added. Just a small piece of example code:

LDA #$19 ; arbitrary factor
STA BUFFER0 ; some byte in the memory, 2^0 * factor
ASL ; doubling of the factor
STA BUFFER1 ; buffer 2^1 * factor
ASL ; doubling
ASL ; doubling, all in all 8 * factor
CLC ; clear carry for addition (without overflow, like here, actually unnecessary)
ADC BUFFER0 ; 2^3 * factor + 2^0 * factor
ADC BUFFER1 ; 2^3 * factor + 2^0 * factor + 2^1 * factor

Simple. Oh, all examples will, just like this one, for the sake of simplicity, refer to unsigned 8-bit integers to be multiplied. By the way, there are some "factors" to be aware of with the multiplication.


#1: After multiplying two factors, the result has the double size, for instance 16 bits with two 8-bit factors. Quite obvious, as for example 255^2 = $FF * $FF = $FE01. Nevertheless, for the sake of simplicity, I'll take appropriately small values in order not to cause any overflows.

#2: Also with computers, the exact result of multiplying two fractions has as many decimals as both the factors have together. So the product of two fixed point numbers with let's say one fraction byte each has two fraction bytes. Together with the squaring of the number range this means, for instance: the product of two 16-bit fixed point numbers with one fraction byte each is 32 bits in size, 16 bits left hand of the point, 16 bits right hand of it.

#3: In order to multiply signed numbers, the signs of both factors are buffered and the absolute value of the result is calculated by multiplying the absolute values of the factors. Afterwards, the buffered signs are evaluated and the result is signed accordingly. Of course, the common rules also apply here, so plus by minus is minus and minus by minus is plus.

Good. The multiplication in general and in particular with constants can be also done in another way, the magic word here is tables.

Tables? Correct. More exactly 'Look Up Tables'. Using these tables, it's possible to calculate products extremely fast, or let's better say "to find them out" extremely fast. For that, a table has to be generated at the beginning of the program, in this case containing the multiplies of the constant in rising order. With the constant 3 ($03, %00000011), it would look like this:

$00, $03, $06, $09, $0C,...

This table is just read out, using the arbitrary factor as index. On principle, this table is as large as the range of the factor is, i.e. 256 bytes with a 1-byte-sized factor. By the way, generating this table, the above mentioned brute force method can be used, as with the initialisation speed plays a minor role compared to memory efficiency.

That's it for the first couple of possibilities to multiply. The next three chapters will be published in "Vandalism News's Ruby Edition", discussing some more ways to multiply in assembly, starting with the most flexible kind of multiplication, the bit-wise multiplication. See you!


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