Non-looping Constant Execution-Time 16-bit Multiply Code for the Z80
====================================================================
The usual method of multiplying a number is to repeatedly add one of the
numbers looping the the count of the other number. However, most
multiplication can be done with a combination of rotations and additions
or subtractions.


; Multiplying by powers of 2
;
ADD HL,HL    ; *2
ADD HL,HL    ; *4
ADD HL,HL    ; *8
ADD HL,HL    ; *16
ADD HL,HL    ; *32
ADD HL,HL    ; *64
ADD HL,HL    ; *128
;
;
; Multiplying by powers of 3
LD  DE,HL
ADD HL,HL
ADD HL,DE    ; *3
ADD HL,HL    ; *6
ADD HL,HL    ; *12
ADD HL,HL    ; *24
ADD HL,HL    ; *48
ADD HL,HL    ; *96
ADD HL,HL    ; *192
;
;
; Multiplying by powers of 5
;
LD  DE,HL
ADD HL,HL
ADD HL,HL
ADD HL,DE    ; *5
ADD HL,HL    ; *10
ADD HL,HL    ; *20
ADD HL,HL    ; *40
ADD HL,HL    ; *80
ADD HL,HL    ; *160
;
;
; Multiplying by powers of 7
;
LD  DE,HL
ADD HL,HL
ADD HL,DE
ADD HL,HL
ADD HL,DE    ; *7
ADD HL,HL    ; *14
ADD HL,HL    ; *28
ADD HL,HL    ; *56
ADD HL,HL    ; *104
ADD HL,HL    ; *208
;
;
; Multiplying by powers of 9
;
LD  DE,HL
ADD HL,HL
ADD HL,HL
ADD HL,HL
ADD HL,DE    ; *9
ADD HL,HL    ; *18
ADD HL,HL    ; *36
ADD HL,HL    ; *72
ADD HL,HL    ; *144
;
;

; This gives this coverage below x50:
;    | 0   1   2   3   4   5   6   7   8   9
; ---|---------------------------------------
;  0 |         2   3   2   5   3   7   2   9
; 10 | 5       3       7       2       9    
; 20 | 5               3               7    
; 30 |         2               9            
; 40 | 5                               3    
;
; and combining gives this coverage below x50:
;    | 0   1   2   3   4   5   6   7   8    9
; ---|----------------------------------------
;  0 |         2   3  2x2  5  3x2  7  2x4   9
; 10 |5x2     3x4     7x2 3x5 2x8     9x2
; 20 |5x4 3x7         3x8 5x5         7x4    
; 30 |5x6     2x16        5x7 9x4            
; 40 |5x8     6x7         5x9         3x16 7x7