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Programming:Integer Multiplication

10,764 bytes added, 17:31, 1 March 2018
/* 16bit * 16bit Unsigned -> 32 bits result */ forgotten label
'''Output:''' HL = ''Product''
'''CPC Cycles:''' 188-244 (216 on average) = 47-61 (54) usec
 
'''Size:''' 33 bytes
 
The additional clocks and bytes maybe required to set up D and L and for RET.
<pre>
sla h ; optimised 1st iterationjr nc,$+3ld l,e
add hl,hl ; unroll 7 timesjr nc,$+3 ; ...add hl,de ; ...
</pre>
 
== Classic 16bit * 8bit Unsigned ==
'''Output:''' A:HL = ''Product''
 
'''CPC Cycles:''' 212-300 (256 on average) = 53-75 (64) usec
 
'''Size:''' 47 bytes
<pre>
add a,a ; optimised 1st iterationjr nc,$+4ld h,dld l,e
add hl,hl ; unroll 7 timesrla ; ...jr nc,$+4 ; ...add hl,de ; ...adc a,c ; ...
</pre>
 
== Fast 8bit * 8bit Unsigned (using log / antilog tables) ==
<pre>
x^a * x^b = x^(a+b)
a * b = x^(logx(a) + logx(b))
</pre>
'''Output:''' DE = ''Product''
'''CPC Cycles:''' 124 = 31 usec
 
'''Size:''' 1024 bytes (768 bytes for the tables)
 
'''Warning:''' It is VERY approximate routine, e.g., 0*7=7, 1*28=27, 10*81=790, 100*100=9742, ...
<pre>
FastMult:
ld l,c ld h,&82 ld d,(hl) ; d = 32 * log_2(c) ld l,b ld a,(hl) ; a = 32 * log_2(b) add a,d ld l,a ld a,0 adc a,0 ld h,a ; hl = d + a add hl,hl set 2,h ; hl = hl + $0400 set 7,h ; hl = hl + &8000 ld e,(hl) inc hl ld d,(hl) ; de = 2^((hl)/32) ret
; 32*Log_2(x) Table
logtable:
db 0 , 0 , 32 , 50 , 64 , 74 , 82 , 89 , 96 , 101 , 106 , 110 , 114 , 118 , 121 db 125 , 128 , 130 , 133 , 135 , 138 , 140 , 142 , 144 , 146 , 148 , 150 , 152 db 153 , 155 , 157 , 158 , 160 , 161 , 162 , 164 , 165 , 166 , 167 , 169 , 170 db 171 , 172 , 173 , 174 , 175 , 176 , 177 , 178 , 179 , 180 , 181 , 182 , 183 db 184 , 185 , 185 , 186 , 187 , 188 , 189 , 189 , 190 , 191 , 192 , 192 , 193 db 194 , 194 , 195 , 196 , 196 , 197 , 198 , 198 , 199 , 199 , 200 , 201 , 201 db 202 , 202 , 203 , 204 , 204 , 205 , 205 , 206 , 206 , 207 , 207 , 208 , 208 db 209 , 209 , 210 , 210 , 211 , 211 , 212 , 212 , 213 , 213 , 213 , 214 , 214 db 215 , 215 , 216 , 216 , 217 , 217 , 217 , 218 , 218 , 219 , 219 , 219 , 220 db 220 , 221 , 221 , 221 , 222 , 222 , 222 , 223 , 223 , 224 , 224 , 224 , 225 db 225 , 225 , 226 , 226 , 226 , 227 , 227 , 227 , 228 , 228 , 228 , 229 , 229 db 229 , 230 , 230 , 230 , 231 , 231 , 231 , 231 , 232 , 232 , 232 , 233 , 233 db 233 , 234 , 234 , 234 , 234 , 235 , 235 , 235 , 236 , 236 , 236 , 236 , 237 db 237 , 237 , 237 , 238 , 238 , 238 , 238 , 239 , 239 , 239 , 239 , 240 , 240 db 240 , 241 , 241 , 241 , 241 , 241 , 242 , 242 , 242 , 242 , 243 , 243 , 243 db 243 , 244 , 244 , 244 , 244 , 245 , 245 , 245 , 245 , 245 , 246 , 246 , 246 db 246 , 247 , 247 , 247 , 247 , 247 , 248 , 248 , 248 , 248 , 249 , 249 , 249 db 249 , 249 , 250 , 250 , 250 , 250 , 250 , 251 , 251 , 251 , 251 , 251 , 252 db 252 , 252 , 252 , 252 , 253 , 253 , 253 , 253 , 253 , 253 , 254 , 254 , 254 db 254 , 254 , 255 , 255 , 255 , 255 , 255
antilog:
dw 1 , 1 , 1 , 1 , 1 , 1 , 1 , 1 , 1 , 1 , 1 , 1 , 1 , 1 , 1 , 1 , 1 , 1 , 1 , 2 dw 2 , 2 , 2 , 2 , 2 , 2 , 2 , 2 , 2 , 2 , 2 , 2 , 2 , 2 , 2 , 2 , 2 , 2 , 2 , 2 dw 2 , 2 , 2 , 3 , 3 , 3 , 3 , 3 , 3 , 3 , 3 , 3 , 3 , 3 , 3 , 3 , 3 , 3 , 4 , 4 dw 4 , 4 , 4 , 4 , 4 , 4 , 4 , 4 , 4 , 4 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 6 dw 6 , 6 , 6 , 6 , 6 , 6 , 6 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 8 , 8 , 8 , 8 , 8 , 9 dw 9 , 9 , 9 , 9 , 10 , 10 , 10 , 10 , 10 , 11 , 11 , 11 , 11 , 12 , 12 , 12 , 12 dw 13 , 13 , 13 , 13 , 14 , 14 , 14 , 15 , 15 , 15 , 16 , 16 , 16 , 17 , 17 , 17 dw 18 , 18 , 19 , 19 , 19 , 20 , 20 , 21 , 21 , 22 , 22 , 23 , 23 , 24 , 24 , 25 dw 25 , 26 , 26 , 27 , 27 , 28 , 29 , 29 , 30 , 31 , 31 , 32 , 33 , 33 , 34 , 35 dw 36 , 36 , 37 , 38 , 39 , 40 , 41 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 dw 50 , 52 , 53 , 54 , 55 , 56 , 57 , 59 , 60 , 61 , 63 , 64 , 65 , 67 , 68 , 70 dw 71 , 73 , 74 , 76 , 78 , 79 , 81 , 83 , 85 , 87 , 89 , 91 , 92 , 95 , 97 , 99 dw 101 , 103 , 105 , 108 , 110 , 112 , 115 , 117 , 120 , 123 , 125 , 128 , 131 dw 134 , 137 , 140 , 143 , 146 , 149 , 152 , 156 , 159 , 162 , 166 , 170 , 173 dw 177 , 181 , 185 , 189 , 193 , 197 , 202 , 206 , 211 , 215 , 220 , 225 , 230 dw 235 , 240 , 245 , 251 , 256 , 262 , 267 , 273 , 279 , 285 , 292 , 298 , 304 dw 311 , 318 , 325 , 332 , 339 , 347 , 354 , 362 , 370 , 378 , 386 , 395 , 403 dw 412 , 421 , 431 , 440 , 450 , 459 , 470 , 480 , 490 , 501 , 512 , 523 , 535 dw 546 , 558 , 571 , 583 , 596 , 609 , 622 , 636 , 650 , 664 , 679 , 693 , 709 dw 724 , 740 , 756 , 773 , 790 , 807 , 825 , 843 , 861 , 880 , 899 , 919 , 939 dw 960 , 981 , 1002 , 1024 , 1046 , 1069 , 1093 , 1117 , 1141 , 1166 , 1192 dw 1218 , 1244 , 1272 , 1300 , 1328 , 1357 , 1387 , 1417 , 1448 , 1480 , 1512 dw 1545 , 1579 , 1614 , 1649 , 1685 , 1722 , 1760 , 1798 , 1838 , 1878 , 1919 dw 1961 , 2004 , 2048 , 2093 , 2139 , 2186 , 2233 , 2282 , 2332 , 2383 , 2435 dw 2489 , 2543 , 2599 , 2656 , 2714 , 2774 , 2834 , 2896 , 2960 , 3025 , 3091 dw 3158 , 3228 , 3298 , 3371 , 3444 , 3520 , 3597 , 3676 , 3756 , 3838 , 3922 dw 4008 , 4096 , 4186 , 4277 , 4371 , 4467 , 4565 , 4664 , 4767 , 4871 , 4978 dw 5087 , 5198 , 5312 , 5428 , 5547 , 5668 , 5793 , 5919 , 6049 , 6182 , 6317 dw 6455 , 6597 , 6741 , 6889 , 7039 , 7194 , 7351 , 7512 , 7677 , 7845 , 8016 dw 8192 , 8371 , 8555 , 8742 , 8933 , 9129 , 9329 , 9533 , 9742 , 9955 , 10173 dw 10396 , 10624 , 10856 , 11094 , 11337 , 11585 , 11839 , 12098 , 12363 , 12634 dw 12910 , 13193 , 13482 , 13777 , 14079 , 14387 , 14702 , 15024 , 15353 , 15689 dw 16033 , 16384 , 16743 , 17109 , 17484 , 17867 , 18258 , 18658 , 19066 , 19484 dw 19911 , 20347 , 20792 , 21247 , 21713 , 22188 , 22674 , 23170 , 23678 , 24196 dw 24726 , 25268 , 25821 , 26386 , 26964 , 27554 , 28158 , 28774 , 29404 , 30048 dw 30706 , 31379 , 32066 , 32768 , 33485 , 34219 , 34968 , 35734 , 36516 , 37316 dw 38133 , 38968 , 39821 , 40693 , 41584 , 42495 , 43425 , 44376 , 45348 , 46341 dw 47356 , 48393 , 49452 , 50535 , 51642 , 52772 , 53928 , 55109 , 56316 , 57549 dw 58809 , 60097 , 61413 , 62757
</pre>
 
== Fastest, accurate 8bit * 8bit Unsigned with 16 KB tables ==
 
'''Input:''' L = Multiplier, C = Multiplicand
 
'''Output:''' DE = Product
 
'''CPC Cycles:''' 80 = 20 usec
 
'''Size:''' 14 bytes of code + 16 KB tables
 
I'm currently working on a new routine, based on a routine by Kirk Meyer [http://www.ticalc.org/pub/86/asm/source/routines/vfmult.asm] which uses nibble multiplication tables.
 
I have a working, tested version which uses 16K of tables and can perform multiplication in 20&mu;s.
 
The code to produce the tables is below:
 
<pre>
.maketabs
ld hl,hltab1
.makelp1
ld a,l
rla
and #1e
add restab / 256
ld (hl),a
inc l
jr nz,makelp1
inc h
.makelp2
ld a,l
rra:rra:rra
and #1e
jr z,usez
add restab2 - restab / 256 - 2
.usez
add restab / 256
ld (hl),a
inc l
jr nz,makelp2
inc h ; restab
.makelp3
ld a,(hl)
inc h
ld d,(hl)
inc h
add l
ld (hl),a
inc h
ld a,0
adc d
ld (hl),a
dec h:dec h:dec h
inc l
jr nz,makelp3
inc h
inc h
ld a,h
cp restab2 / 256 - 2
jr nz,makelp3
ld b,h
ld c,l
inc b
inc b
ld h,restab / 256 + 2
.makelp4
ld e,(hl)
inc h
ld d,(hl)
dec h
ex de,hl
add hl,hl:add hl,hl:add hl,hl:add hl,hl
ex de,hl
ld a,e
ld (bc),a
inc b
ld a,d
ld (bc),a
dec b
inc l
inc c
jr nz,makelp4
inc h
inc h
inc b
inc b
ld a,h
cp restab2 / 256
jr nz,makelp4
ret
 
ds -$ and #ff
.hltab1
ds 256
.hltab2
ds 256
.restab
ds 512 * 16
.restab2
ds 512 * 15
</pre>
 
The code to perform the multiplication (DE = L * C):
 
<pre>
ld h,hltab1 / 256 ; 2
ld b,(hl) ; 4
inc h ; 5
ld h,(hl) ; 7
ld l,c ; 8
ld a,(bc) ; 10
add (hl) ; 12
ld e,a ; 13
inc b ; 14
inc h ; 15
ld a,(bc) ; 17
adc (hl) ; 19
ld d,a ; 20
</pre>
 
== Fast, accurate 8bit * 8bit Unsigned with 8 KB tables ==
 
'''Input:''' L = Multiplier, C = Multiplicand
 
'''Output:''' DE = Product
 
'''CPC Cycles:''' 100 = 25 usec
 
'''Size:''' 19 bytes of code + 8 KB tables
 
16K is a lot of memory to use for tables, but I'm working on a way to reduce this to 8K while maintaining similar performance (around 26&mu;s). The idea is rather than using tables for the low and high nibbles, use tables for alternate bits. So there would be 16 tables for the values #00, #01, #04, #05, #10, #11, #14, #15, #40, #41, #44, #45, #50, #51, #54, #55. The values can be shifted by 1 (rather than 4) to give values for the alternate bits using a simple ADD HL,HL.
 
This routine should be easy to adapt to a 16bit * 8bit unsigned multiplication or even a 16bit * 16bit, using simple additions.
 
[[User:Executioner|Executioner]] 03:58, 8 May 2007 (CEST)
 
Ok, here is the new 8K routine, first the routine to build the tables:
 
<pre>
.maketabs
ld hl,hltab1
.makelp5
ld a,l
ld bc,0
rla:rl b:rla:rl c
rla:rl b:rla:rl c
rla:rl b:rla:rl c
rla:rl b:rla:rl c
ld a,b
add a
add restab / 256
ld (hl),a
inc h
ld a,c
add a
add restab / 256
ld (hl),a
dec h
inc l
jr nz,makelp5
ld h,restab / 256 + 2
.makelp6
ld (hl),l
inc l
jr nz,makelp6
inc h:inc h
ld b,2
.makelp8
push bc
xor a
rr b:rra:rrca
rr b:rra:rrca
rr b:rra:rrca
rr b:rra:rrca
.makelp7
push hl
push af
call mulLbyA
ex de,hl
pop af
pop hl
ld (hl),e
inc h
ld (hl),d
dec h
inc l
jr nz,makelp7
inc h:inc h
pop bc
inc b
bit 4,b
jr z,makelp8
ret
 
.mulLbyA ; MAX times ; MIN times
ld e,l ; 1 ; 1
ld d,0 ; 3 ; 3
add a ; 4 ; 4
ld h,a ; 5 ; 5
jr c,ncad0 ; 8 ; 8
ld l,d
.ncad0
add hl,hl ; 11 ; 11
jr nc,ncad1 ; 13 ; 14
add hl,de ; 16
.ncad1
add hl,hl ; 19 ; 17
jr nc,ncad2 ; 21 ; 20
add hl,de ; 24
.ncad2
add hl,hl ; 27 ; 23
jr nc,ncad3 ; 29 ; 26
add hl,de ; 32
.ncad3
add hl,hl ; 35 ; 29
jr nc,ncad4 ; 37 ; 32
add hl,de ; 40
.ncad4
add hl,hl ; 43 ; 35
jr nc,ncad5 ; 45 ; 38
add hl,de ; 48
.ncad5
add hl,hl ; 51 ; 41
jr nc,ncad6 ; 53 ; 44
add hl,de ; 56
.ncad6
add hl,hl ; 59 ; 47
ld a,h ; 60 ; 48
ret nc ; 61 ; 50?
add hl,de ; 64
ld a,h ; 65
ret
 
ds -$ and #ff
 
.hltab1
ds 256
.hltab2
ds 256
.restab
ds 512 * 16
</pre>
 
And the routine to do the multiplication:
 
<pre>
.umultCL ; Using 8 bit only (25us - DE = L * C)
ld h,hltab2 / 256 ; 2
ld b,(hl) ; 4
dec h ; 5
ld h,(hl) ; 7
ld l,c ; 8
ld a,(hl) ; 10
add a ; 11
inc h ; 12
ld d,(hl) ; 14
rl d ; 16
ld h,b ; 17
add (hl) ; 19
ld e,a ; 20
inc h ; 21
ld a,(hl) ; 23
adc d ; 24
ld d,a ; 25
ret
</pre>
 
== Very fast 8bit * 8bit Unsigned with only 1K of tables ==
 
'''Input:''' A = ''Multiplier'', L = ''Multiplicand''
 
'''Output:''' DE = ''Product''
 
'''CPC Cycles''': 104-112 (108 on average) = 26-28 (27) usec
 
'''Size''': 24 bytes of code and 1024 bytes for the tables
 
Here's a new routine I've developed which uses the formula ab = ((a + b)<sup>2</sup> - (a - b)<sup>2</sup>) / 4. It's based on a routine for the 6502 by Stephen Judd in a C= Hacking article. Because of differences between the way the 6502 does register indexing it was quite difficult to actually get this working, but it's a great compromise between speed and space since it only uses 1K of tables (as opposed to the 16K or 8K table routines above), and can still manage to do the job in a maximum of 28 microseconds.
 
<br>Firstly, once again, we need some code to generate the tables. These tables contain values for
 
<br>x<sup>2</sup>/4 for 9 bit values of x, with the LSB when bit 8 is zero first followed by the MSB.
<pre>.gen_sq4
xor a
ld de,umul_tab + #1ff
ld (de),a
dec d
ld (de),a
ld h,d
ld l,e
inc e
ld c,e
ld b,2
 
.sq4_lp
ld a,b
cp 2
ld a,e
rra
add (hl)
ld (de),a
inc h
inc d
ld a,(hl)
adc c
ld (de),a
dec d
ld h,d
inc l
inc e
jr nz,sq4_lp
inc d
inc d
djnz sq4_lp
ret
 
align #100
.umul_tab ds #400
 
</pre>
Now for the actual multiply routine:
 
<pre>
ld h,umul_tab_lo / #100 ; 2
ld b,h ; 3
add l ; 4
ld c,a ; 5
jr nc,@noovf ; 7
inc b ; 8
inc b ; 9
.@noovf
sub l ; 10
sub l ; 11
jr nc,@noneg ; 13
neg ; 15
.@noneg
ld l,a ; 16
ld a,(bc) ; 18
sub (hl) ; 20
ld e,a ; 21
inc b ; 22
inc h ; 23
ld a,(bc) ; 25
sbc (hl) ; 27
ld d,a ; 28
 
</pre>
This code could easily be converted to a macro as it's only 24 bytes. I've tried to optimize it further but with no luck!
 
[[User:Executioner|Executioner]] 06:25, 4 April 2008 (CEST)
 
== Fast 8bit * 8bit Unsigned with only 512 bytes of tables ==
 
'''Input''': A = Multiplier, B = Multiplicant
 
'''Output''': A:E = Product
 
'''CPC Cycles''': 136-172 (154 on average) = 34-43 (38.5) usec
 
'''Size''': 44 bytes of code and 512 bytes for the tables
 
Fast 8 bit unsigned multiplication with 16 bit result. It uses formula
<br>x*y = ((x+y)/2)<sup>2</sup> - ((x-y)/2)<sup>2</sup>, if x+y is even
<br> = ((x+y-1)/2)<sup>2</sup> - ((x-y-1)/2)<sup>2</sup> + y, if x+y is odd and x>=y
 
<pre> cp b
jr nc,l1
 
ld e,a
ld a,b
ld b,e
 
l1 ld c,a
sub b
rra
ld d,a
ld a,c
add a,b
rra
ld l,a
ld h,high(sqrlo)
ld a,(hl)
ld e,l
ld l,d
jr nc,l2
 
sub (hl) ;odd
ld l,e
ld e,a
inc h ;loads high(sqrhi)
ld a,(hl)
ld l,d
sbc a,(hl)
ld d,a
 
ld a,e
add a,b
ld e,a
ld a,d
adc a,0
ret
 
l2 sub (hl) ;even
ld l,e
ld e,a
inc h
ld a,(hl)
ld l,d
sbc a,(hl)
ret
 
sqrlo ;low(x*x) should be at the page border
db 0,1,4,9,$10,$19,$24,$31,$40,$51,$64,$79,$90,$a9,$c4,$e1
db 0,$21,$44,$69,$90,$b9,$e4,$11,$40,$71,$a4,$d9,$10,$49,$84,$c1
db 0,$41,$84,$c9,$10,$59,$a4,$f1,$40,$91,$e4,$39,$90,$e9,$44,$a1
db 0,$61,$c4,$29,$90,$f9,$64,$d1,$40,$b1,$24,$99,$10,$89,4,$81
db 0,$81,4,$89,$10,$99,$24,$b1,$40,$d1,$64,$f9,$90,$29,$c4,$61
db 0,$a1,$44,$e9,$90,$39,$e4,$91,$40,$f1,$a4,$59,$10,$c9,$84,$41
db 0,$c1,$84,$49,$10,$d9,$a4,$71,$40,$11,$e4,$b9,$90,$69,$44,$21
db 0,$e1,$c4,$a9,$90,$79,$64,$51,$40,$31,$24,$19,$10,9,4,$1
db 0,1,4,9,$10,$19,$24,$31,$40,$51,$64,$79,$90,$a9,$c4,$e1
db 0,$21,$44,$69,$90,$b9,$e4,$11,$40,$71,$a4,$d9,$10,$49,$84,$c1
db 0,$41,$84,$c9,$10,$59,$a4,$f1,$40,$91,$e4,$39,$90,$e9,$44,$a1
db 0,$61,$c4,$29,$90,$f9,$64,$d1,$40,$b1,$24,$99,$10,$89,4,$81
db 0,$81,4,$89,$10,$99,$24,$b1,$40,$d1,$64,$f9,$90,$29,$c4,$61
db 0,$a1,$44,$e9,$90,$39,$e4,$91,$40,$f1,$a4,$59,$10,$c9,$84,$41
db 0,$c1,$84,$49,$10,$d9,$a4,$71,$40,$11,$e4,$b9,$90,$69,$44,$21
db 0,$e1,$c4,$a9,$90,$79,$64,$51,$40,$31,$24,$19,$10,9,4,$1
sqrhi ;high(x*x)
db 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
db 1,1,1,1,1,1,1,2,2,2,2,2,3,3,3,3
db 4,4,4,4,5,5,5,5,6,6,6,7,7,7,8,8
db 9,9,9,$a,$a,$a,$b,$b,$c,$c,$d,$d,$e,$e,$f,$f
db $10,$10,$11,$11,$12,$12,$13,$13,$14,$14,$15,$15,$16,$17,$17,$18
db $19,$19,$1a,$1a,$1b,$1c,$1c,$1d,$1e,$1e,$1f,$20,$21,$21,$22,$23
db $24,$24,$25,$26,$27,$27,$28,$29,$2a,$2b,$2b,$2c,$2d,$2e,$2f,$30
db $31,$31,$32,$33,$34,$35,$36,$37,$38,$39,$3a,$3b,$3c,$3d,$3e,$3f
db $40,$41,$42,$43,$44,$45,$46,$47,$48,$49,$4a,$4b,$4c,$4d,$4e,$4f
db $51,$52,$53,$54,$55,$56,$57,$59,$5a,$5b,$5c,$5d,$5f,$60,$61,$62
db $64,$65,$66,$67,$69,$6a,$6b,$6c,$6e,$6f,$70,$72,$73,$74,$76,$77
db $79,$7a,$7b,$7d,$7e,$7f,$81,$82,$84,$85,$87,$88,$8a,$8b,$8d,$8e
db $90,$91,$93,$94,$96,$97,$99,$9a,$9c,$9d,$9f,$a0,$a2,$a4,$a5,$a7
db $a9,$aa,$ac,$ad,$af,$b1,$b2,$b4,$b6,$b7,$b9,$bb,$bd,$be,$c0,$c2
db $c4,$c5,$c7,$c9,$cb,$cc,$ce,$d0,$d2,$d4,$d5,$d7,$d9,$db,$dd,$df
db $e1,$e2,$e4,$e6,$e8,$ea,$ec,$ee,$f0,$f2,$f4,$f6,$f8,$fa,$fc,$fe
</pre>
 
[[User:Litwr|Litwr]] 10:25, 24 October 2015 (CEST)
 
== 16bit * 16bit Unsigned -> 32 bits result ==
 
'''Input:''' BC = ''Multiplier'', DE = ''Multiplicand''
 
'''Output:''' DE,HL = ''Product''
<pre>ld hl,0
ld a,16
muluw
add hl,hl
rl e
rl d
jr nc,muluw_cont
add hl,bc
jr nc,muluw_cont
inc de
muluw_cont
dec a
jr nz,muluw
</pre>
 
== 16bit * 16bit Unsigned -> 24 bits result ==
 
'''Input:''' BC = ''Multiplier'', DE = ''Multiplicand''
 
'''Output:''' A,HL = ''Product''
<pre>ld ix,0
ld hl,0
.mul24b1:
ld a,c
or b
jr z,mul24b3
srl b
rr c
jr nc,mul24b2
add ix,de
ld a,h
adc l
ld h,a
.mul24b2:
sla e
rl d
rl l
jr mul24b1:
.mul24b3:
ld a,h
db #dd:ld e,l
db #dd:ld d,h
ex de,hl
ret</pre>
 
== Web links ==
 
* [http://map.tni.nl/articles/mult_div_shifts.php Multiplications and divisions on the MSX Assembly Page]
* [http://www.mccw.hetlab.tk/92/Multiplication/en.html "Multiplication on a Z80" ( MSX Computer & Club Webmagazine)]
* [http://www.andreadrian.de/oldcpu/Z80_number_cruncher.html The Z80 number cruncher (Andre Adrian)] (16/32bit multiplication, addition, right shift)
 
[[Category:Programming]]
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