📄 umul.asm
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;******************************************************************************
;* *
;* Functions for arithmetic and number theory with large integers in C *
;* Software supplement to the book "Cryptography in C and C++" *
;* by Michael Welschenbach *
;* *
;* Module umul.asm Revision: 18.12.2000 *
;* *
;* Copyright (C) 1998-2005 by Michael Welschenbach *
;* Copyright (C) 2001-2005 by Springer-Verlag Berlin, Heidelberg *
;* Copyright (C) 2001-2005 by Apress L.P., Berkeley, CA *
;* Copyright (C) 2002-2005 by Wydawnictwa MIKOM, Poland *
;* Copyright (C) 2002-2005 by PHEI, P.R.China *
;* Copyright (C) 2002-2005 by InfoBook, Korea *
;* Copyright (C) 2002-2005 by Triumph Publishing, Russia *
;* *
;* All Rights Reserved *
;* *
;* The software may be used for noncommercial purposes and may be altered, *
;* as long as the following conditions are accepted without any *
;* qualification: *
;* *
;* (1) All changes to the sources must be identified in such a way that the *
;* changed software cannot be misinterpreted as the original software. *
;* *
;* (2) The statements of copyright may not be removed or altered. *
;* *
;* (3) The following DISCLAIMER is accepted: *
;* *
;* DISCLAIMER: *
;* *
;* There is no warranty for the software contained in this distribution, to *
;* the extent permitted by applicable law. The copyright holders provide the *
;* software `as is' without warranty of any kind, either expressed or *
;* implied, including, but not limited to, the implied warranty of fitness *
;* for a particular purpose. The entire risk as to the quality and *
;* performance of the program is with you. *
;* *
;* In no event unless required by applicable law or agreed to in writing *
;* will the copyright holders, or any of the individual authors named in *
;* the source files, be liable to you for damages, including any general, *
;* special, incidental or consequential damages arising out of any use of *
;* the software or out of inability to use the software (including but not *
;* limited to any financial losses, loss of data or data being rendered *
;* inaccurate or losses sustained by you or by third parties as a result of *
;* a failure of the software to operate with any other programs), even if *
;* such holder or other party has been advised of the possibility of such *
;* damages. *
;* *
;******************************************************************************
;* *
;* Multiplication, interface compatible with C function umul_l() *
;* *
;* Product := a * u *
;* *
;* Stack on calling of umul: SP+12 ---> Offset product *
;* SP+ 8 ---> Offset a (CLINT) *
;* SP+ 4 ---> Offset u (USHORT) *
;* SP ---> Return address *
;* *
;* Return Value: ax = 0 *
;* *
;******************************************************************************
PUBLIC umul
.386
.MODEL FLAT
.CODE
umul proc near
;
a equ 0000 ; CLINT factor (working copy)
p equ 0520 ; Product (working copy)
;
_a equ [ebp+8] ; CLINT factor (parameter)
_u equ [ebp+12] ; USHORT factor (parameter)
_p equ [ebp+16] ; Product (storage space for result)
;
WORKSP equ 1560 ; Working memory in stack area
;
umult: push ebp ; Store value for calling procdure
mov ebp,esp
sub esp,WORKSP ; Local memory
mov eax,esp ; Store starting address
push edi
push esi
push ebx
mov ebx,eax
;
xor eax,eax
xor edx,edx
mov dx,_u ; USHORT factor into dx
cmp dx,0
jne @F
jmp umultz
@@: mov esi,_a ; Segment:Offset of variable a
mov ax,[esi]
cmp ax,0 ; a = 0 ?
jne umult01
jmp umultz
;
;>>>>>> Load operands
;
umult01:
lea edi,a[ebx] ; Destination offset is a[ebx]
mov ecx,0
mov cx,ax ; l(a) into ecx
cld
rep movsw ; Load a as chunks of USHORT into a
movsw ; plus one word
mov word ptr [edi],0
;
;>>>>>> Prepare access to local memory
;
push ebp ; Store base pointer bp
mov ebp,ebx ; Indexed addressing based on ss
;
;>>>>>>> Remove leading zeros from operands
;
xor eax,eax
mov ax,a[ebp] ; #USHORTs into ax
cmp ax,0
je end1
shl eax,1 ; #Bytes
mov esi,eax
@@: cmp word ptr a[ebp][esi],0 ; USHORT = 0 ?
jne @F ; If not, we're done
sub esi,2 ; Else: Step back one USHORT
cmp esi,0 ; Index = 0?
je mazer1 ; Then argument = 0
jmp @B ; Compare next digit
@@:
mazer1: shr esi,1 ; #USHORTs
mov eax,esi ; Store number of digits
mov a[ebp],ax
;
@@:
end1: cmp ax,0
jne @F
pop ebp
jmp umultz
;
;>>>>>> Start of multiplication
;
;-----> Initialize working memory
;
@@: mov esi,2 ; Initialization of index i
mov cx,a[ebp] ; Length l(a)
shr cx,1
jnc umult0
inc cx
umult0: mov eax,0
umult1: mov p[ebp][esi],eax
add esi,4
loop umult1
;
;-----> Inner loop
;
mov edi,2 ; Index j
mov cx,a[ebp] ; Initialize counter inner loop
shr cx,1
jnc umult2a
inc cx
umult2a:
xor ebx,ebx
mov bx,dx ; Load USHORT-Faktor into bx
mov edx,0 ; Dummy carry
mov esi,2
mov edi,2
umult3: push ecx ; Store counter
mov ecx,edx ; Store carry in dx
mov eax,ebx ; Load USHORT factor into ax
mul dword ptr a[ebp][edi] ; ax=lo, dx=hi
add eax,ecx ; ax + u
adc edx,0
add p[ebp][esi],eax ; p[i+j-1]= ax + p[i+j-1]
adc edx,0 ; New carry u into dx
add edi,4 ; j=j+4
add esi,4 ; i+j=i+j+4
pop ecx ; Get counter for inner loop
loop umult3
;
;-----> End of inner loop
;
mov p[ebp][esi],edx ; p[i+j-1]=u
add esi,2
umult3a:
cmp word ptr p[ebp][esi],0
jne short umult4
sub esi,2
jmp umult3a
umult4: mov ecx,esi
shr ecx,1
mov p[ebp],cx
;
;-----> Store product
;
lea esi,p[ebp]
mov ebx,ebp
pop ebp
mov edi,_p
cmp cx,0
je @F
cld ; Length l(p) is still in cx
rep movsw
@@: movsw
;
;*******************************************************************************
; Security: Purge stack
muret: mov edi,ebx
mov ecx,(WORKSP-4)/4
cld
mov eax,0 ; Return-Value = 0: Everything is OK
rep stosd ; Overwrite with 0
;*******************************************************************************
;
pop ebx
pop esi ; Restore registers
pop edi
mov esp,ebp
pop ebp
ret
;
umultz: mov esi,_p
mov word ptr [esi],0
jmp short muret
;
umul endp
END
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