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- ;
- ; Command & Conquer Red Alert(tm)
- ; Copyright 2025 Electronic Arts Inc.
- ;
- ; This program is free software: you can redistribute it and/or modify
- ; it under the terms of the GNU General Public License as published by
- ; the Free Software Foundation, either version 3 of the License, or
- ; (at your option) any later version.
- ;
- ; This program is distributed in the hope that it will be useful,
- ; but WITHOUT ANY WARRANTY; without even the implied warranty of
- ; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
- ; GNU General Public License for more details.
- ;
- ; You should have received a copy of the GNU General Public License
- ; along with this program. If not, see <http://www.gnu.org/licenses/>.
- ;
- ;***************************************************************************
- ;** C O N F I D E N T I A L --- W E S T W O O D A S S O C I A T E S **
- ;***************************************************************************
- ;* *
- ;* Project Name : Library routine *
- ;* *
- ;* File Name : RANDOM.ASM *
- ;* *
- ;* Programmer : Christopher Yates *
- ;* *
- ;* Last Update : 20 August, 1990 [CY] *
- ;* *
- ;*-------------------------------------------------------------------------*
- ;* Functions: *
- ;* *
- ;* UBYTE Random(VOID); *
- ;* *
- ;* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - *
- IDEAL
- P386
- MODEL USE32 FLAT
- Global Random :NEAR
- Global Get_Random_Mask :NEAR
- Global RandNumb :DWORD
- DATASEG
- RandNumb DD 12349876H
- CODESEG
- ; ----------------------------------------------------------------
- ;
- ; Here are prototypes for the routines defined within this module:
- ;
- ; UBYTE Random(VOID);
- ; int Get_Random_Mask(int maxval);
- ;
- ; ----------------------------------------------------------------
- ;-----------------------------------------------------------------
- ; RANDOM
- ;
- ; UBYTE Random(VOID);
- ;
- ;*
- PROC Random C near
- USES esi
- lea esi, [RandNumb] ; get offset in segment of RandNumb
- xor eax,eax
- mov al,[esi]
- shr al,1 ; shift right 1 bit (bit0 in carry)
- shr al,1
- rcl [BYTE PTR esi+2],1 ; rcl byte 3 of RandNumb
- rcl [BYTE PTR esi+1],1 ; rcl byte 2 of RandNumb
- cmc ; complement carry
- sbb al,[esi] ; sbb byte 1 of RandNumb
- shr al,1 ; sets carry
- rcr [BYTE PTR esi],1 ; rcr byte 1 of RandNumb
- mov al,[esi] ; reload byte 1 of RandNumb
- xor al,[esi+1] ; xor with byte 2 of RandNumb
- ret
- ENDP Random
- ;-----------------------------------------------------------------
- ; GET_RANDOM_MASK - returns an AND value that is large enough that it
- ; encloses the 'maxval' parameter.
- ;
- ; int Get_Random_Mask(int maxval);
- ;
- ;*
- PROC Get_Random_Mask C near
- USES ecx
- ARG maxval:DWORD
- ; This function takes as a parameter a maximum value, for example, 61. It
- ; then tries to create an AND mask that is big enough to enclose that number.
- ; For our example case, that AND mask would be 0x3F. It does this by scanning
- ; for the highest bit in the number, then making an all-1's mask from that
- ; bit position down to bit 0.
- bsr ecx,[maxval] ; put bit position of highest bit in ecx
- mov eax,1 ; set one bit on in eax
- jz ??invalid ; if BSR shows maxval==0, return eax=1
- inc ecx ; get one bit higher than count showed
- shl eax,cl ; move our EAX bit into position
- dec eax ; dec it to create the mask.
- ??invalid:
- ret
- ENDP Get_Random_Mask
- ;----------------------------------------------------------------------------
- END
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