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