penney.asm

;  Inputs: A and B are bit-strings of length N,
;  right-justified in EAX and EBX respectively;
;  N ≤ 30 is in ECX, and A ≠ B.
;
;  Output:  ESI/EDI (a fraction) is the probability
;  that bit-string  A  beats bit-string  B.

Magic:
        PUSH EAX
        PUSH EBX
        MOV EBX, EAX
        CALL InnerMagic
        MOV ESI, EDX
        POP EBX
        CALL InnerMagic
        SUB ESI, EDX 
        MOV EAX,EBX
        CALL InnerMagic
        MOV EDI, EDX
        POP EBX
        CALL InnerMagic
        SUB EDI, EDX
        ADD EDI, ESI
        RET

InnerMagic:
        PUSHAD 
        NEG ECX
        SHL EBX, CL
        MOV EDI, 1
        MOV ESI,80000000H
        XOR EBP,EBP
next:   ROR EAX, 1
        MOV EDX, EAX
        XOR EDX, EBX
        AND EDX, ESI
        JNZ skip
        ADD EBP, EDI
skip:   SAR ESI, 1
        SHL EDI, 1
        INC ECX
        JNZ next
        MOV 20[ESP], EBP
        POPAD
        RET