Random number generation in assembly NASM

Question

I am working on a university project in assembly nasm. The only problem is i am not able to generate an even random number between 162 and 278. I have tried many algorithms but cant seem to restrict the number in range.

Is there a small trick or tweak to get the desired number in range? The aim is to display a fruit randomly on the screen(It is mainly a snake game).

Answer 1

generate an even random number between 162 and 278

Step 1

"generate random number"
The task of displaying a fruit in a snake game is not too demanding, so I think using the TimeStampCounter is quite ok. You'll continu with the low dword found in EAX.

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rdtsc                        ; -> EDX:EAX

Step 2

"number between 162 and 278"
To restrict the number to your desired range of [162,278], you calculate
(RandomNumber mod NumbersInTheRange) + StartOfTheRange.

• RandomNumber is the EAX register from the 1st step.
• NumbersInTheRange is upper bound - lower bound + 1. Don't forget that +1.
• StartOfTheRange is its lower bound.

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xor     edx, edx             ; Required because there's no division of EAX solely
mov     ecx, 278 - 162 + 1   ; 117 possible values
div     ecx                  ; EDX:EAX / ECX --> EAX quotient, EDX remainder
mov     eax, edx             ; -> EAX = [0,116]
add     eax, 162             ; -> EAX = [162,278]

Step 3

"even number"
Make the number even by dropping its lowest bit.

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and     eax, -2              ; -> EAX = {162,164,...,276,278}

Answer 2

I look a little bit in this post: nasm random number generator function. Maybe you can try (rdtsc % (278 - 162) / 2 + 162 / 2) * 2. I hope I helped you :)