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How to Compare the Numerical value of the Second-lowest Byte of L to the Second Lowest Byte of M?

I have a pseudo-random number generator, but I am unsure how to sort the bytes. I need to find the second lowest byte of "L" and "M" and then increment the value of the variable "lHigh" or "mHigh" depending on which one has the greater "second-lowest" byte.

So, for example, the second-lowest Byte of the 32-bit Hex number 0xAABBCCDD would be the “CC” value.

If L is higher than M i would want to increment the variable "lHigh"

So basically, I am comparing bits 8-15 of L to bits 8-15 of M.

Dylan de Hoyos
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2 Answers2

1

With unsigned values and lowest means least significant: the 2nd least significant bytes are

#include <limits.h>
unsigned char L2 = L >> CHAR_BIT;
unsigned char M2 = M >> CHAR_BIT;

// Form a 1,0,-1 depending on compare result.
int compare = (L2 > M2) - (L2 < < M2);
chux - Reinstate Monica
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  • this would work, but I need it in assembly language – Dylan de Hoyos Sep 19 '22 at 03:54
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    @DylandeHoyos This answer answers the question posted. If you have additional requirements, they should have been detailed in the original post. Here simple compile the C code and look at its assembly output. – chux - Reinstate Monica Sep 19 '22 at 03:57
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    @DylandeHoyos: Then use a C compiler for whatever ISA you want assembly for. Such as https://godbolt.org/ (use `-O2`). Keil makes assemblers for at least a couple, I think, and your question didn't specific which architecture even if someone wanted to write an assembly answer. – Peter Cordes Sep 20 '22 at 02:31
0

To elminate the bits of the "lowest byte" so they have no consequence you can 'knock those bits down' by ANDing with the ones' complement of 0xFF.
( x & ~0xFF )

Below, two integer values are shown and then compared.

int mmain() {
    // test 5 values crossing a threshold
    for( int i = 0; i < 5; i++ ) {
        int x = (1 << 8) + 254; // a start with 1 in "2nd lowest byte"
        int y = (1 << 8) + 4;

        x += i; // "lowest byte" of 'x' increases on each loop

        // show values
        printf( "%d : x(%d) y(%d) :: special-x(%d) special-y(%d) :: ",
            i, x, y, x&~0xFF, y&~0xFF );

        // show comparison
        puts( (x&~0xFF) == (y&~0xFF) ? "x==y" : "x > y" );
    }

    return 0;
}

Output

0 : x(510) y(260) :: special-x(256) special-y(256) :: x==y
1 : x(511) y(260) :: special-x(256) special-y(256) :: x==y
2 : x(512) y(260) :: special-x(512) special-y(256) :: x > y
3 : x(513) y(260) :: special-x(512) special-y(256) :: x > y
4 : x(514) y(260) :: special-x(512) special-y(256) :: x > y
Fe2O3
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