Arduino inline ASM not working as expected

Question

I'm trying to build a clock, so I'm working with ASM and an arduino. For most parts, plain C will be fine, but for preparing the time to be output to BCD to Decimal converters I decided to go with ASM. I wrote the following code in 8086 C++/ASM and it runs fine on my computer:

#include <iostream>

using namespace std;

int main(int argc, char **argv)
{
    for(int num = 0; num < 16; num++) {
        int bin[4];
        bin[0] = bin[1] = bin[2] = bin[3] = 0;
        asm("movl %%ebx, %%eax;"
            "andl $8, %%eax;"
            "cmp $0, %%eax;"
            "je a;"
            "movl $1, %0;"
            "jmp b;"
            "a: movl $0, %0;"
            "b: movl %%ebx, %%eax;"
            "andl $4, %%eax;"
            "cmp $0, %%eax;"
            "je c;"
            "movl $1, %1;"
            "jmp d;"
            "c: movl $0, %1;"
            "d: movl %%ebx, %%eax;"
            "andl $2, %%eax;"
            "cmp $0, %%eax;"
            "je e;"
            "movl $1, %2;"
            "jmp f;"
            "e: movl $0, %2;"
            "f: movl %%ebx, %%eax;"
            "andl $1, %%eax;"
            "cmp $0, %%eax;"
            "je g;"
            "movl $1, %3;"
            "jmp h;"
            "g: movl $0, %3;"
            "h: nop"
            : "=r" (bin[0]), "=r" (bin[1]), "=r" (bin[2]), "=b" (bin[3])
            : "b" (num)
            : "%eax"
            );
        cout << num << ": ";
        for(int i = 0; i < 4; i++) {
            cout << bin[i];
        }
        cout << endl;
    }
    return 0;
}

However, when I modified it to run on the Arduino, things stop working entirely:

  for(uint8_t num = 0; num < 16; num++) {
  uint8_t bin[4];
  bin[0] = bin[1] = bin[2] = bin[3] = 0;
  asm("mov __tmp_reg__, %[val];"
    "and __tmp_reg__, $8;"
    "cmp __tmp_reg__, $0;"
    "je a;"
    "mov %[bit8], $1;"
    "rjmp b;"
    "a: mov $0, %[bit8];"
    "b: mov %[val], __tmp_reg__;"
    "and __tmp_reg__, $4;"
    "cmp __tmp_reg__, $0;"
    "je c;"
    "mov %[bit4], $1;"
    "rjmp d;"
    "c: mov $0, %[bit4];"
    "d: mov %[val], __tmp_reg__;"
    "and __tmp_reg__, $2;"
    "cmp __tmp_reg__, $0;"
    "je e;"
    "mov %[bit2], $1;"
    "rjmp f;"
    "e: mov $0, %[bit2];"
    "f: mov %[val], __tmp_reg__;"
    "and __tmp_reg__, $1;"
    "cmp __tmp_reg__, $0;"
    "je g;"
    "mov %[bit1], $1;"
    "rjmp h;"
    "g: mov $0, %[bit1];"
    "h: nop"
    : [bit8] "=r" (bin[0]), [bit4] "=r" (bin[1]), [bit2] "=r" (bin[2]), [bit1] "=r" (bin[3])
    : [val] "r" (num)
    : "r0"
    );

The 8086 code gives the output you'd expect:

0: 0000
1: 0001
2: 0010
3: 0011
...
13: 1101
14: 1110
15: 1111

But the code run on the Arduino gives a different output:

0: 5000
1: 0000
2: 0000
3: 0000
... (zeros continue)
13: 0000
14: 0000
15: 0000

As you can imagine, the code becomes useless if it returns... five. And I'm clueless as to how it could return 5 when nothing is anywhere close to 5 in the source. I'm at a loss as to what to do here, so I could really use some help.

I'm using the Arduino Leonardo, which has an ATMega32U processor. I've tried disassembling the executable generated by the Arduino software (which compiles it with AVR-GCC), but I can't seem to get anywhere in my efforts to find the code I put in.

Thanks for your time, Stack Overflow.

Answer 1

The code you have can EASILY be written in C++, like this:

 int bin[4] = {};

 bin[0] = !!(num & 8);
 bin[1] = !!(num & 4);
 bin[2] = !!(num & 2);
 bin[3] = !!(num & 1);

or:

int bin[4];
int bit = 8;
for(int i = 0; i < 4; i++)
{
    bin[i] = !!(num & bit);
    bit >>= 1;
}

If you don't like !! (which makes "take the next value and make it either 0 [if it's false] or 1 [if it's true]), you could replace it with:

for(int i = 0; i < 4; i++)
{
    bin[i] = (num >> 3 - i) & 1;
}

I take it you intentionally want the highest bit in the lowest bin index, rather than the usual case of highest bit in the highest index.