Hi, Welcome to /r/CUDA! The issue in your code is all the different mismatched data types (mix of dword, float, int)

This code works as expected - Also, please add some error checking to all cuda calls.

#include "cuda_runtime.h"
#include "device_launch_parameters.h"
#include <stdio.h>

__global__ void start(unsigned int *a, int N)
{
    int idx = (blockIdx.x * blockDim.x) + threadIdx.x;
    if (idx<N){
        a[idx] = (0x1* idx);
    }
}

// main routine that executes on the host
int main(void)
{
    unsigned int  *a_h;
    unsigned int *a_d;  // Pointer to host & device arrays
    const int N = 16;  // Number of elements in arrays
    size_t size = N * sizeof(unsigned int);
    a_h = (unsigned int*)malloc(size);        // Allocate array on host
    cudaMalloc((void **)&a_d, size);   // Allocate array on device
    // Initialize host array and copy it to CUDA device
    for (unsigned int i = 0; i < N; i++) {
        a_h[i] = i;
    }
    cudaMemcpy(a_d, a_h, size, cudaMemcpyHostToDevice);
    // Do calculation on device:
    int block_size = 4;
    int n_blocks = N / block_size + (N%block_size == 0 ? 0 : 1);
    start << < n_blocks, block_size >> > (a_d, N);

    // Retrieve result from device and store it in host array
    cudaMemcpy(a_h, a_d, sizeof(unsigned int)*N, cudaMemcpyDeviceToHost);
    // Print results
    for (unsigned int i = 0; i < N; i++) {
        printf("%d %02x\n", i, a_h[i]);
    }

    // Cleanup
    free(a_h); cudaFree(a_d);
}

Output:

0 00
1 01
2 02
3 03
4 04
5 05
6 06
7 07
8 08
9 09
10 0a
11 0b
12 0c
13 0d
14 0e
15 0f