code:

#include <stdint.h>
#include <stdlib.h>

enum
{
    Memory_Size = 2048,
    Word_Size = sizeof(void *),
    Word_Size_Mask = Word_Size - 1
};

static char memory[Memory_Size];
static uint32_t allocated = 0;

void allocator_reset()
{
    uint32_t x;

    for(x = 0; x < Memory_Size; x++) {
        memory[x] = 0;
    }

    allocated = 0;
}

void * allocator(size_t num_bytes)
{
    void * result;

    if ((allocated + num_bytes) >= Memory_Size) {
        return NULL;
    }

    result = (void *)(memory + allocated);
    allocated += num_bytes;

    while (allocated & Word_Size_Mask) {
        allocated++;
    }

    return result;
}