我正试图弄清楚内存是如何在操作系统的最低级别分配的。据我所知，操作系统只是对可用和不可用的内存进行簿记，而 C 编程语言将在最低级别进行分配。
因此，第一个示例是我提出的一个简单的内存分配系统，然后我从以下资源中获取示例:https://github.com/levex/osdev .
示例 1:
struct heap_elements {
int start_address;
int end_address;
int size;
int reservation;
};
struct heap_elements heap[25];
// Write len copies of val into dest.
void memset(int *dest, int val, int len)
{
int *temp = (int *)dest;
for ( ; len != 0; len--) *temp++ = val;
}
/*
* This function will take a source and destination and copy n amount
* - of bytes from the source to the destination address.
*/
void memory_copy(unsigned char *source, unsigned char *destination, int bytes) {
for (int i = 0; i < bytes; i++) {
*(destination + i) = *(source + i);
}
}
int find_memory_hole(int size) {
for (int i = 0; i < total_elements; i++) {
if (heap[i].reservation == 0) {
if (heap[i].size >= size || heap[i].size == 0) {
return i;
}
}
}
return -1;
}
int * malloc(int size) {
int hole = find_memory_hole(size);
if (hole != -1) {
if (heap[hole].start_address == 0) {
heap[hole].start_address = ending_address;
ending_address += size;
heap[hole].end_address = ending_address;
heap[hole].size = size;
heap[hole].reservation = 1;
kprintf("Starting address: %d\n", heap[hole].start_address);
kprintf("Ending address: %d\n", heap[hole].end_address);
} else {
heap[hole].size = size;
heap[hole].reservation = 1;
}
memset((int*)heap[hole].start_address, 0, size);
return (int*)heap[hole].start_address;
} else {
kprintf("FREE SOME MEMORY~!\n");
kprintf("WE NEED ROOM IN HERE~!\n");
return 0;
}
}
void heap_install() {
total_elements = 25;
starting_address = 0x100000; // 1 - MB
ending_address = 0x100000; // 1 - MB
max_memory_address = 0xEEE00000; // 4 - GB
for (int i = 0; i < total_elements; i++) {
heap[i].start_address = 0;
heap[i].end_address = 0;
heap[i].size = 0;
heap[i].reservation = 0;
}
return;
}
void free(void * pointer) {
int memory_found = 0;
kprintf("Address %d\n", &pointer);
int memory_address = &pointer;
for (int i = 0; i < total_elements; i++) {
if (heap[i].start_address == memory_address) {
heap[i].size = 0;
heap[i].reservation = 0;
memory_found = 1;
break;
}
}
if (memory_found == 0)
kprintf("Memory could not bee free'd (NOT FOUND).\n");
return;
}
示例 2:
void mm_init(unsigned kernel_end)
{
kprintf("The kernel end is: %d\n", kernel_end);
last_alloc = kernel_end + 0x1000; // Set our starting point.
heap_begin = last_alloc;
heap_end = 0x5B8D80; // Set the bar to 6 MB
memset((char *)heap_begin, 0, heap_end - heap_begin);
}
void mm_print_out()
{
kprintf("Memory used: %d bytes\n", memory_used);
kprintf("Memory free: %d bytes\n", heap_end - heap_begin - memory_used);
kprintf("Heap size: %d bytes\n", heap_end - heap_begin);
kprintf("Heap start: 0x%x\n", heap_begin);
kprintf("Heap end: 0x%x\n", heap_end);
}
void free(void *mem)
{
alloc_t *alloc = (mem - sizeof(alloc_t));
memory_used -= alloc->size + sizeof(alloc_t);
alloc->status = 0;
}
char* malloc(unsigned size)
{
if(!size) return 0;
/* Loop through blocks and find a block sized the same or bigger */
unsigned char *mem = (unsigned char *)heap_begin;
while((unsigned)mem < last_alloc)
{
alloc_t *a = (alloc_t *)mem;
/* If the alloc has no size, we have reaced the end of allocation */
if(!a->size)
goto nalloc;
/* If the alloc has a status of 1 (allocated), then add its size
* and the sizeof alloc_t to the memory and continue looking.
*/
if(a->status) {
mem += a->size;
mem += sizeof(alloc_t);
mem += 4;
continue;
}
/* If the is not allocated, and its size is bigger or equal to the
* requested size, then adjust its size, set status and return the location.
*/
if(a->size >= size)
{
/* Set to allocated */
a->status = 1;
memset(mem + sizeof(alloc_t), 0, size);
memory_used += size + sizeof(alloc_t);
return (char *)(mem + sizeof(alloc_t));
}
/* If it isn't allocated, but the size is not good, then
* add its size and the sizeof alloc_t to the pointer and
* continue;
*/
mem += a->size;
mem += sizeof(alloc_t);
mem += 4;
}
nalloc:;
if(last_alloc+size+sizeof(alloc_t) >= heap_end)
{
panic("From Memory.c", "Something", 0);
}
alloc_t *alloc = (alloc_t *)last_alloc;
alloc->status = 1;
alloc->size = size;
last_alloc += size;
last_alloc += sizeof(alloc_t);
last_alloc += 4;
memory_used += size + 4 + sizeof(alloc_t);
memset((char *)((unsigned)alloc + sizeof(alloc_t)), 0, size);
return (char *)((unsigned)alloc + sizeof(alloc_t));
}
从这两个示例中，我预计我从 malloc() 分配的内存将具有与我分配它的位置相同的起始地址，如果这有意义的话？如果我知道内核的末尾位于 0x9000 标记处，并且我想在 1 MB 标记处开始分配内存。是的，我知道我的内核在内存中的位置很奇怪而且不符合常规，但我知道超过 1 MB 标记的内存是空闲的。
所以，如果我知道以下内容:
kernel_end = 0x9000;
heap_starts = 0x100000;
heap_ends = 0x5B8D80;
我希望这样:
char * ptr = malloc(5)
printf("The memory address for this pointer is at: %d\n", &ptr);
应该在 0x100000 内存地址附近，但它不是。这是一些完全不同的地方，这就是为什么我认为我没有在物理上告诉 char 指针在内存中的位置，而是 C 编程语言将它放在不同的地方。我不知道我做错了什么，理解这一点应该不难。另外，我查看了 OSDev Wiki，但没有找到任何东西。
最佳答案
I'm trying to figure out how memory is allocated at the lowest level in an operating system. From what I can gather is that the operating system is just doing the book keeping of the memory that is available and not available, and it is the C programming language that will do the allocation at the lowest level.
操作系统当然会记录哪些内存可用，哪些不可用，但用这些术语来描述会大大简化，而且我怀疑您对“内存”的含义有不同的理解，而不是最合适的理解。
操作系统的虚拟内存管理子系统管理如何将物理内存和其他存储资源(例如基于磁盘的交换空间)映射到每个进程的虚拟地址空间，包括多少以及内存的哪些部分进程的虚拟地址空间映射到可用内存。它服务于增加和减少进程可用虚拟内存的请求，以及创建内存映射的显式请求，例如基于普通文件的请求。
就在用户空间程序中为 malloc() 调用提供服务而言，您或多或少是正确的。程序通常以相当大的 block 从操作系统获取内存，这些内存由 malloc()、free() 和 friend 瓜分和管理。通常，只有当进程填满了可用的内存并且需要向内核请求更多内存时，这些细节才会涉及内核。
但是最底层肯定是在内核中。 C 库的内存管理函数只能使用操作系统分配给进程的内存。
From both examples I expected memory that I allocated from malloc() would have the same starting address as to where I allocated it at, if that makes sense? If I know the end of my kernel is at the 0x9000 mark, and I want to start allocating memory at the 1 MB mark. Yes, I know where my kernel is in memory is weird and not conventional, but I know that memory is free past the 1 MB mark.
内核对内存的看法与用户空间进程不同。每个进程都在自己的虚拟地址空间中运行，看不到它正在使用哪个物理地址。
关于c - 内存分配如何在操作系统的最低级别发生？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/55940833/