# 物理内存探测

操作系统怎样知道物理内存所在的那段物理地址呢？在 RISC-V 中，这个一般是由 bootloader ，即 OpenSBI 来完成的。它来完成对于包括物理内存在内的各外设的扫描，将扫描结果以 DTB(Device Tree Blob) 的格式保存在物理内存中的某个地方。随后 OpenSBI 会将其地址保存在 `a1` 寄存器中，给我们使用。

这个扫描结果描述了所有外设的信息，当中也包括 Qemu 模拟的 RISC-V 计算机中的物理内存。

{% hint style="success" %}
**Qemu 模拟的 RISC-V virt 计算机中的物理内存**

通过查看[virt.c](https://github.com/qemu/qemu/blob/master/hw/riscv/virt.c)的**virt\_memmap\[]**的定义，可以了解到 Qemu 模拟的 RISC-V virt 计算机的详细物理内存布局。可以看到，整个物理内存中有不少内存空洞（即含义为**unmapped**的地址空间），也有很多外设特定的地址空间，现在我们看不懂没有关系，后面会慢慢涉及到。目前只需关心最后一块含义为**DRAM**的地址空间，这就是 OS 将要管理的 128MB 的内存空间。
{% endhint %}

| 起始地址       | 终止地址       | 含义                                                  |
| ---------- | ---------- | --------------------------------------------------- |
| 0x0        | 0x100      | QEMU VIRT\_DEBUG                                    |
| 0x100      | 0x1000     | unmapped                                            |
| 0x1000     | 0x12000    | QEMU MROM (包括 hard-coded reset vector; device tree) |
| 0x12000    | 0x100000   | unmapped                                            |
| 0x100000   | 0x101000   | QEMU VIRT\_TEST                                     |
| 0x101000   | 0x2000000  | unmapped                                            |
| 0x2000000  | 0x2010000  | QEMU VIRT\_CLINT                                    |
| 0x2010000  | 0x3000000  | unmapped                                            |
| 0x3000000  | 0x3010000  | QEMU VIRT\_PCIE\_PIO                                |
| 0x3010000  | 0xc000000  | unmapped                                            |
| 0xc000000  | 0x10000000 | QEMU VIRT\_PLIC                                     |
| 0x10000000 | 0x10000100 | QEMU VIRT\_UART0                                    |
| 0x10000100 | 0x10001000 | unmapped                                            |
| 0x10001000 | 0x10002000 | QEMU VIRT\_VIRTIO                                   |
| 0x10002000 | 0x20000000 | unmapped                                            |
| 0x20000000 | 0x24000000 | QEMU VIRT\_FLASH                                    |
| 0x24000000 | 0x30000000 | unmapped                                            |
| 0x30000000 | 0x40000000 | QEMU VIRT\_PCIE\_ECAM                               |
| 0x40000000 | 0x80000000 | QEMU VIRT\_PCIE\_MMIO                               |
| 0x80000000 | 0x88000000 | DRAM 缺省 128MB，大小可配置                                 |


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