2.6 Code: starting xv6, the first process and system call
# boot
1. boot loader
2. boot loader 把 xv6 kernal 搬到 memory
3. 在 machine mode，CPU 開始執行 `kernel/entry.S: _entryo`
* 直接使用 physical address

loader 把 xv6 kernel 放到 0x80000000
kernel 放到 0x80000000 是因為較低的 memory 位置 0x0 ~ 0x80000000 包含了 I/O devices

有點看不懂這裡是在做什麼:

        # qemu -kernel loads the kernel at 0x80000000
        # and causes each hart (i.e. CPU) to jump there.
        # kernel.ld causes the following code to
        # be placed at 0x80000000.
.section .text
.global _entry
_entry:
        # set up a stack for C.
        # stack0 is declared in start.c,
        # with a 4096-byte stack per CPU.
        # sp = stack0 + (hartid * 4096)
        la sp, stack0
        li a0, 1024*4
        csrr a1, mhartid
        addi a1, a1, 1
        mul a0, a0, a1
        add sp, sp, a0
        # jump to start() in start.c
        call start
spin:
        j spin

## reference
[2.6 Code: starting xv6, the first process and system call](https://pdos.csail.mit.edu/6.S081/2022/xv6/book-riscv-rev3.pdf)