MikeOS 引导加载程序中的堆栈段

发布于 2024-09-09 00:26:03 字数 490 浏览 3 评论 0原文

我不明白这段代码：

mov ax, 07C0h   ; Set up 4K of stack space above buffer
add ax, 544     ; 8k buffer = 512 paragraphs + 32 paragraphs (loader)
cli             ; Disable interrupts while changing stack
mov ss, ax
mov sp, 4096
sti             ; Restore interrupts

mov ax, 07C0h - 这里 BIOS 加载我们的代码。但什么是“4K”？千字节？我没明白:)
添加 ax, 544 - 为什么又是“8K”？为什么我们要加上 544？为什么不是512？
mov sp, 4096 - 在这里我们设置堆栈指针。

在设置堆栈指针之前，我们为什么要做所有这些操作呢？

原文

I don't understand this piece of code:

mov ax, 07C0h   ; Set up 4K of stack space above buffer
add ax, 544     ; 8k buffer = 512 paragraphs + 32 paragraphs (loader)
cli             ; Disable interrupts while changing stack
mov ss, ax
mov sp, 4096
sti             ; Restore interrupts

mov ax, 07C0h - here BIOS loads our
code. But what is '4K'? Kilobytes? I
didn't get it :)
add ax, 544 - Why again '8K'? And why we add 544? Why not 512?
mov sp, 4096 - Here we set stack pointer.

What for do we do all these manipulations, before we set stack pointer?

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━╋う一瞬間旳綻放 2024-09-16 00:26:04

我认为最后一行的评论总结了这一点：

buffer:             ; Disk buffer begins (8k after this, stack starts)

内存布局看起来像这样：

+-------------------+ <-- 07C0:0000, where the BIOS loads the boot sector
| 512 bytes of code |
+-------------------+
| 8KB set aside for |
|   a disk buffer   |
+-------------------+ <-- SS:0000
|   4KB of stack    |
+-------------------+ <-- SS:1000 = SS:SP

关于段落的评论有点迟钝；我发现用字节来思考更容易，16 个字节组成一个段落。

这些幻数的原因：

从段 07C0 开始，BIOS 在此加载代码
跳过 512 字节，以说明代码本身（512 字节 = 32 段）
跳过 8KB，为磁盘缓冲区留出空间（8,192字节 = 512 段）
将 SS 放在 4KB 块的开头。 512+8192 = 8,704 字节 = 544 个段落
将 SP 放在该块的末尾。放在最后是因为栈需要在内存中向上增长。

请注意，数字 4096 = 4KB 在代码中正常显示，因为 SP 寄存器需要一个以字节为单位的值。所有其他值都在段落中，因为它们与 SS（段寄存器）相关。

I think the comment on the last line sums it up:

buffer:             ; Disk buffer begins (8k after this, stack starts)

The memory layout looks like this:

+-------------------+ <-- 07C0:0000, where the BIOS loads the boot sector
| 512 bytes of code |
+-------------------+
| 8KB set aside for |
|   a disk buffer   |
+-------------------+ <-- SS:0000
|   4KB of stack    |
+-------------------+ <-- SS:1000 = SS:SP

The comment about paragraphs is slightly obtuse; I find it easier to think in bytes, where 16 bytes makes one paragraph.

The reason for these magic numbers:

Start at segment 07C0, where the BIOS loads the code
Skip past 512 bytes, to account for the code itself (512 bytes = 32 paragraphs)
Skip past 8KB, to set aside space for the disk buffer (8,192 bytes = 512 paragraphs)
Put SS at the start of a 4KB block. 512+8192 = 8,704 bytes = 544 paragraphs
Put SP at the end of that block. Put it at the end because the stack needs to grow upwards in memory.

Note that the number 4096 = 4KB appears as normal in the code, because the SP register needs a value in bytes. All the other values are in paragraphs because they relate to SS, which is a segment register.

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