Question

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?

Answer 1

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.