I have problems reading the superblock structure of an ext2 file system. Why does my code read the wrong value for the log2(block_size) field?

Question

I try to write a bootloader for a ext2 formatted drive. I loaded the superblock to 0x7e00. Then I determined the block size by reading 24th byte. I shifted 1024 left with this value. But the cl register contains the wrong value.

Memory dump (At 0x7e00 + 24 for 4 bytes):

24>: 0x02 0x00 0x00 0x00

Shifted value: 0x400 (eax)
cl: 0x00

My Code:

mov     eax, 1024
mov     cl, byte [0x7e00 + 24]
shl     eax, cl
mov     [BLOCK_SIZE], eax

What have I done wrong?

Bochs is a virtual machine. I entered in its debugger: x /1024bx 0x7e00. It's the part of the memory dump — Xorda, Jun 25 '20 at 14:01
I know what BOCHS is, it wasn't clear to me what the value you're showing there is. (and still isn't really) — Thomas Jager, Jun 25 '20 at 14:04
That is the content of my memory at 0x7e00 + 24 for 4 bytes. It's a part of the superblock I loaded into memory. By the way I confused the value in cl. It's 0x00. I expect the content of cl to be 0x02 and eax to be 0x1000 — Xorda, Jun 25 '20 at 14:14
Are you absolutely sure that you're dumping these values after the execution of `mov cl, byte [0x7e00 + 24]` has happened? — Thomas Jager, Jun 25 '20 at 14:15
Yes I am sure. I set a breakpoint and after executing mov cl, byte [0x7e00 + 24], cl contains 0x00. I read with a hex editor the drive and I dumped the memory. At this place is the value 0x02 but it read 0x00 — Xorda, Jun 25 '20 at 14:20

score 2 · Answer 1 · answered Jun 25 '20 at 14:33

2

Solution: The problem was that I forgot my DS register. I changed the org directive to org 0x7c00 and set the segment registers to 0x0

answered Jun 25 '20 at 14:33

Xorda

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4

I have problems reading the superblock structure of an ext2 file system. Why does my code read the wrong value for the log2(block_size) field?

1 Answers1