Raphael S. Carvalho's blog

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Dissecting the ROM BIOS Shipped With QEMU for Fun

I’m currently studying OS development with MIT course 6.828 mostly for fun. I highly recommend it for everyone wanting to have a better understanding of how computer works. It will teach you how an operating system works like xv6 and you will also build your own OS. Check it out here: https://pdos.csail.mit.edu/6.828/2016/schedule.html

After you complete the first lab, you will better understand how hardware initialization is done by firmware BIOS, then how bootloader is loaded, and in turn the operating system.

Earlier IBM PCs couldn’t address more than 1MB of memory, and only the first 640k chunk was actually RAM. The 640k-1MB range was reserved for special use, like mapping devices like VGA display, 16-bit PCI devices, and BIOS ROM.

The layout is more or less as follow:

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+------------------+  <- 0x00100000 (1MB)
|     BIOS ROM     |
+------------------+  <- 0x000F0000 (960KB)
|  16-bit devices, |
|  expansion ROMs  |
+------------------+  <- 0x000C0000 (768KB)
|   VGA Display    |
+------------------+  <- 0x000A0000 (640KB)
|                  |
|    Low Memory    |
|                  |
+------------------+  <- 0x00000000

It gets more complex for 32-bit physical address space. It’s also amazing how the hardware engineers left holes in the physical address space of the machine for backward compatibility.

When computer is turned on, BIOS is mapped at that 64k region reserved to it and control is transferred over to it. I want to better understand how QEMU emulates that. GDB and strace will be my friends in this journey. The former will be used to step through BIOS instructions, and the latter to see what QEMU is doing internally, like memory maps and files opened.

Using strace first

one of the most interesting things in strace output is the following:

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access("/usr/share/qemu/bios-256k.bin", R_OK) = 0
open("/usr/share/qemu/bios-256k.bin", O_RDONLY) = 13
lseek(13, 0, SEEK_END)                  = 262144
lseek(13, 0, SEEK_SET)                  = 0
read(13, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 262144) = 262144
close(13)                               = 0

It’s very interesting that file storing BIOS is 256k in size, even though area reserved for it is 64k. I suppose that only some part of it is actually mapped. There are also some interesting files such as /usr/share/qemu/kvmvapic.bin and /usr/share/qemu/vgabios-stdvga.bin which are opened and read. They all will be used to emulate the layout figure shown above.

QEMU also reports the following through ‘info roms’:

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fw=genroms/kvmvapic.bin size=0x002400 name="kvmvapic.bin"
addr=00000000fffc0000 size=0x040000 mem=rom name="bios-256k.bin"
/rom@etc/acpi/tables size=0x200000 name="etc/acpi/tables"
/rom@etc/table-loader size=0x001000 name="etc/table-loader"
/rom@etc/acpi/rsdp size=0x000024 name="etc/acpi/rsdp"

Actual dissection of BIOS using GDB

GDB can remotely connect to QEMU instance which waits for it, and that’s what I do here. Let’s get started…

QEMU starts executing at physical address 0x000FF000 and in real mode, which is at the very top of the area reserved for BIOS. CS and IP registers are 0xF000 and 0xFFF0, respectively. CS and IP were both used for addressing memory because registers were only 16 bits, thus the segmented addressing mode which multiples segment by 16 and adds offset. So PC is able to address up to 1MB of memory. The problem is that different values for segment and offset may refer to same physical memory which may easily lead to bugs. That was later addressed in protected mode which had bigger registers and MMU at its disposal.

We have a very basic idea of what BIOS will accomplish which is memory check, device initialization, and lately find a bootable sector to load into a predefined address (0x7C00) for which it will transfer control to. The purpose of this article is to go into the very specific details, because we already know the higher-level overview.

Dissecting…

I’ll only show the most important instructions because this article would get long and boring otherwise. On top of each instruction, there will be a comment to explain it and also possibly share something interesting.

http://web.archive.org/web/20040304063834/http://members.iweb.net.au:80/~pstorr/pcbook/book2/ioassign.htm is used as a reference for I/O addresses. I’m happy wayback machine allows me to access that great website again.

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# PC starts at 0x0FF000 and jumps to physical address 0xFE05B. This way BIOS
# assumes control after power up or system restart:
[f000:fff0]    0xffff0:   ljmp   $0xf000,$0xe05b

# Cleans stack segment register
[f000:e066]    0xfe066:   xor    %dx,%dx
[f000:e068]    0xfe068:   mov    %dx,%ss
# Sets up stack point to 0x7000, meaning stack address is actually 0x7000
[f000:e06a]    0xfe06a:   mov    $0x7000,%esp

# 0xf3513 is probably an address which is moved to EDX, let's see what BIOS
# want to achieve with that...
[f000:e070]    0xfe070:   mov    $0xf3513,%edx

# Interrupt flag is reset
[f000:d15d]    0xfd15d:   cli
# So is direction flag:
[f000:d15e]    0xfd15e:   cld 

# Select register 0xF from CMOS with NMI disabled. The lower order 7 bits are
# used to select register, and the most significant bit determines whether to
# disable NMI. Register 0xF is CMOS Shutdown Status.
[f000:d15f]    0xfd15f:   mov    $0x8f,%eax
[f000:d165]    0xfd165:   out    %al,$0x70
# Now we will get CMOS Shutdown Status 
[f000:d167]    0xfd167:   in     $0x71,%al

# A20 control via System Control Port A. Used to control memory 1MB barrier.
# Bit 1 (rw): 0: disable A20, 1: enable A20
[f000:d169]    0xfd169:   in     $0x92,%al
[f000:d16b]    0xfd16b:   or     $0x2,%al
[f000:d16d]    0xfd16d:   out    %al,$0x92

# Load Global/Interrupt Descriptor Table Register
# Sorry I'm tired. I won't go into details of what exactly is stored in tables
[f000:d16f]    0xfd16f:   lidtw  %cs:0x6af8
[f000:d175]    0xfd175:   lgdtw  %cs:0x6ab8

# Wow, it looks like protected mode was set up by BIOS. Did I get that right?
[f000:d17b]    0xfd17b:   mov    %cr0,%eax
[f000:d17e]    0xfd17e:   or     $0x1,%eax
[f000:d182]    0xfd182:   mov    %eax,%cr0

# Looks like so. GDT descriptor is now used for addressing memory.
[f000:d185]    0xfd185:   ljmpl  $0x8,$0xfd18d
The target architecture is assumed to be i386
# And registers for data will use its own GDT descriptor:
=> 0xfd18d:    mov    $0x10,%eax
=> 0xfd192:    mov    %eax,%ds
=> 0xfd194:    mov    %eax,%es
=> 0xfd196:    mov    %eax,%ss
=> 0xfd198:    mov    %eax,%fs
=> 0xfd19a:    mov    %eax,%gs

# Now an indirect jump for value we moved to EDX in the beginning
=> 0xfd19e:    jmp    *%edx
# Setting up stack frame according to x86 calling convention
=> 0xf3513:    push   %ebx
=> 0xf3514:    sub    $0x20,%esp

# Pushing two values as arguments for a function
# Let use variables for them: A=0xf5b6c and B=0xf430b
=> 0xf3517:    push   $0xf5b6c
=> 0xf351c:    push   $0xf430b
=> 0xf3521:    call   0xf096f
# Loading address of A into ecx, and value of B into edx
=> 0xf096f:    lea    0x8(%esp),%ecx
=> 0xf0973:    mov    0x4(%esp),%edx
=> 0xf0977:    mov    $0xf5b68,%eax
# If I understood it correctly, it will now initialize PCI devices...

# and it goes on until bootloader takes over...

I’m really tired now, and the article will also be boring if I keep going. I think it’s better to stop at this point. As we know, the BIOS will also keep initializing things and afterwards will load boot sector into 0x7c00 and start executing it. I hope you had lots of fun reading this article. My main idea is to have people interested in operating system engineering. I’ll also try to keep posting as I go through the OS course offered by MIT.

Bye for now!

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