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Chapter 8. Frequently Asked Questions

Here are frequently asked questions (with answers) about Linux assembly programming. Some of the questions (and the answers) were taken from the the linux-assembly mailing list.

8.1. How do I do graphics programming in Linux?
8.2. How do I debug pure assembly code under Linux?
8.3. Any other useful debugging tools?
8.4. How do I access BIOS functions from Linux (BSD, BeOS, etc)?
8.5. Is it possible to write kernel modules in assembly?
8.6. How do I allocate memory dynamically?
8.7. I can't understand how to use select system call!

8.1. How do I do graphics programming in Linux?

An answer from Paul Furber:

Ok you have a number of options to graphics in Linux. Which one you use
depends on what you want to do. There isn't one Web site with all the
information but here are some tips:

SVGALib: This is a C library for console SVGA access.
Pros: very easy to learn, good coding examples, not all that different
from equivalent gfx libraries for DOS, all the effects you know from DOS
can be converted with little difficulty.
Cons: programs need superuser rights to run since they write directly to
the hardware, doesn't work with all chipsets, can't run under X-Windows.
Search for svgalib-1.4.x on http://ftp.is.co.za

Framebuffer: do it yourself graphics at SVGA res
Pros: fast, linear mapped video access, ASM can be used if you want :)
Cons: has to be compiled into the kernel, chipset-specific issues, must
switch out of X to run, relies on good knowledge of linux system calls
and kernel, tough to debug
Examples: asmutils (http://www.linuxassembly.org) and the leaves example
and my own site for some framebuffer code and tips in asm
(http://ma.verick.co.za/linux4k/)

Xlib: the application and development libraries for XFree86.
Pros: Complete control over your X application
Cons: Difficult to learn, horrible to work with and requires quite a bit
of knowledge as to how X works at the low level. 
Not recommended but if you're really masochistic go for it. All the
include and lib files are probably installed already so you have what
you need. 

Low-level APIs: include PTC, SDL, GGI and Clanlib
Pros: very flexible, run under X or the console, generally abstract away
the video hardware a little so you can draw to a linear surface, lots of
good coding examples, can link to other APIs like OpenGL and sound libs,
Windows DirectX versions for free
Cons: Not as fast as doing it yourself, often in development so versions
can (and do) change frequently.
Examples: PTC and GGI have excellent demos, SDL is used in sdlQuake,
Myth II, Civ CTP and Clanlib has been used for games as well.

High-level APIs: OpenGL - any others?
Pros: clean api, tons of functionality and examples, industry standard
so you can learn from SGI demos for example
Cons: hardware acceleration is normally a must, some quirks between
versions and platforms
Examples: loads - check out www.mesa3d.org under the links section.

To get going try looking at the svgalib examples and also install SDL
and get it working. After that, the sky's the limit.

8.2. How do I debug pure assembly code under Linux?

There's an early version of the Assembly Language Debugger, which is designed to work with assembly code, and is portable enough to run on Linux and *BSD. It is already functional and should be the right choice, check it out!

You can also try gdb ;). Although it is source-level debugger, it can be used to debug pure assembly code, and with some trickery you can make gdb to do what you need (unfortunately, nasm '-g' switch does not generate proper debug info for gdb; this is nasm bug, I think). Here's an answer from Dmitry Bakhvalov:

Personally, I use gdb for debugging asmutils. Try this:
 
1) Use the following stuff to compile:
   $ nasm -f elf -g smth.asm
   $ ld -o smth smth.o

2) Fire up gdb:
   $ gdb smth

3) In gdb:
   (gdb) disassemble _start
   Place a breakpoint at _start+1 (If placed at _start the breakpoint
   wouldnt work, dunno why)
   (gdb) b *0x8048075

   To step thru the code I use the following macro:
   (gdb)define n
   >ni
   >printf "eax=%x ebx=%x ...etc...",$eax,$ebx,...etc...
   >disassemble $pc $pc+15
   >end

   Then start the program with r command and debug with n.

   Hope this helps.

An additional note from ???:

    I have such a macro in my .gdbinit for quite some time now, and it
    for sure makes life easier. A small difference : I use "x /8i $pc",
    which guarantee a fixed number of disassembled instructions. Then,
    with a well chosen size for my xterm, gdb output looks like it is
    refreshed, and not scrolling.

If you want to set breakpoints across your code, you can just use int 3 instruction as breakpoint (instead of entering address manually in gdb).

If you're using gas, you should consult gas and gdb related tutorials.

8.3. Any other useful debugging tools?

Definitely strace can help a lot (ktrace and kdump on FreeBSD), it is used to trace system calls and signals. Read its manual page (man strace) and strace --help output for details.

8.4. How do I access BIOS functions from Linux (BSD, BeOS, etc)?

Short answer is -- noway. This is protected mode, use OS services instead. Again, you can't use int 0x10, int 0x13, etc. Fortunately almost everything can be implemented by means of system calls or library functions. In the worst case you may go through direct port access, or make a kernel patch to implement needed functionality, or use LRMI library to access BIOS functions.

8.5. Is it possible to write kernel modules in assembly?

Yes, indeed it is. While in general it is not a good idea (it hardly will speedup anything), there may be a need of such wizardy. The process of writing a module itself is not that hard -- a module must have some predefined global function, it may also need to call some external functions from the kernel. Examine kernel source code (that can be built as module) for details.

Meanwhile, here's an example of a minimum dumb kernel module (module.asm) (source is based on example by mammon_ from APJ #8):

section .text

        global init_module
        global cleanup_module
        global kernel_version

        extern printk

init_module:
        push    dword str1
        call    printk
        pop     eax
        xor     eax,eax
        ret

cleanup_module:
        push    dword str2
        call    printk
        pop     eax
        ret
        
str1            db      "init_module done",0xa,0
str2            db      "cleanup_module done",0xa,0

kernel_version  db      "2.2.18",0

The only thing this example does is reporting its actions. Modify kernel_version to match yours, and build module with:

$ nasm -f elf -o module.m module.asm
$ ld -r -o module.o module.m

Now you can play with it using insmod/rmmod/lsmod (root privilidged are required); a lot of fun, huh?

8.6. How do I allocate memory dynamically?

A laconic answer from H-Peter Recktenwald:

       ebx := 0        (in fact, any value below .bss seems to do)
        sys_brk
        eax := current top (of .bss section)

        ebx := [ current top < ebx < (esp - 16K) ]
        sys_brk
        eax := new top of .bss

An extensive answer from Tiago Gasiba:

section        .bss

var1    resb    1

section .text

;
;allocate memory
;

%define LIMIT   0x4000000                       ; about 100Megs

        mov     ebx,0                           ; get bottom of data segment
        call    sys_brk

        cmp     eax,-1                          ; ok?
        je      erro1

        add     eax,LIMIT                       ; allocate +LIMIT memory
        mov     ebx,eax
        call    sys_brk
        
        cmp     eax,-1                          ; ok?
        je      erro1

        cmp     eax,var1+1                      ; has the data segment grown?
        je      erro1

;
;use allocated memory
;
                                                ; now eax contains bottom of
                                                ; data segment
        mov     ebx,eax                         ; save bottom
        mov     eax,var1                        ; eax=beginning of data segment
repeat: 
        mov     word    [eax],1                 ; fill up with 1's
        inc     eax
        cmp     ebx,eax                         ; current pos = bottom?
        jne     repeat

;
;free memory
;

        mov     ebx,var1                        ; deallocate memory
        call    sys_brk                         ; by forcing its beginning=var1

        cmp     eax,-1                          ; ok?
        je      erro2

8.7. I can't understand how to use select system call!

An answer from Patrick Mochel:

When you call sys_open, you get back a file descriptor, which is simply an
index into a table of all the open file descriptors that your process has.
stdin, stdout, and stderr are always 0, 1, and 2, respectively, because
that is the order in which they are always open for your process from there.
Also, notice that the first file descriptor that you open yourself (w/o first
closing any of those magic three descriptors) is always 3, and they increment
from there.

Understanding the index scheme will explain what select does. When you
call select, you are saying that you are waiting certain file descriptors
to read from, certain ones to write from, and certain ones to watch from
exceptions from. Your process can have up to 1024 file descriptors open,
so an fd_set is just a bit mask describing which file descriptors are valid
for each operation. Make sense?

Since each fd that you have open is just an index, and it only needs to be
on or off for each fd_set, you need only 1024 bits for an fd_set structure.
1024 / 32 = 32 longs needed to represent the structure.

Now, for the loose example.
Suppose you want to read from a file descriptor (w/o timeout).

- Allocate the equivalent to an fd_set.  

.data

my_fds: times 32 dd 0

- open the file descriptor that you want to read from.

- set that bit in the fd_set structure.

   First, you need to figure out which of the 32 dwords the bit is in.  

   Then, use bts to set the bit in that dword. bts will do a modulo 32
   when setting the bit. That's why you need to first figure out which
   dword to start with.

   mov edx, 0
   mov ebx, 32
   div ebx

   lea ebx, my_fds
   bts ebx[eax * 4], edx

- repeat the last step for any file descriptors you want to read from.

- repeat the entire exercise for either of the other two fd_sets if you want action from them.

That leaves two other parts of the equation - the n paramter and the timeout
parameter. I'll leave the timeout parameter as an exercise for the reader
(yes, I'm lazy), but I'll briefly talk about the n parameter.

It is the value of the largest file descriptor you are selecting from (from
any of the fd_sets), plus one. Why plus one? Well, because it's easy to
determine a mask from that value. Suppose that there is data available on
x file descriptors, but the highest one you care about is (n - 1). Since
an fd_set is just a bitmask, the kernel needs some efficient way for
determining whether to return or not from select. So, it masks off the bits
that you care about, checks if anything is available from the bits that are
still set, and returns if there is (pause as I rummage through kernel source).
Well, it's not as easy as I fantasized it would be. To see how the kernel
determines that mask, look in fs/select.c in the kernel source tree.

Anyway, you need to know that number, and the easiest way to do it is to save
the value of the last file descriptor open somewhere so you don't lose it.

Ok, that's what I know. A warning about the code above (as always) is that
it is not tested. I think it should work, but if it doesn't let me know.
But, if it starts a global nuclear meltdown, don't call me. ;-)

That's all for now, folks.