Chapter 8. Frequently Asked Questions

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.

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.

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.

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.

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?

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

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. ;-)