Linux kernel code syntax doubt Post: 302534370

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Top Forums Programming Linux kernel code syntax doubt Post 302534370 by ygemici on Monday 27th of June 2011 05:19:11 PM

06-27-2011

Registered User

Quote:

Originally Posted by alister

I believe you are absolutely mistaken regarding this being a performance optimization. The cpu registers are the fastest, operating system-visible processor-storage locations available, faster than any level of cache and most definitely faster than main memory (RAM). Performance optimizations typically attempt to stuff as many things into registers as possible.

I'm not a big hardware person, but if I'm not mistaken, the x86 32bit architecture has comparitively few registers. asmlinkage is a workaround for this. Note that the stack is not used for this purpose on any other platform. Except for x86 32-bit which includes the gcc function attribute code in the definition of asmlinkage, by default asmlinkage is nothing more than "extern C".

Regards,
Alister

generally you are right.registers are faster therefore are part of the microprocessor.yes stack access is slower than register access.
Additional, CPU often access the stack and therefore stack(return addres of functions,parameters,registers,tmp infos,local variables..) datas usually is
saved in processor's cache.however cpu registers are very fast beceause of registers are very near and wired directly alu.So I meant just optimization not fast my fault.

i want to add some information about this issue.
asmlinkage tells to compiler that the arguments are on the stack(gcc takes it (i386 specific __attribute__((regparm(0))) and related in below asm code)
(actually,is a tag for assembly code convention for compiler (like asm/nasm/..))

# define asmlinkage CPP_ASMLINKAGE __attribute__((regparm(0))) provides like about below..
* userspace application executes
* (c) library routines puts arguments in registers from userspace.
( about register i want to tell something , since Linux 2.3.31 that is supported six system call parameters,but before
linux could handle 4(for some ver..5) system call parametres, if more than four arguments are needed, they are lef on the stack but i mentioned for new
version)
* enter kernel mode (executing trap --> int 0x80/sysenter) and executed kernel syscall handler(system_call() f)
* save current syscall number to eax.
* executed SAVE_ALL macro (%macro SAVE_ALL)
1) save address of kernel data(ds) and extra segment(to es)
2) all arguments(ebx,ecx,edx,esi,edi,ebp actually correct order will be reverse of this for C function call ) saves to stack
3) copy user_data_segment to ds register.
* %endmacro
* call (C) function (arguments are passed on the stack to c function (with asmlinkage))
* RESTORE_ALL(restores ebx, ecx, edx, esi, edi, ebp, eax, ds, es) and then go back to usermode (ret)

And asmlinkage format is like..
On i386
__attribute__((regparm(0)))
On ia64
__attribute__((syscall_linkage))

regards
ygemici

ygemici

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ppmtosixel

ppmtosixel(1)                                                 General Commands Manual                                                ppmtosixel(1)

NAME

       ppmtosixel - convert a portable pixmap into DEC sixel format

SYNOPSIS

       ppmtosixel [-raw] [-margin] [ppmfile]

DESCRIPTION

       Reads  a  portable  pixmap  as input.  Produces sixel commands (SIX) as output.  The output is formatted for color printing, e.g. for a DEC
       LJ250 color inkjet printer.

       If RGB values from the PPM file do not have maxval=100, the RGB values are rescaled.  A printer control header and a color assignment table
       begin the SIX file.  Image data is written in a compressed format by default.  A printer control footer ends the image file.

OPTIONS

       -raw   If  specified,  each  pixel  will  be explicitly described in the image file.  If -raw is not specified, output will default to com-
              pressed format in which identical adjacent pixels are replaced by "repeat pixel" commands.  A raw file is often an order  of  magni-
              tude larger than a compressed file and prints much slower.

       -margin
              If  -margin  is not specified, the image will be start at the left margin (of the window, paper, or whatever).  If -margin is speci-
              fied, a 1.5 inch left margin will offset the image.

PRINTING

       Generally, sixel files must reach the printer unfiltered.  Use the lpr -x option or cat filename > /dev/tty0?.

BUGS

       Upon rescaling, truncation of the least significant bits of RGB values may result in poor color conversion.  If the original PPM maxval was
       greater  than  100,  rescaling  also reduces the image depth.  While the actual RGB values from the ppm file are more or less retained, the
       color palette of the LJ250 may not match the colors on your screen.  This seems to be a printer limitation.

SEE ALSO

       ppm(5)

AUTHOR

       Copyright (C) 1991 by Rick Vinci.

                                                                   26 April 1991                                                     ppmtosixel(1)