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<HTML
><HEAD
><TITLE
>linux/arch/i386/boot/compressed/head.S</TITLE
><META
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>Linux i386 Boot Code HOWTO</TH
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><HR
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><DIV
CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="compressed_head"
></A
>5. linux/arch/i386/boot/compressed/head.S</H1
><P
>&#13; We are in <EM
>bvmlinux</EM
> now!
With the help of <EM
>misc.c:decompress_kernel()</EM
>,
we are going to decompress <EM
>piggy.o</EM
>
to get the resident kernel image <TT
CLASS="filename"
>linux/vmlinux</TT
>.
</P
><P
>&#13; This file is of pure 32-bit startup code.
Unlike previous two files, it has no ".code16" statement in the source file.
Refer to
<A
HREF="http://www.gnu.org/software/binutils/manual/gas-2.9.1/html_chapter/as_16.html#SEC205"
TARGET="_top"
>&#13; Using as: Writing 16-bit Code</A
> for details.
</P
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="decompress_kernel"
></A
>5.1. Decompress Kernel</H2
><P
>&#13; The segment base addresses in segment descriptors (which correspond to
segment selector __KERNEL_CS and __KERNEL_DS) are equal to 0;
therefore, the logical address offset (in segment:offset format) will
be equal to its linear address if either of these segment selectors
is used.
For <EM
>zImage</EM
>, CS:EIP is at logical address 10:1000
(linear address 0x1000) now;
for <EM
>bzImage</EM
>, 10:100000 (linear address 0x100000).
</P
><P
>&#13; As paging is not enabled, linear address is identical to physical address.
Check IA-32 Manual (Vol.1. Ch.3.3. Memory Organization, and
Vol.3. Ch.3. Protected-Mode Memory Management) and
<A
HREF="http://www.xml.com/ldd/chapter/book/ch13.html#t1"
TARGET="_top"
>&#13; Linux Device Drivers: Memory Management in Linux</A
> for address issue.
</P
><P
>&#13; It comes from <TT
CLASS="filename"
>setup.S</TT
> that BX=0 and
ESI=INITSEG&#60;&#60;4.
</P
><P
>&#13;<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>.text
///////////////////////////////////////////////////////////////////////////////
startup_32()
{
cld;
cli;
DS = ES = FS = GS = __KERNEL_DS;
SS:ESP = *stack_start; // end of user_stack[], defined in misc.c
// all segment registers are reloaded after protected mode is enabled
// check that A20 really IS enabled
EAX = 0;
do {
1: DS:[0] = ++EAX;
} while (DS:[0x100000]==EAX);
EFLAGS = 0;
clear BSS; // from _edata to _end
struct moveparams mp; // subl $16,%esp
if (!decompress_kernel(&#38;mp, ESI)) { // return value in AX
restore ESI from stack;
EBX = 0;
goto __KERNEL_CS:100000;
// see linux/arch/i386/kernel/head.S:startup_32
}
/*
* We come here, if we were loaded high.
* We need to move the move-in-place routine down to 0x1000
* and then start it with the buffer addresses in registers,
* which we got from the stack.
*/
3: move move_rountine_start..move_routine_end to 0x1000;
// move_routine_start &#38; move_routine_end are defined below
// prepare move_routine_start() parameters
EBX = real mode pointer; // ESI value passed from setup.S
ESI = mp.low_buffer_start;
ECX = mp.lcount;
EDX = mp.high_buffer_star;
EAX = mp.hcount;
EDI = 0x100000;
cli; // make sure we don't get interrupted
goto __KERNEL_CS:1000; // move_routine_start();
}
/* Routine (template) for moving the decompressed kernel in place,
* if we were high loaded. This _must_ PIC-code ! */
///////////////////////////////////////////////////////////////////////////////
move_routine_start()
{
move mp.low_buffer_start to 0x100000, mp.lcount bytes,
in two steps: (lcount &#62;&#62; 2) words + (lcount &#38; 3) bytes;
move/append mp.high_buffer_start, ((mp.hcount + 3) &#62;&#62; 2) words
// 1 word == 4 bytes, as I mean 32-bit code/data.
ESI = EBX; // real mode pointer, as that from setup.S
EBX = 0;
goto __KERNEL_CS:100000;
// see linux/arch/i386/kernel/head.S:startup_32()
move_routine_end:
}</PRE
></FONT
></TD
></TR
></TABLE
>
For the meaning of "je 1b" and "jnz 3f", refer to
<A
HREF="http://www.gnu.org/software/binutils/manual/gas-2.9.1/html_chapter/as_5.html#SEC48"
TARGET="_top"
>&#13; Using as: Local Symbol Names</A
>.
</P
><P
>&#13; Didn't find <EM
>_edata</EM
> and
<EM
>_end</EM
> definitions?
No problem, they are defined in the "internal linker script".
Without -T (--script=) option specified, <B
CLASS="command"
>ld</B
> uses
this builtin script to link <EM
>compressed/bvmlinux</EM
>.
Use "<B
CLASS="command"
>ld --verbose</B
>" to display this script, or check
Appendix B. <A
HREF="internel_lds.html"
><I
>Internal Linker Script</I
></A
>.
</P
><P
>&#13; Refer to
<A
HREF="http://www.gnu.org/software/binutils/manual/ld-2.9.1/html_chapter/ld_2.html#SEC3"
TARGET="_top"
>&#13; Using LD, the GNU linker: Command Line Options</A
> for
-T (--script=), -L (--library-path=) and --verbose
option description.
"<B
CLASS="command"
>man ld</B
>" and "<B
CLASS="command"
>info ld</B
>" may help too.
</P
><P
>&#13; <EM
>piggy.o</EM
> has been unzipped and control is passed to
__KERNEL_CS:100000, i.e.
<EM
>linux/arch/i386/kernel/head.S:startup_32()</EM
>.
See <A
HREF="kernel_head.html"
>Section 6</A
>.
</P
><P
>&#13;<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>#define LOW_BUFFER_START 0x2000
#define LOW_BUFFER_MAX 0x90000
#define HEAP_SIZE 0x3000
///////////////////////////////////////////////////////////////////////////////
asmlinkage int decompress_kernel(struct moveparams *mv, void *rmode)
|-- setup real_mode(=rmode), vidmem, vidport, lines and cols;
|-- if (is_zImage) setup_normal_output_buffer() {
| output_data = 0x100000;
| free_mem_end_ptr = real_mode;
| } else (is_bzImage) setup_output_buffer_if_we_run_high(mv) {
| output_data = LOW_BUFFER_START;
| low_buffer_end = MIN(real_mode, LOW_BUFFER_MAX) &#38; ~0xfff;
| low_buffer_size = low_buffer_end - LOW_BUFFER_START;
| free_mem_end_ptr = &#38;end + HEAP_SIZE;
| // get mv-&#62;low_buffer_start and mv-&#62;high_buffer_start
| mv-&#62;low_buffer_start = LOW_BUFFER_START;
| /* To make this program work, we must have
| * high_buffer_start &#62; &#38;end+HEAP_SIZE;
| * As we will move low_buffer from LOW_BUFFER_START to 0x100000
| * (max low_buffer_size bytes) finally, we should have
| * high_buffer_start &#62; 0x100000+low_buffer_size; */
| mv-&#62;high_buffer_start = high_buffer_start
| = MAX(&#38;end+HEAP_SIZE, 0x100000+low_buffer_size);
| mv-&#62;hcount = 0 if (0x100000+low_buffer_size &#62; &#38;end+HEAP_SIZE);
| = -1 if (0x100000+low_buffer_size &#60;= &#38;end+HEAP_SIZE);
| /* mv-&#62;hcount==0 : we need not move high_buffer later,
| * as it is already at 0x100000+low_buffer_size.
| * Used by close_output_buffer_if_we_run_high() below. */
| }
|-- makecrc(); // create crc_32_tab[]
| puts("Uncompressing Linux... ");
|-- gunzip();
| puts("Ok, booting the kernel.\n");
|-- if (is_bzImage) close_output_buffer_if_we_run_high(mv) {
| // get mv-&#62;lcount and mv-&#62;hcount
| if (bytes_out &#62; low_buffer_size) {
| mv-&#62;lcount = low_buffer_size;
| if (mv-&#62;hcount)
| mv-&#62;hcount = bytes_out - low_buffer_size;
| } else {
| mv-&#62;lcount = bytes_out;
| mv-&#62;hcount = 0;
| }
| }
`-- return is_bzImage; // return value in AX</PRE
></FONT
></TD
></TR
></TABLE
>
<EM
>end</EM
> is defined in the "internal linker script" too.
</P
><P
>&#13; <EM
>decompress_kernel()</EM
> has an "asmlinkage" modifer.
In <TT
CLASS="filename"
>linux/include/linux/linkage.h</TT
>:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>#ifdef __cplusplus
#define CPP_ASMLINKAGE extern "C"
#else
#define CPP_ASMLINKAGE
#endif
#if defined __i386__
#define asmlinkage CPP_ASMLINKAGE __attribute__((regparm(0)))
#elif defined __ia64__
#define asmlinkage CPP_ASMLINKAGE __attribute__((syscall_linkage))
#else
#define asmlinkage CPP_ASMLINKAGE
#endif</PRE
></FONT
></TD
></TR
></TABLE
>
Macro "asmlinkage" will force the compiler to
pass all function arguments on the stack, in case
some optimization method may try to change this convention.
Check
<A
HREF="http://gcc.gnu.org/onlinedocs/gcc-3.3.2/gcc/Function-Attributes.html#Function%20Attributes"
TARGET="_top"
>Using the GNU Compiler Collection (GCC): Declaring Attributes of Functions</A
> (regparm) and
<A
HREF="http://kernelnewbies.org/faq/index.php3#asmlinkage"
TARGET="_top"
>Kernelnewbies FAQ: What is asmlinkage</A
> for more details.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="gunzip"
></A
>5.2. gunzip()</H2
><P
>&#13; <EM
>decompress_kernel()</EM
> calls
<EM
>gunzip() -&#62; inflate()</EM
>, which are defined in
<TT
CLASS="filename"
>linux/lib/inflate.c</TT
>,
to decompress resident kernel image to
low buffer (pointed by <EM
>output_data</EM
>) and
high buffer (pointed by <EM
>high_buffer_start</EM
>, for
<EM
>bzImage</EM
> only).
</P
><P
>&#13; The gzip file format is specified in
<A
HREF="http://www.ietf.org/rfc/rfc1952.txt"
TARGET="_top"
>RFC 1952</A
>.
<DIV
CLASS="table"
><A
NAME="AEN857"
></A
><P
><B
>Table 6. gzip file format</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><THEAD
><TR
><TH
ALIGN="LEFT"
VALIGN="MIDDLE"
>Component</TH
><TH
ALIGN="LEFT"
VALIGN="MIDDLE"
>Meaning</TH
><TH
ALIGN="LEFT"
VALIGN="MIDDLE"
>Byte</TH
><TH
ALIGN="LEFT"
VALIGN="MIDDLE"
>Comment</TH
></TR
></THEAD
><TBODY
><TR
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>ID1</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>IDentification 1</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>1</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>31 (0x1f, \037)</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>ID2</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>IDentification 2</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>1</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>139 (0x8b, \213)
<A
NAME="AEN877"
HREF="#FTN.AEN877"
><SPAN
CLASS="footnote"
>[a]</SPAN
></A
>
</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>CM</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>Compression Method</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>1</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>8 - denotes the "deflate" compression method</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>FLG</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>FLaGs</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>1</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>0 for most cases</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>MTIME</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>Modification TIME</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>4</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>modification time of the original file</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>XFL</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>eXtra FLags</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>1</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>2 - compressor used maximum compression, slowest algorithm
<A
NAME="AEN899"
HREF="#FTN.AEN899"
><SPAN
CLASS="footnote"
>[b]</SPAN
></A
>
</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>OS</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>Operating System</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>1</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>3 - Unix</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>extra fields</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>-</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>-</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>variable length, field indicated by FLG
<A
NAME="AEN911"
HREF="#FTN.AEN911"
><SPAN
CLASS="footnote"
>[c]</SPAN
></A
>
</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>compressed blocks</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>-</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>-</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>variable length</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>CRC32</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>-</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>4</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>CRC value of the uncompressed data</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>ISIZE</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>Input SIZE</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>4</TD
><TD
ALIGN="LEFT"
VALIGN="MIDDLE"
>the size of the uncompressed input data modulo 2^32</TD
></TR
></TBODY
><TR
><TD
COLSPAN="4"
>Notes:<BR><A
NAME="FTN.AEN877"
>a. </A
>
ID2 value can be 158 (0x9e, \236) for gzip 0.5;
<BR><A
NAME="FTN.AEN899"
>b. </A
>
XFL value 4 - compressor used fastest algorithm;
<BR><A
NAME="FTN.AEN911"
>c. </A
>
FLG bit 0, FTEXT, does not indicate any "extra field".
<BR></TD
></TR
></TABLE
></DIV
>
</P
><P
>&#13; We can use this file format knowledge to find out
the beginning of gzipped <TT
CLASS="filename"
>linux/vmlinux</TT
>.
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="screen"
><B
CLASS="command"
>[root@localhost boot]# hexdump -C /boot/vmlinuz-2.4.20-28.9 | grep '1f 8b 08 00'</B
>
00004c50 1f 8b 08 00 01 f6 e1 3f 02 03 ec 5d 7d 74 14 55 |.......?...]}t.U|
<B
CLASS="command"
>[root@localhost boot]# hexdump -C /boot/vmlinuz-2.4.20-28.9 -s 0x4c40 -n 64</B
>
00004c40 00 80 0b 00 00 fc 21 00 68 00 00 00 1e 01 11 00 |......!.h.......|
00004c50 1f 8b 08 00 01 f6 e1 3f 02 03 ec 5d 7d 74 14 55 |.......?...]}t.U|
00004c60 96 7f d5 a9 d0 1d 4d ac 56 93 35 ac 01 3a 9c 6a |......M.V.5..:.j|
00004c70 4d 46 5c d3 7b f8 48 36 c9 6c 84 f0 25 88 20 9f |MF\.{.H6.l..%. .|
00004c80
<B
CLASS="command"
>[root@localhost boot]# hexdump -C /boot/vmlinuz-2.4.20-28.9 | tail -n 4</B
>
00114d40 bd 77 66 da ce 6f 3d d6 33 5c 14 a2 9f 7e fa e9 |.wf..o=.3\...~..|
00114d50 a7 9f 7e fa ff 57 3f 00 00 00 00 00 d8 bc ab ea |..~..W?.........|
00114d60 44 5d 76 d1 fd 03 33 58 c2 f0 00 51 27 00 |D]v...3X...Q'.|
00114d6e</PRE
></FONT
></TD
></TR
></TABLE
>
We can see that the gzipped file begins at 0x4c50 in the above example.
The four bytes before "1f 8b 08 00" is <EM
>input_len</EM
>
(0x0011011e, in little endian), and 0x4c50+0x0011011e=0x114d6e equals to
the size of <EM
>bzImage</EM
>
(<TT
CLASS="filename"
>/boot/vmlinuz-2.4.20-28.9</TT
>).
</P
><P
>&#13;<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>static uch *inbuf; /* input buffer */
static unsigned insize = 0; /* valid bytes in inbuf */
static unsigned inptr = 0; /* index of next byte to be processed in inbuf */
///////////////////////////////////////////////////////////////////////////////
static int gunzip(void)
{
Check input buffer for {ID1, ID2, CM}, must be
{0x1f, 0x8b, 0x08} (normal case), or
{0x1f, 0x9e, 0x08} (for gzip 0.5);
Check FLG (flag byte), must not set bit 1, 5, 6 and 7;
Ignore {MTIME, XFL, OS};
Handle optional structures, which correspond to FLG bit 2, 3 and 4;
inflate(); // handle compressed blocks
Validate {CRC32, ISIZE};
}</PRE
></FONT
></TD
></TR
></TABLE
>
When <EM
>get_byte()</EM
>, defined in
<TT
CLASS="filename"
>linux/arch/i386/boot/compressed/misc.c</TT
>,
is called for the first time,
it calls <EM
>fill_inbuf()</EM
> to setup input buffer
<EM
>inbuf=input_data</EM
> and
<EM
>insize=input_len</EM
>.
Symbol <EM
>input_data</EM
> and
<EM
>input_len</EM
> are defined in
<EM
>piggy.o</EM
> linker script.
See <A
HREF="makefiles.html#i386_boot_compressed_makefile"
>Section 2.5</A
>.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="inflate"
></A
>5.3. inflate()</H2
><P
>&#13;<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>// some important definitions in misc.c
#define WSIZE 0x8000 /* Window size must be at least 32k,
* and a power of two */
static uch window[WSIZE]; /* Sliding window buffer */
static unsigned outcnt = 0; /* bytes in output buffer */
// linux/lib/inflate.c
#define wp outcnt
#define flush_output(w) (wp=(w),flush_window())
STATIC unsigned long bb; /* bit buffer */
STATIC unsigned bk; /* bits in bit buffer */
STATIC unsigned hufts; /* track memory usage */
static long free_mem_ptr = (long)&#38;end;
///////////////////////////////////////////////////////////////////////////////
STATIC int inflate()
{
int e; /* last block flag */
int r; /* result code */
unsigned h; /* maximum struct huft's malloc'ed */
void *ptr;
wp = bb = bk = 0;
// inflate compressed blocks one by one
do {
hufts = 0;
gzip_mark() { ptr = free_mem_ptr; };
if ((r = inflate_block(&#38;e)) != 0) {
gzip_release() { free_mem_ptr = ptr; };
return r;
}
gzip_release() { free_mem_ptr = ptr; };
if (hufts &#62; h)
h = hufts;
} while (!e);
/* Undo too much lookahead. The next read will be byte aligned so we
* can discard unused bits in the last meaningful byte. */
while (bk &#62;= 8) {
bk -= 8;
inptr--;
}
/* write the output window window[0..outcnt-1] to output_data,
* update output_ptr/output_data, crc and bytes_out accordingly, and
* reset outcnt to 0. */
flush_output(wp);
/* return success */
return 0;
}</PRE
></FONT
></TD
></TR
></TABLE
>
<EM
>free_mem_ptr</EM
> is used in
<EM
>misc.c:malloc()</EM
> for dynamic memory allocation.
Before inflating each compressed block, <EM
>gzip_mark()</EM
>
saves the value of <EM
>free_mem_ptr</EM
>;
After inflation, <EM
>gzip_release()</EM
> will
restore this value.
This is how it "<EM
>free()</EM
>" the memory allocated in
<EM
>inflate_block()</EM
>.
</P
><P
>&#13; <A
HREF="http://www.gzip.org"
TARGET="_top"
>Gzip</A
> uses
Lempel-Ziv coding (LZ77) to compress files.
The compressed data format is specified in
<A
HREF="http://www.ietf.org/rfc/rfc1951.txt"
TARGET="_top"
>RFC 1951</A
>.
<EM
>inflate_block()</EM
> will inflate compressed blocks,
which can be treated as a bit sequence.
</P
><P
>&#13; The data structure of each compressed block is outlined below:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>BFINAL (1 bit)
0 - not the last block
1 - the last block
BTYPE (2 bits)
00 - no compression
remaining bits until the byte boundary;
LEN (2 bytes);
NLEN (2 bytes, the one's complement of LEN);
data (LEN bytes);
01 - compressed with fixed Huffman codes
{
literal (7-9 bits, represent code 0..287, excluding 256);
// See RFC 1951, table in Paragraph 3.2.6.
length (0-5 bits if literal &#62; 256, represent length 3..258);
// See RFC 1951, 1st alphabet table in Paragraph 3.2.5.
data (of literal bytes if literal &#60; 256);
distance (5 plus 0-13 extra bits if literal == 257..285, represent
distance 1..32768);
/* See RFC 1951, 2nd alphabet table in Paragraph 3.2.5,
* but statement in Paragraph 3.2.6. */
/* Move backward "distance" bytes in the output stream,
* and copy "length" bytes */
}* // can be of multiple instances
literal (7 bits, all 0, literal == 256, means end of block);
10 - compressed with dynamic Huffman codes
HLIT (5 bits, # of Literal/Length codes - 257, 257-286);
HDIST (5 bits, # of Distance codes - 1, 1-32);
HCLEN (4 bits, # of Code Length codes - 4, 4 - 19);
Code Length sequence ((HCLEN+4)*3 bits)
/* The following two alphabet tables will be decoded using
* the Huffman decoding table which is generated from
* the preceeding Code Length sequence. */
Literal/Length alphabet (HLIT+257 codes)
Distance alphabet (HDIST+1 codes)
// Decoding tables will be built from these alphpabet tables.
/* The following is similar to that of fixed Huffman codes portion,
* except that they use different decoding tables. */
{
literal/length
(variable length, depending on Literal/Length alphabet);
data (of literal bytes if literal &#60; 256);
distance (variable length if literal == 257..285, depending on
Distance alphabet);
}* // can be of multiple instances
literal (literal value 256, which means end of block);
11 - reserved (error)</PRE
></FONT
></TD
></TR
></TABLE
>
Note that data elements are packed into bytes starting from
Least-Significant Bit (LSB) to Most-Significant Bit (MSB), while
Huffman codes are packed starting with MSB.
Also note that <EM
>literal</EM
> value 286-287 and
<EM
>distance</EM
> codes 30-31 will never actually occur.
</P
><P
>&#13; With the above data structure in mind and RFC 1951 by hand,
it is not too hard to understand <EM
>inflate_block()</EM
>.
Refer to related paragraphs in RFC 1951 for Huffman coding and
alphabet table generation.
</P
><P
>&#13; For more details, refer to <TT
CLASS="filename"
>linux/lib/inflate.c</TT
>,
gzip source code (many in-line comments) and related reference materials.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="chead_ref"
></A
>5.4. Reference</H2
><P
>&#13; <P
></P
><UL
><LI
><P
>&#13; <A
HREF="http://www.gnu.org/software/binutils/manual/gas-2.9.1/"
TARGET="_top"
>&#13; Using as</A
>
</P
></LI
><LI
><P
>&#13; <A
HREF="http://www.gnu.org/software/binutils/manual/ld-2.9.1/"
TARGET="_top"
>&#13; Using LD, the GNU linker</A
>
</P
></LI
><LI
><P
><A
HREF="http://developer.intel.com/design/pentium4/manuals/"
TARGET="_top"
>&#13; IA-32 Intel Architecture Software Developer's Manual</A
></P
></LI
><LI
><P
><A
HREF="http://www.gzip.org"
TARGET="_top"
>&#13; The gzip home page</A
></P
></LI
><LI
><P
><A
HREF="http://freshmeat.net/projects/gzip"
TARGET="_top"
>&#13; gzip (freshmeat.net)</A
></P
></LI
><LI
><P
>&#13; <A
HREF="http://www.ietf.org/rfc/rfc1951.txt"
TARGET="_top"
>&#13; RFC 1951: DEFLATE Compressed Data Format Specification version 1.3
</A
>
</P
></LI
><LI
><P
>&#13; <A
HREF="http://www.ietf.org/rfc/rfc1952.txt"
TARGET="_top"
>&#13; RFC 1952: GZIP file format specification version 4.3
</A
>
</P
></LI
></UL
>
</P
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