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><DIV
CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="bootsect"
></A
>3. linux/arch/i386/boot/bootsect.S</H1
><P
> Given that we are booting up <EM
>bzImage</EM
>, which is
composed of <EM
>bbootsect</EM
>, <EM
>bsetup</EM
>
and <EM
>bvmlinux (head.o, misc.o, piggy.o)</EM
>,
the first floppy sector, <EM
>bbootsect</EM
> (512 bytes),
which is compiled from <TT
CLASS="filename"
>linux/arch/i386/boot/bootsect.S</TT
>,
is loaded by BIOS to 07C0:0.
The reset of <EM
>bzImage</EM
> (<EM
>bsetup</EM
>
and <EM
>bvmlinux</EM
>) has not been loaded yet.
</P
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="move_bootsect"
></A
>3.1. Move Bootsect</H2
><P
> <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>SETUPSECTS = 4 /* default nr of setup-sectors */
BOOTSEG = 0x07C0 /* original address of boot-sector */
INITSEG = DEF_INITSEG (0x9000) /* we move boot here - out of the way */
SETUPSEG = DEF_SETUPSEG (0x9020) /* setup starts here */
SYSSEG = DEF_SYSSEG (0x1000) /* system loaded at 0x10000 (65536) */
SYSSIZE = DEF_SYSSIZE (0x7F00) /* system size: # of 16-byte clicks */
/* to be loaded */
ROOT_DEV = 0 /* ROOT_DEV is now written by "build" */
SWAP_DEV = 0 /* SWAP_DEV is now written by "build" */
.code16
.text
///////////////////////////////////////////////////////////////////////////////
_start:
{
// move ourself from 0x7C00 to 0x90000 and jump there.
move BOOTSEG:0 to INITSEG:0 (512 bytes);
goto INITSEG:go;
}</PRE
></FONT
></TD
></TR
></TABLE
>
<EM
>bbootsect</EM
> has been moved to INITSEG:0 (0x9000:0).
Now we can forget BOOTSEG.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="get_disk_para"
></A
>3.2. Get Disk Parameters</H2
><P
> <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>///////////////////////////////////////////////////////////////////////////////
// prepare stack and disk parameter table
go:
{
SS:SP = INITSEG:3FF4; // put stack at INITSEG:0x4000-12
/* 0x4000 is an arbitrary value >=
* length of bootsect + length of setup + room for stack;
* 12 is disk parm size. */
copy disk parameter (pointer in 0:0078) to INITSEG:3FF4 (12 bytes);
// <A
HREF="http://www.ctyme.com/intr/rb-2445.htm"
TARGET="_top"
>int1E: SYSTEM DATA - DISKETTE PARAMETERS</A
>
patch sector count to 36 (offset 4 in parameter table, 1 byte);
set disk parameter table pointer (0:0078, int1E) to INITSEG:3FF4;
}</PRE
></FONT
></TD
></TR
></TABLE
>
Make sure SP is initialized immediately after SS register.
The recommended method of modifying SS is to use "lss" instruction
according to
<A
HREF="http://developer.intel.com/design/pentium4/manuals/"
TARGET="_top"
> IA-32 Intel Architecture Software Developer's Manual</A
>
(Vol.3. Ch.5.8.3. Masking Exceptions and Interrupts When Switching Stacks).
</P
><P
> Stack operations, such as push and pop, will be OK now.
First 12 bytes of disk parameter have been copied to INITSEG:3FF4.
</P
><P
> <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>///////////////////////////////////////////////////////////////////////////////
// get disk drive parameters, specifically number of sectors/track.
char disksizes[] = {36, 18, 15, 9};
int sectors;
{
SI = disksizes; // i = 0;
do {
probe_loop:
sectors = DS:[SI++]; // sectors = disksizes[i++];
if (SI>=disksizes+4) break; // if (i>=4) break;
int13/AH=02h(AL=1, ES:BX=INITSEG:0200, CX=sectors, DX=0);
// <A
HREF="http://www.ctyme.com/intr/rb-0607.htm"
TARGET="_top"
>int13/AH=02h: DISK - READ SECTOR(S) INTO MEMORY</A
>
} while (failed to read sectors);
}</PRE
></FONT
></TD
></TR
></TABLE
>
"lodsb" loads a byte from DS:[SI] to AL and increases SI automatically.
</P
><P
> The number of sectors per track has been saved in variable
<EM
>sectors</EM
>.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="load_setup"
></A
>3.3. Load Setup Code</H2
><P
> <EM
>bsetup</EM
> (<EM
>setup_sects</EM
> sectors)
will be loaded right after <EM
>bbootsect</EM
>, i.e. SETUPSEG:0.
Note that INITSEG:0200==SETUPSEG:0 and
<EM
>setup_sects</EM
> has been changed
by <B
CLASS="command"
>tools/build</B
> to match
<EM
>bsetup</EM
> size
in <A
HREF="makefiles.html#i386_tools_build.c"
>Section 2.6</A
>.
</P
><P
> <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>///////////////////////////////////////////////////////////////////////////////
got_sectors:
word sread; // sectors read for current track
char setup_sects; // overwritten by tools/build
{
print out "Loading";
/* <A
HREF="http://www.ctyme.com/intr/rb-0088.htm"
TARGET="_top"
>int10/AH=03h(BH=0): VIDEO - GET CURSOR POSITION AND SIZE</A
>
* <A
HREF="http://www.ctyme.com/intr/rb-0210.htm"
TARGET="_top"
>int10/AH=13h(AL=1, BH=0, BL=7, CX=9, DH=DL=0, ES:BP=INITSEG:$msg1):</A
>
* <A
HREF="http://www.ctyme.com/intr/rb-0210.htm"
TARGET="_top"
>VIDEO - WRITE STRING</A
> */
// load setup-sectors directly after the moved bootblock (at 0x90200).
SI = &sread; // using SI to index sread, head and track
sread = 1; // the boot sector has already been read
int13/AH=00h(DL=0); // <A
HREF="http://www.ctyme.com/intr/rb-0605.htm"
TARGET="_top"
>reset FDC</A
>
BX = 0x0200; // read bsetup right after bbootsect (512 bytes)
do {
next_step:
/* to prevent cylinder crossing reading,
* calculate how many sectors to read this time */
uint16 pushw_ax = AX = MIN(sectors-sread, setup_sects);
no_cyl_crossing:
read_track(AL, ES:BX); // AX is not modified
// set ES:BX, sread, head and track for next read_track()
set_next(AX);
setup_sects -= pushw_ax; // rest - for next step
} while (setup_sects);
}</PRE
></FONT
></TD
></TR
></TABLE
>
SI is set to the address of <EM
>sread</EM
> to index
variables <EM
>sread</EM
>, <EM
>head</EM
> and
<EM
>track</EM
>, as they are contiguous in memory.
Check <A
HREF="bootsect.html#read_disk"
>Section 3.6</A
> for read_track() and set_next() details.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="load_compressed"
></A
>3.4. Load Compressed Image</H2
><P
> <EM
>bvmlinux (head.o, misc.o, piggy.o)</EM
> will be loaded
at 0x100000, <EM
>syssize</EM
>*16 bytes.
</P
><P
> <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>///////////////////////////////////////////////////////////////////////////////
// load vmlinux/bvmlinux (head.o, misc.o, piggy.o)
{
read_it(ES=SYSSEG);
kill_motor(); // turn off floppy drive motor
print_nl(); // print CR LF
}</PRE
></FONT
></TD
></TR
></TABLE
>
Check <A
HREF="bootsect.html#read_disk"
>Section 3.6</A
> for read_it() details.
If we are booting up <EM
>zImage</EM
>,
<EM
>vmlinux</EM
> is loaded at 0x10000 (SYSSEG:0).
</P
><P
> <EM
>bzImage (bbootsect, bsetup, bvmlinux)</EM
> is
in the memory as a whole now.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="go_setup"
></A
>3.5. Go Setup</H2
><P
> <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>///////////////////////////////////////////////////////////////////////////////
// check which root-device to use and jump to setup.S
int root_dev; // overwritten by tools/build
{
if (!root_dev) {
switch (sectors) {
case 15: root_dev = 0x0208; // /dev/ps0 - 1.2Mb
break;
case 18: root_dev = 0x021C; // /dev/PS0 - 1.44Mb
break;
case 36: root_dev = 0x0220; // /dev/fd0H2880 - 2.88Mb
break;
default: root_dev = 0x0200; // /dev/fd0 - auto detect
break;
}
}
// jump to the setup-routine loaded directly after the bootblock
goto SETUPSEG:0;
}</PRE
></FONT
></TD
></TR
></TABLE
>
It passes control to <EM
>bsetup</EM
>.
See <EM
>linux/arch/i386/boot/setup.S:start</EM
> in
<A
HREF="setup.html"
>Section 4</A
>.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="read_disk"
></A
>3.6. Read Disk</H2
><P
> The following functions are used to load <EM
>bsetup</EM
>
and <EM
>bvmlinux</EM
> from disk.
Note that <EM
>syssize</EM
> has been changed
by <B
CLASS="command"
>tools/build</B
> in
<A
HREF="makefiles.html#i386_tools_build.c"
>Section 2.6</A
> too.
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>sread: .word 0 # sectors read of current track
head: .word 0 # current head
track: .word 0 # current track
///////////////////////////////////////////////////////////////////////////////
// load the system image at address SYSSEG:0
read_it(ES=SYSSEG)
int syssize; /* system size in 16-bytes,
* overwritten by tools/build */
{
if (ES & 0x0fff) die; // not 64KB aligned
BX = 0;
for (;;) {
rp_read:
#ifdef __BIG_KERNEL__
bootsect_helper(ES:BX);
/* INITSEG:0220==SETUPSEG:0020 is bootsect_kludge,
* which contains pointer SETUPSEG:bootsect_helper().
* This function initializes some data structures
* when it is called for the first time,
* and moves SYSSEG:0 to 0x100000, 64KB each time,
* in the following calls.
* See <A
HREF="bootsect.html#bootsect_helper"
>Section 3.7</A
>. */
#else
AX = ES - SYSSEG + ( BX >> 4); // how many 16-bytes read
#endif
if (AX > syssize) return; // everything loaded
ok1_read:
/* Get proper AL (sectors to read) for this time
* to prevent cylinder crossing reading and BX overflow. */
AX = sectors - sread;
CX = BX + (AX << 9); // 1 sector = 2^9 bytes
if (CX overflow && CX!=0) { // > 64KB
AX = (-BX) >> 9;
}
ok2_read:
read_track(AL, ES:BX);
set_next(AX);
}
}
///////////////////////////////////////////////////////////////////////////////
// read disk with parameters (sread, track, head)
read_track(AL sectors, ES:BX destination)
{
for (;;) {
printf(".");
// <A
HREF="http://www.ctyme.com/intr/rb-0106.htm"
TARGET="_top"
>int10/AH=0Eh: VIDEO - TELETYPE OUTPUT</A
>
// set CX, DX according to (sread, track, head)
DX = track;
CX = sread + 1;
CH = DL;
DX = head;
DH = DL;
DX &= 0x0100;
int13/AH=02h(AL, ES:BX, CX, DX);
// <A
HREF="http://www.ctyme.com/intr/rb-0607.htm"
TARGET="_top"
>int13/AH=02h: DISK - READ SECTOR(S) INTO MEMORY</A
>
if (read disk success) return;
// "addw $8, %sp" is to cancel previous 4 "pushw" operations.
bad_rt:
print_all(); // print error code, AX, BX, CX and DX
int13/AH=00h(DL=0); // <A
HREF="http://www.ctyme.com/intr/rb-0605.htm"
TARGET="_top"
>reset FDC</A
>
}
}
///////////////////////////////////////////////////////////////////////////////
// set ES:BX, sread, head and track for next read_track()
set_next(AX sectors_read)
{
CX = AX; // sectors read
AX += sread;
if (AX==sectors) {
head = 1 ^ head; // flap head between 0 and 1
if (head==0) track++;
ok4_set:
AX = 0;
}
ok3_set:
sread = AX;
BX += CX && 9;
if (BX overflow) { // > 64KB
ES += 0x1000;
BX = 0;
}
set_next_fn:
}</PRE
></FONT
></TD
></TR
></TABLE
>
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="bootsect_helper"
></A
>3.7. Bootsect Helper</H2
><P
> <EM
>setup.S:bootsect_helper()</EM
> is only used by
<EM
>bootsect.S:read_it()</EM
>.
</P
><P
> Because <EM
>bbootsect</EM
> and <EM
>bsetup</EM
>
are linked separately, they use offsets relative to
their own code/data segments.
We have to "call far" (lcall) for <EM
>bootsect_helper()</EM
>
in different segment, and it must "return far" (lret) then.
This results in CS change in calling, which makes CS!=DS, and
we have to use segment modifier to specify variables in
<TT
CLASS="filename"
>setup.S</TT
>.
</P
><P
> <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>///////////////////////////////////////////////////////////////////////////////
// called by bootsect loader when loading bzImage
bootsect_helper(ES:BX)
bootsect_es = 0; // defined in setup.S
type_of_loader = 0; // defined in setup.S
{
if (!bootsect_es) { // called for the first time
type_of_loader = 0x20; // bootsect-loader, version 0
AX = ES >> 4;
*(byte*)(&bootsect_src_base+2) = AH;
bootsect_es = ES;
AX = ES - SYSSEG;
return;
}
bootsect_second:
if (!BX) { // 64KB full
// move from SYSSEG:0 to destination, 64KB each time
int15/AH=87h(CX=0x8000, ES:SI=CS:bootsect_gdt);
// <A
HREF="http://www.ctyme.com/intr/rb-1527.htm"
TARGET="_top"
>int15/AH=87h: SYSTEM - COPY EXTENDED MEMORY</A
>
if (failed to copy) {
bootsect_panic() {
prtstr("INT15 refuses to access high mem, "
"giving up.");
bootsect_panic_loop: goto bootsect_panic_loop; // never return
}
}
ES = bootsect_es; // reset ES to always point to 0x10000
*(byte*)(&bootsect_dst_base+2)++;
}
bootsect_ex:
// have the number of moved frames (16-bytes) in AX
AH = *(byte*)(&bootsect_dst_base+2) << 4;
AL = 0;
}
///////////////////////////////////////////////////////////////////////////////
// data used by bootsect_helper()
bootsect_gdt:
.word 0, 0, 0, 0
.word 0, 0, 0, 0
bootsect_src:
.word 0xffff
bootsect_src_base:
.byte 0x00, 0x00, 0x01 # base = 0x010000
.byte 0x93 # typbyte
.word 0 # limit16,base24 =0
bootsect_dst:
.word 0xffff
bootsect_dst_base:
.byte 0x00, 0x00, 0x10 # base = 0x100000
.byte 0x93 # typbyte
.word 0 # limit16,base24 =0
.word 0, 0, 0, 0 # BIOS CS
.word 0, 0, 0, 0 # BIOS DS
bootsect_es:
.word 0
bootsect_panic_mess:
.string "INT15 refuses to access high mem, giving up."</PRE
></FONT
></TD
></TR
></TABLE
>
Note that <EM
>type_of_loader</EM
> value is changed.
It will be referenced in <A
HREF="setup.html#check_loader"
>Section 4.3</A
>.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="bootsect_misc"
></A
>3.8. Miscellaneous</H2
><P
> The rest are supporting functions, variables
and part of "real-mode kernel header".
Note that data is in .text segment as code, thus it can be
properly initialized when loaded.
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>///////////////////////////////////////////////////////////////////////////////
// some small functions
print_all(); /* print error code, AX, BX, CX and DX */
print_nl(); /* print CR LF */
print_hex(); /* print the word pointed to by SS:BP in hexadecimal */
kill_motor() /* turn off floppy drive motor */
{
#if 1
int13/AH=00h(DL=0); // <A
HREF="http://www.ctyme.com/intr/rb-0605.htm"
TARGET="_top"
>reset FDC</A
>
#else
outb(0, 0x3F2); // outb(val, port)
#endif
}
///////////////////////////////////////////////////////////////////////////////
sectors: .word 0
disksizes: .byte 36, 18, 15, 9
msg1: .byte 13, 10
.ascii "Loading"</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
> Bootsect trailer, which is a part of "real-mode kernel header",
begins at offset 497.
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>.org 497
setup_sects: .byte SETUPSECS // overwritten by tools/build
root_flags: .word ROOT_RDONLY
syssize: .word SYSSIZE // overwritten by tools/build
swap_dev: .word SWAP_DEV
ram_size: .word RAMDISK
vid_mode: .word SVGA_MODE
root_dev: .word ROOT_DEV // overwritten by tools/build
boot_flag: .word 0xAA55</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
> This "header" must conform to the layout pattern in
<TT
CLASS="filename"
>linux/Documentation/i386/boot.txt</TT
>:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="programlisting"
>Offset Proto Name Meaning
/Size
01F1/1 ALL setup_sects The size of the setup in sectors
01F2/2 ALL root_flags If set, the root is mounted readonly
01F4/2 ALL syssize DO NOT USE - for bootsect.S use only
01F6/2 ALL swap_dev DO NOT USE - obsolete
01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
01FA/2 ALL vid_mode Video mode control
01FC/2 ALL root_dev Default root device number
01FE/2 ALL boot_flag 0xAA55 magic number</PRE
></FONT
></TD
></TR
></TABLE
>
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="bootsect_ref"
></A
>3.9. Reference</H2
><P
> <P
></P
><UL
><LI
><P
>THE LINUX/I386 BOOT PROTOCOL:
<TT
CLASS="filename"
>linux/Documentation/i386/boot.txt</TT
></P
></LI
><LI
><P
><A
HREF="http://developer.intel.com/design/pentium4/manuals/"
TARGET="_top"
> IA-32 Intel Architecture Software Developer's Manual</A
></P
></LI
><LI
><P
><A
HREF="http://www.cs.cmu.edu/~ralf/files.html"
TARGET="_top"
> Ralf Brown's Interrupt List</A
></P
></LI
></UL
>
As <IA-32 Intel Architecture Software Developer's Manual>
is widely referenced in this document, I will call it "IA-32 Manual"
for short.
</P
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