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(2006-08-06) rescue-bootcd

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Valeriano A.R.
2006-08-06 00:00:00 +02:00
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$Id: comboot.doc,v 1.34 2005/01/20 18:43:22 hpa Exp $
COMBOOT and COM32 files
SYSLINUX supports simple standalone programs, using a file format
similar to DOS ".com" files. A 32-bit version, called COM32, is also
provided. A simple API provides access to a limited set of filesystem
and console functions.
++++ COMBOOT file format ++++
A COMBOOT file is a raw binary file containing 16-bit code. It should
be linked to run at offset 0x100, and contain no absolute segment
references. It is run in 16-bit real mode.
A COMBOOT image can be written to be compatible with MS-DOS. Such a
file will usually have extension ".com". A COMBOOT file which is not
compatible with MS-DOS will usually have extension ".cbt".
Before running the program, SYSLINUX sets up the following fields in
the Program Segment Prefix (PSP), a structure at offset 0 in the
program segment:
Offset Size Meaning
0 word Contains an INT 20h instruction
2 word Contains the paragraph (16-byte "segment" address) at
the end of memory available to the program.
128 byte Length of the command line arguments, including the leading
space but not including the final CR character.
129 127b Command line arguments, starting with a space and ending
with a CR character (ASCII 13).
The program is allowed to use memory between the PSP paragraph (which
all the CS, DS, ES and SS registers point to at program start) and the
paragraph value given at offset 2.
On startup, SP is set up to point to the end of the 64K segment, at
0xfffe. Under DOS it is possible for SP to contain a smaller
value if memory is very tight; this is never the case under SYSLINUX.
The program should make no assumptions about what segment address it
will be loaded at; instead it should look at the segment registers on
program startup. Both DOS and SYSLINUX will guarantee CS == DS == ES
== SS on program start; the program should not assume anything about
the values of FS or GS.
To exit, a program can either execute a near RET (which will jump to
offset 0 which contains an INT 20h instruction, terminating the
program), or execute INT 20h or INT 21h AH=00h or INT 21h AH=4Ch.
If compatiblity with SYSLINUX 1.xx is desired, use INT 20h.
++++ COM32 file format ++++
A COM32 file is a raw binary file containing 32-bit code. It should
be linked to run at address 0x101000, and should not contain any
segment references. It will be run in flat-memory 32-bit protected
mode. Under SYSLINUX, it will be run in CPL 0, however, since it may
be possible to create a COM32 execution engine that would run under
something like Linux DOSEMU, it is recommended that the code does not
assume CPL 0 unless absolutely necessary.
It is highly recommended that every COM32 program begins with the byte
sequence B8 FF 4C CD 21 (mov eax,21cd4cffh) as a magic number.
A COM32 file should have extension ".c32".
On startup, CS will be set up as a flat 32-bit code segment, and DS ==
ES == SS will be set up as the equivalent flat 32-bit data segment.
FS and GS are reserved for future use and are currently initialized to
zero. A COM32 image should not assume any particular values of
segment selectors.
ESP is set up at the end of available memory and also serves as
notification to the program how much memory is available.
The following arguments are passed to the program on the stack:
Address Size Meaning
[ESP] dword Return (termination) address
[ESP+4] dword Number of additional arguments (currently 5)
[ESP+8] dword Pointer to the command line arguments (null-terminated string)
[ESP+12] dword Pointer to INT call helper function
[ESP+16] dword Pointer to low memory bounce buffer
[ESP+20] dword Size of low memory bounce buffer
[ESP+24] dword Pointer to FAR call helper function (new in 2.05)
This corresponds to the following C prototype, available in the file
com32/include/com32.h:
/* The standard prototype for _start() */
int _start(unsigned int __nargs,
char *__cmdline,
void (*__intcall)(uint8_t, com32sys_t *, com32sys_t *),
void *__bounce_ptr,
unsigned int __bounce_len,
void (*__farcall)(uint32_t, uint16_t, com32sys_t *, com32sys_t *));
The intcall helper function can be used to issue BIOS or SYSLINUX API
calls, and takes the interrupt number as first argument. The second
argument is a pointer to the input register definition, an instance of
the following structure (also available in com32.h):
typedef union {
uint32_t l;
uint16_t w[2];
uint8_t b[4];
} reg32_t;
typedef struct {
uint16_t gs; /* Offset 0 */
uint16_t fs; /* Offset 2 */
uint16_t es; /* Offset 4 */
uint16_t ds; /* Offset 6 */
reg32_t edi; /* Offset 8 */
reg32_t esi; /* Offset 12 */
reg32_t ebp; /* Offset 16 */
reg32_t _unused; /* Offset 20 */
reg32_t ebx; /* Offset 24 */
reg32_t edx; /* Offset 28 */
reg32_t ecx; /* Offset 32 */
reg32_t eax; /* Offset 36 */
reg32_t eflags; /* Offset 40 */
} com32sys_t;
The third argument is a pointer to the output register definition, an
instance of the same structure. The third argument can also be zero
(NULL).
Since BIOS or SYSLINUX API calls can generally only manipulate data
below address 0x100000, a "bounce buffer" in low memory, at least 64K
in size, is available, to copy data in and out.
The farcall helper function behaves similarly, but takes as its first
argument the CS:IP (in the form (CS << 16) + IP) of procedure to be
invoked via a FAR CALL.
++++ SYSLINUX API CALLS +++
SYSLINUX provides the following API calls. SYSLINUX 1.xx only
supported INT 20h - terminate program. [] indicates the first version
of SYSLINUX which supported this feature (correctly.)
NOTE: Most of the API functionality is still experimental. Expect to
find bugs.
++++ DOS-COMPATIBLE API CALLS ++++
INT 20h [1.48] Terminate program
INT 21h AH=00h [2.00] Terminate program
INT 21h AH=4Ch [2.00] Terminate program
All of these terminate the program.
INT 21h AH=01h [2.01] Get Key with Echo
Reads a key from the console input, with echo to the console
output. The read character is returned in AL. Extended
characters received from the keyboard are returned as NUL (00h)
+ the extended character code.
INT 21h AH=02h [2.01] Write Character
Writes a character in DL to the console (video and serial)
output.
INT 21h AH=04h [2.01] Write Character to Serial Port
Writes a character in DL to the serial console output
(if enabled.) If no serial port is configured, this routine
does nothing.
INT 21h AH=08h [2.09] Get Key without Echo
Reads a key fron the console input, without echoing it to the
console output. The read character is returned in AL.
INT 21h AH=09h [2.01] Write DOS String to Console
Writes a DOS $-terminated string in DS:DX to the console.
INT 21h AH=0Bh [2.00] Check Keyboard
Returns AL=FFh if there is a keystroke waiting (which can then
be read with INT 21h, AH=01h or AH=08h), otherwise AL=00h.
INT 21h AH=30h [2.00] Check DOS Version
This function returns AX=BX=CX=DX=0, corresponding to a
hypothetical "DOS 0.0", but the high parts of EAX-EBX-ECX-EDX
spell "SYSLINUX":
EAX=59530000h EBX=4C530000h ECX=4E490000h EDX=58550000h
This function can thus be used to distinguish running on
SYSLINUX from running on DOS.
++++ SYSLINUX-SPECIFIC API CALLS ++++
SYSLINUX-specific API calls are executed using INT 22h, with a
function number in AX. INT 22h is used by DOS for internal purposes;
do not execute INT 22h under DOS.
DOS-compatible function INT 21h, AH=30h can be used to detect if the
SYSLINUX API calls are available.
Any register not specifically listed as modified is preserved;
however, future versions of SYSLINUX may add additional output
registers to existing calls.
All calls return CF=0 on success, CF=1 on failure. The noted outputs
apply if CF=0 only unless otherwise noted. All calls clobber the
arithmetric flags (CF, PF, AF, ZF, SF and OF) but leave all other
flags unchanged unless otherwise noted.
AX=0001h [2.00] Get Version
Input: AX 0001h
Output: AX number of INT 22h API functions available
CH SYSLINUX major version number
CL SYSLINUX minor version number
DL SYSLINUX derivative ID (e.g. 32h = PXELINUX)
ES:SI SYSLINUX version string
ES:DI SYSLINUX copyright string
This API call returns the SYSLINUX version and API
information.
AX=0002h [2.01] Write String
Input: AX 0002h
ES:BX null-terminated string
Output: None
Writes a null-terminated string on the console.
AX=0003h [2.01] Run command
Input: AX 0003h
ES:BX null-terminated command string
Output: Does not return
This API call terminates the program and executes the command
string as if the user had entered it at the SYSLINUX command
line. This API call does not return.
AX=0004h [2.01] Run default command
Input: AX 0004h
Output: Does not return
This API call terminates the program and executes the default
command string as if the user had pressed Enter alone on the
SYSLINUX command line. This API call does not return.
AX=0005h [2.00] Force text mode
Input: AX 0005h
Output: None
If the screen was in graphics mode (due to displaying a splash
screen using the <Ctrl-X> command in a message file, or
similar), return to text mode.
AX=0006h [2.08] Open file
Input: AX 0006h
ES:SI null-terminated filename
Output: SI file handle
EAX length of file in bytes
CX file block size
Open a file for reading. The exact syntax of the filenames
allowed depends on the particular SYSLINUX derivative.
The SYSLINUX file system is block-oriented. The size of a
block will always be a power of two and no greater than 16K.
Note: SYSLINUX considers a zero-length file to be nonexistent.
AX=0007h [2.08] Read file
Input: AX 0007h
SI file handle
ES:BX buffer
CX number of blocks to read
Output: SI file handle, or 0 if EOF was reached
Read blocks from a file. Note that the file handle that is
returned in SI may not be the same value that was passed in.
If end of file was reached (SI=0), the file was automatically
closed.
The address of the buffer (ES:BX) should be at least 512-byte
aligned. SYSLINUX guarantees at least this alignment for the
COMBOOT load segment or the COM32 bounce buffer.
Keep in mind that a "file" may be a TFTP connection, and that
leaving a file open for an extended period of time may result
in a timeout.
WARNING: Calling this function with an invalid file handle
will probably crash the system.
AX=0008h [2.08] Close file
Input: AX 0008h
SI file handle
Output: None
Close a file before reaching the end of file.
WARNING: Calling this function with an invalid file handle
will probably crash the system.
AX=0009h [2.00] Call PXE Stack [PXELINUX ONLY]
Input: AX 0009h
BX PXE function number
ES:DI PXE data buffer
Output: AX PXE return status code
Invoke an arbitrary PXE stack function. On SYSLINUX/ISOLINUX,
this function returns with an error (CF=1) and no action is
taken. On PXELINUX, this function always returns with CF=0
indicating that the PXE stack was successfully invoked; check
the status code in AX and in the first word of the data buffer
to determine if the PXE call succeeded or not.
The PXE stack will have the UDP stack OPEN; if you change that
you cannot call any of the file-related API functions, and
must restore UDP OPEN before returning to PXELINUX.
PXELINUX reserves UDP port numbers from 49152 to 65535 for its
own use; port numbers below that range is available.
AX=000Ah [2.00] Get Derivative-Specific Information
[SYSLINUX, EXTLINUX]
Input: AX 000Ah
Output: AL 31h (SYSLINUX), 34h (EXTLINUX)
DL drive number
ES:BX pointer to partition table entry (if DL >= 80h)
Note: This function was broken in EXTLINUX 3.00-3.02.
[PXELINUX]
Input: AX 000Ah
Output: AL 32h (PXELINUX)
DX PXE API version detected (DH=major, DL=minor)
ES:BX pointer to PXENV+ or !PXE structure
FS:SI pointer to original stack with invocation record
Note: DX notes the API version detected by PXELINUX,
which may be more conservative than the actual version
available. For exact information examine the API
version entry in the PXENV+ structure, or the API
version entries in the ROMID structures pointed from
the !PXE structure.
PXELINUX will use, and provide, the !PXE structure
over the PXENV+ structure. Examine the structure
signature to determine which particular structure was
provided.
The FS:SI pointer points to the top of the original stack
provided by the PXE stack, with the following values
pushed at the time PXELINUX is started:
[fs:si+0] GS <- top of stack
[fs:si+2] FS
[fs:si+4] ES
[fs:si+6] DS
[fs:si+8] EDI
[fs:si+12] ESI
[fs:si+16] EBP
[fs:si+20] -
[fs:si+24] EBX
[fs:si+28] EDX
[fs:si+32] ECX
[fs:si+36] EAX
[fs:si+40] EFLAGS
[fs:si+44] PXE return IP <- t.o.s. when PXELINUX invoked
[fs:si+46] PXE return CS
[ISOLINUX]
Input: AX 000Ah
Output: AL 33h (ISOLINUX)
DL drive number
ES:BX pointer to El Torito spec packet
Note: Some very broken El Torito implementations do
not provide the spec packet information. If so, ES:BX
may point to all zeroes or to garbage. Call INT 13h,
AX=4B01h to obtain the spec packet directly from the
BIOS if necessary.
This call gives information specific to a particular SYSLINUX
derivative. The value returned in AL is the same as is
returned in DL by INT 22h AX=0001h.
AX=000Bh [2.00] Get Serial Console Configuration
Input: AX 000Bh
Output: DX serial port I/O base (e.g. 3F8h = COM1...)
CX baud rate divisor (1 = 115200 bps, 2 = 57600 bps...)
BX flow control configuration bits (see syslinux.doc)
-> bit 15 is set if the video console is disabled
If no serial port is configured, DX will be set to 0 and the
other registers are undefined.
AX=000Ch [2.00] Perform final cleanup
Input: AX 000Ch
DX derivative-specific flags (0000h = clean up all)
Output: Does not return
This routine performs any "final cleanup" the boot loader
would normally perform before loading a kernel, such as
unloading the PXE stack in the case of PXELINUX. AFTER
INVOKING THIS CALL, NO OTHER API CALLS MAY BE INVOKED, NOR MAY
THE PROGRAM TERMINATE AND RETURN TO THE BOOT LOADER. This
call basically tells the boot loader "get out of the way, I'll
handle it from here."
For COM32 images, the boot loader will continue to provide
interrupt and BIOS call thunking services as long its memory
areas (0x0800-0xffff, 0x100000-0x100fff) are not overwritten.
MAKE SURE TO DISABLE INTERRUPTS, AND INSTALL NEW GDT AND IDTS
BEFORE OVERWRITING THESE MEMORY AREAS.
The permissible values for DX is an OR of these values:
SYSLINUX: 0000h Normal cleanup
PXELINUX: 0000h Normal cleanup
0003h Keep UNDI and PXE stacks loaded
ISOLINUX: 0000h Normal cleanup
All other values are undefined, and may have different
meanings in future versions of SYSLINUX.
AX=000Dh [2.08] Cleanup and replace bootstrap code
Input: AX 000Dh
DX derivative-specific flags (see previous function)
EDI bootstrap code (linear address, can be in high memory)
ECX bootstrap code length in bytes (must fit in low mem)
EBX(!) initial value of EDX after bootstrap
ESI initial value of ESI after bootstrap
DS initial value of DS after bootstrap
Output: Does not return
This routine performs final cleanup, then takes a piece of
code, copies it over the primary bootstrap at address 7C00h,
and jumps to it. This can be used to chainload boot sectors,
MBRs, bootstraps, etc.
Normal boot sectors expect DL to contain the drive number,
and, for hard drives (DL >= 80h) DS:SI to contain a pointer to
the 16-byte partition table entry. The memory between
600h-7FFh is available to put the partition table entry in.
For PXELINUX, if the PXE stack is not unloaded, all registers
(except DS, ESI and EDX) and the stack will be set up as they
were set up by the PXE ROM.
AX=000Eh [2.11] Get configuration file name
Input: AX 0000Eh
Output: ES:BX null-terminated file name string
Returns the name of the configuration file. Note that it is
possible that the configuration file doesn't actually exist.
AX=000Fh [3.00] Get IPAPPEND strings [PXELINUX]
Input: AX 000Fh
Output: CX number of strings (currently 2)
ES:BX pointer to an array of NEAR pointers in
the same segment, one for each of the above
strings
Returns the same strings that the "ipappend" option would have
added to the command line, one for each bit of the "ipappend"
flag value, so entry 0 is the "ip=" string and entry 1 is the
"BOOTIF=" string.
AX=0010h [3.00] Resolve hostname [PXELINUX]
Input: ES:BX pointer to null-terminated hostname
Output: EAX IP address of hostname (zero if not found)
Queries the DNS server(s) for a specific hostname. If the
hostname does not contain a dot (.), the local domain name
is automatically appended.
This function only return CF=1 if the function is not
supported. If the function is supported, but the hostname did
not resolve, it returns with CF=0, EAX=0.
The IP address is returned in network byte order, i.e. if the
IP address is 1.2.3.4, EAX will contain 0x04030201. Note that
all uses of IP addresses in PXE are also in network byte order.
AX=0011h [3.05] Maximum number of shuffle descriptors
Input: AX 0011h
Output: CX maximum number of descriptors
This routine reports the maximum number of shuffle descriptors
permitted in a call to function 0012h.
Typical values are 682 and 1365.
AX=0012h [3.05] Cleanup, shuffle and boot
Input: AX 0012h
DX derivative-specific flags (see previous function)
ES:DI shuffle descriptor list (must be in low memory)
CX number of shuffle descriptors
EBX(!) initial value of EDX after bootstrap
ESI initial value of ESI after bootstrap
DS initial value of DS after bootstrap
EBP CS:IP of routine to jump to
Output: Does not return
(if CX is too large the routine returns with CF=1)
This routine performs final cleanup, then performs a sequence
of copies, and jumps to a specified real mode entry point.
This is a more general version of function 000Dh, which can
also be used to load other types of programs.
The copies must not touch memory below address 7C00h.
ES:DI points to a list of CX descriptors each of the form:
Offset Size Meaning
0 dword destination address
4 dword source address
8 dword length in bytes
The copies are overlap-safe, like memmove().
Normal boot sectors expect DL to contain the drive number,
and, for hard drives (DL >= 80h) DS:SI to contain a pointer to
the 16-byte partition table entry. The memory between
600h-7FFh is available to put the partition table entry in.
For PXELINUX, if the PXE stack is not unloaded, all registers
(except DS, ESI and EDX) and the stack will be set up as they
were set up by the PXE ROM.
AX=0013h [3.08] Idle loop call
Input: AX 0013h
Output: None
Call this routine while sitting in an idle loop. It performs
any periodic activities required by the filesystem code. At
the moment, this is a no-op on all derivatives except
PXELINUX, where it executes PXE calls to answer ARP queries.