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SubjectRe: Killing off the boot sector (was: [STATUS 2.5] January 8, 2002)
Date
Followup to:  <3E1C2208.6727.5370CB@localhost>
By author: "Guillaume Boissiere" <boissiere@adiglobal.com>
In newsgroup: linux.dev.kernel
>
> 179 nor bugme-janitors@lists.osdl.org OPEN boot from 21
> sec/track floppy
>

Can we *please* kill off the stupid in-kernel boot sector? The
probing method it uses to determine geometry is unreliable (doesn't
work on anything but true legacy floppies, not IDE, not USB, not
firewire); it generates these kinds of requests; doesn't handle
large-size kernels; hard-codes the use of address 0x90000 which isn't
available on all machines; and overall promotes what's fundamentally
bad practice.

People keep asking what's the harm in keeping it, and the answer is,
quite simply: "because people continue to try to use it."

Here is a patch that guts it to print an error message. It's even
tested.

--- linux-2.5.54/arch/i386/boot/bootsect.S.dist Wed Jan 8 11:35:52 2003
+++ linux-2.5.54/arch/i386/boot/bootsect.S Wed Jan 8 11:52:16 2003
@@ -4,29 +4,13 @@
* modified by Drew Eckhardt
* modified by Bruce Evans (bde)
* modified by Chris Noe (May 1999) (as86 -> gas)
- *
- * 360k/720k disk support: Andrzej Krzysztofowicz <ankry@green.mif.pg.gda.pl>
+ * gutted by H. Peter Anvin (Jan 2003)
*
* BIG FAT NOTE: We're in real mode using 64k segments. Therefore segment
* addresses must be multiplied by 16 to obtain their respective linear
* addresses. To avoid confusion, linear addresses are written using leading
* hex while segment addresses are written as segment:offset.
*
- * bde - should not jump blindly, there may be systems with only 512K low
- * memory. Use int 0x12 to get the top of memory, etc.
- *
- * It then loads 'setup' directly after itself (0x90200), and the system
- * at 0x10000, using BIOS interrupts.
- *
- * NOTE! currently system is at most (8*65536-4096) bytes long. This should
- * be no problem, even in the future. I want to keep it simple. This 508 kB
- * kernel size should be enough, especially as this doesn't contain the
- * buffer cache as in minix (and especially now that the kernel is
- * compressed :-)
- *
- * The loader has been made as simple as possible, and continuous
- * read errors will result in a unbreakable loop. Reboot by hand. It
- * loads pretty fast by getting whole tracks at a time whenever possible.
*/

#include <asm/boot.h>
@@ -59,359 +43,47 @@
.global _start
_start:

-# First things first. Move ourself from 0x7C00 -> 0x90000 and jump there.
+ # Normalize the start address
+ jmpl $BOOTSEG, $start2

- movw $BOOTSEG, %ax
- movw %ax, %ds # %ds = BOOTSEG
- movw $INITSEG, %ax
- movw %ax, %es # %ax = %es = INITSEG
- movw $256, %cx
- subw %si, %si
- subw %di, %di
- cld
- rep
- movsw
- ljmp $INITSEG, $go
-
-# bde - changed 0xff00 to 0x4000 to use debugger at 0x6400 up (bde). We
-# wouldn't have to worry about this if we checked the top of memory. Also
-# my BIOS can be configured to put the wini drive tables in high memory
-# instead of in the vector table. The old stack might have clobbered the
-# drive table.
-
-go: movw $0x4000-12, %di # 0x4000 is an arbitrary value >=
- # length of bootsect + length of
- # setup + room for stack;
- # 12 is disk parm size.
- movw %ax, %ds # %ax and %es already contain INITSEG
+start2:
+ movw %cs, %ax
+ movw %ax, %ds
+ movw %ax, %es
movw %ax, %ss
- movw %di, %sp # put stack at INITSEG:0x4000-12.
+ movw $0x7c00, %sp
+ sti
+ cld

-# Many BIOS's default disk parameter tables will not recognize
-# multi-sector reads beyond the maximum sector number specified
-# in the default diskette parameter tables - this may mean 7
-# sectors in some cases.
-#
-# Since single sector reads are slow and out of the question,
-# we must take care of this by creating new parameter tables
-# (for the first disk) in RAM. We will set the maximum sector
-# count to 36 - the most we will encounter on an ED 2.88.
-#
-# High doesn't hurt. Low does.
-#
-# Segments are as follows: %cs = %ds = %es = %ss = INITSEG, %fs = 0,
-# and %gs is unused.
-
- movw %cx, %fs # %fs = 0
- movw $0x78, %bx # %fs:%bx is parameter table address
- pushw %ds
- ldsw %fs:(%bx), %si # %ds:%si is source
- movb $6, %cl # copy 12 bytes
- pushw %di # %di = 0x4000-12.
- rep # don't worry about cld
- movsw # already done above
- popw %di
- popw %ds
- movb $36, 0x4(%di) # patch sector count
- movw %di, %fs:(%bx)
- movw %es, %fs:2(%bx)
-
-# Get disk drive parameters, specifically number of sectors/track.
-
-# It seems that there is no BIOS call to get the number of sectors.
-# Guess 36 sectors if sector 36 can be read, 18 sectors if sector 18
-# can be read, 15 if sector 15 can be read. Otherwise guess 9.
-# Note that %cx = 0 from rep movsw above.
+ movw $bugger_off_msg, %si

- movw $disksizes, %si # table of sizes to try
-probe_loop:
+msg_loop:
lodsb
- cbtw # extend to word
- movw %ax, sectors
- cmpw $disksizes+4, %si
- jae got_sectors # If all else fails, try 9
-
- xchgw %cx, %ax # %cx = track and sector
- xorw %dx, %dx # drive 0, head 0
- movw $0x0200, %bx # address = 512, in INITSEG (%es = %cs)
- movw $0x0201, %ax # service 2, 1 sector
- int $0x13
- jc probe_loop # try next value
-
-got_sectors:
- movb $0x03, %ah # read cursor pos
- xorb %bh, %bh
- int $0x10
- movw $9, %cx
- movb $0x07, %bl # page 0, attribute 7 (normal)
- # %bh is set above; int10 doesn't
- # modify it
- movw $msg1, %bp
- movw $0x1301, %ax # write string, move cursor
- int $0x10 # tell the user we're loading..
-
-# Load the setup-sectors directly after the moved bootblock (at 0x90200).
-# We should know the drive geometry to do it, as setup may exceed first
-# cylinder (for 9-sector 360K and 720K floppies).
-
- movw $0x0001, %ax # set sread (sector-to-read) to 1 as
- movw $sread, %si # the boot sector has already been read
- movw %ax, (%si)
-
- call kill_motor # reset FDC
- movw $0x0200, %bx # address = 512, in INITSEG
-next_step:
- movb setup_sects, %al
- movw sectors, %cx
- subw (%si), %cx # (%si) = sread
- cmpb %cl, %al
- jbe no_cyl_crossing
- movw sectors, %ax
- subw (%si), %ax # (%si) = sread
-no_cyl_crossing:
- call read_track
- pushw %ax # save it
- call set_next # set %bx properly; it uses %ax,%cx,%dx
- popw %ax # restore
- subb %al, setup_sects # rest - for next step
- jnz next_step
-
- pushw $SYSSEG
- popw %es # %es = SYSSEG
- call read_it
- call kill_motor
- call print_nl
-
-# After that we check which root-device to use. If the device is
-# defined (!= 0), nothing is done and the given device is used.
-# Otherwise, one of /dev/fd0H2880 (2,32) or /dev/PS0 (2,28) or /dev/at0 (2,8)
-# depending on the number of sectors we pretend to know we have.
-
-# Segments are as follows: %cs = %ds = %ss = INITSEG,
-# %es = SYSSEG, %fs = 0, %gs is unused.
-
- movw root_dev, %ax
- orw %ax, %ax
- jne root_defined
-
- movw sectors, %bx
- movw $0x0208, %ax # /dev/ps0 - 1.2Mb
- cmpw $15, %bx
- je root_defined
-
- movb $0x1c, %al # /dev/PS0 - 1.44Mb
- cmpw $18, %bx
- je root_defined
-
- movb $0x20, %al # /dev/fd0H2880 - 2.88Mb
- cmpw $36, %bx
- je root_defined
-
- movb $0, %al # /dev/fd0 - autodetect
-root_defined:
- movw %ax, root_dev
-
-# After that (everything loaded), we jump to the setup-routine
-# loaded directly after the bootblock:
-
- ljmp $SETUPSEG, $0
-
-# These variables are addressed via %si register as it gives shorter code.
-
-sread: .word 0 # sectors read of current track
-head: .word 0 # current head
-track: .word 0 # current track
-
-# This routine loads the system at address SYSSEG, making sure
-# no 64kB boundaries are crossed. We try to load it as fast as
-# possible, loading whole tracks whenever we can.
-
-read_it:
- movw %es, %ax # %es = SYSSEG when called
- testw $0x0fff, %ax
-die: jne die # %es must be at 64kB boundary
- xorw %bx, %bx # %bx is starting address within segment
-rp_read:
-#ifdef __BIG_KERNEL__ # look in setup.S for bootsect_kludge
- bootsect_kludge = 0x220 # 0x200 + 0x20 which is the size of the
- lcall *bootsect_kludge # bootsector + bootsect_kludge offset
-#else
- movw %es, %ax
- subw $SYSSEG, %ax
- movw %bx, %cx
- shr $4, %cx
- add %cx, %ax # check offset
-#endif
- cmpw syssize, %ax # have we loaded everything yet?
- jbe ok1_read
-
- ret
-
-ok1_read:
- movw sectors, %ax
- subw (%si), %ax # (%si) = sread
- movw %ax, %cx
- shlw $9, %cx
- addw %bx, %cx
- jnc ok2_read
-
- je ok2_read
-
- xorw %ax, %ax
- subw %bx, %ax
- shrw $9, %ax
-ok2_read:
- call read_track
- call set_next
- jmp rp_read
-
-read_track:
- pusha
- pusha
- movw $0xe2e, %ax # loading... message 2e = .
- movw $7, %bx
- int $0x10
- popa
-
-# Accessing head, track, sread via %si gives shorter code.
-
- movw 4(%si), %dx # 4(%si) = track
- movw (%si), %cx # (%si) = sread
- incw %cx
- movb %dl, %ch
- movw 2(%si), %dx # 2(%si) = head
- movb %dl, %dh
- andw $0x0100, %dx
- movb $2, %ah
- pushw %dx # save for error dump
- pushw %cx
- pushw %bx
- pushw %ax
- int $0x13
- jc bad_rt
-
- addw $8, %sp
- popa
- ret
-
-set_next:
- movw %ax, %cx
- addw (%si), %ax # (%si) = sread
- cmp sectors, %ax
- jne ok3_set
- movw $0x0001, %ax
- xorw %ax, 2(%si) # change head
- jne ok4_set
- incw 4(%si) # next track
-ok4_set:
- xorw %ax, %ax
-ok3_set:
- movw %ax, (%si) # set sread
- shlw $9, %cx
- addw %cx, %bx
- jnc set_next_fin
- movw %es, %ax
- addb $0x10, %ah
- movw %ax, %es
+ andb %al, %al
+ jz die
+ mov $0xe, %ah
xorw %bx, %bx
-set_next_fin:
- ret
-
-bad_rt:
- pushw %ax # save error code
- call print_all # %ah = error, %al = read
- xorb %ah, %ah
- xorb %dl, %dl
- int $0x13
- addw $10, %sp
- popa
- jmp read_track
-
-# print_all is for debugging purposes.
-#
-# it will print out all of the registers. The assumption is that this is
-# called from a routine, with a stack frame like
-#
-# %dx
-# %cx
-# %bx
-# %ax
-# (error)
-# ret <- %sp
-
-print_all:
- movw $5, %cx # error code + 4 registers
- movw %sp, %bp
-print_loop:
- pushw %cx # save count remaining
- call print_nl # <-- for readability
- cmpb $5, %cl
- jae no_reg # see if register name is needed
-
- movw $0xe05 + 'A' - 1, %ax
- subb %cl, %al
- int $0x10
- movb $'X', %al
int $0x10
- movb $':', %al
- int $0x10
-no_reg:
- addw $2, %bp # next register
- call print_hex # print it
- popw %cx
- loop print_loop
- ret
+ jmp msg_loop

-print_nl:
- movw $0xe0d, %ax # CR
- int $0x10
- movb $0xa, %al # LF
- int $0x10
- ret
-
-# print_hex is for debugging purposes, and prints the word
-# pointed to by %ss:%bp in hexadecimal.
-
-print_hex:
- movw $4, %cx # 4 hex digits
- movw (%bp), %dx # load word into %dx
-print_digit:
- rolw $4, %dx # rotate to use low 4 bits
- movw $0xe0f, %ax # %ah = request
- andb %dl, %al # %al = mask for nybble
- addb $0x90, %al # convert %al to ascii hex
- daa # in only four instructions!
- adc $0x40, %al
- daa
- int $0x10
- loop print_digit
- ret
+die:
+ # Allow the user to press a key, then reboot
+ xorw %ax, %ax
+ int $0x16
+ int $0x19

-# This procedure turns off the floppy drive motor, so
-# that we enter the kernel in a known state, and
-# don't have to worry about it later.
-# NOTE: Doesn't save %ax or %dx; do it yourself if you need to.
-
-kill_motor:
-#if 1
- xorw %ax, %ax # reset FDC
- xorb %dl, %dl
- int $0x13
-#else
- movw $0x3f2, %dx
- xorb %al, %al
- outb %al, %dx
-#endif
- ret

-sectors: .word 0
-disksizes: .byte 36, 18, 15, 9
-msg1: .byte 13, 10
- .ascii "Loading"
+bugger_off_msg:
+ .ascii "Direct booting from floppy is no longer supported.\r\n"
+ .ascii "Please use a boot loader program instead.\r\n"
+ .ascii "\n"
+ .ascii "Remove disk and press any key to reboot . . .\r\n"
+ .byte 0
+

-# XXX: This is a fairly snug fit.
+ # Kernel attributes; used by setup

-.org 497
+ .org 497
setup_sects: .byte SETUPSECTS
root_flags: .word ROOT_RDONLY
syssize: .word SYSSIZE
--- linux-2.5.54/arch/i386/boot/tools/build.c.dist Wed Jan 1 19:22:04 2003
+++ linux-2.5.54/arch/i386/boot/tools/build.c Wed Jan 8 11:53:32 2003
@@ -150,13 +150,10 @@
sz = sb.st_size;
fprintf (stderr, "System is %d kB\n", sz/1024);
sys_size = (sz + 15) / 16;
- /* 0x28000*16 = 2.5 MB, conservative estimate for the current maximum */
- if (sys_size > (is_big_kernel ? 0x28000 : DEF_SYSSIZE))
+ /* 0x40000*16 = 4.0 MB, reasonable estimate for the current maximum */
+ if (sys_size > (is_big_kernel ? 0x40000 : DEF_SYSSIZE))
die("System is too big. Try using %smodules.",
is_big_kernel ? "" : "bzImage or ");
- if (sys_size > 0xefff)
- fprintf(stderr,"warning: kernel is too big for standalone boot "
- "from floppy\n");
while (sz > 0) {
int l, n;

--
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"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt <amsp@zytor.com>
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