/* NOTE: this boot sector contains instructions that need at least an 80186.
* Yes, as86 has a bug somewhere in the valid instruction set checks.
*
*/
/* floppyload.S Copyright (C) 1991, 1992 Linus Torvalds
* modified by Drew Eckhardt
* modified by Bruce Evans (bde)
*
* floppyprefix.S is loaded at 0x0000:0x7c00 by the bios-startup routines.
*
* It then loads the system at SYSSEG<<4, using BIOS interrupts.
*
* 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.
*/
FILE_LICENCE ( GPL2_ONLY )
#include <librm.h>
.equ BOOTSEG, 0x07C0 /* original address of boot-sector */
.equ SYSSEG, 0x1000 /* system loaded at SYSSEG<<4 */
.section ".note.GNU-stack", "", @progbits
.org 0
.arch i386
.text
.section ".prefix", "ax", @progbits
.code16
.globl _dsk_start
_dsk_start:
jmp $BOOTSEG, $go /* reload cs:ip to match relocation addr */
go:
movw $0x2000-12, %di /* 0x2000 is arbitrary value >= length */
/* of bootsect + room for stack + 12 for */
/* saved disk parm block */
movw $BOOTSEG, %ax
movw %ax,%ds
movw %ax,%es
movw %ax,%ss /* put stack at BOOTSEG:0x4000-12. */
movw %di,%sp
/* 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: ds=es=ss=cs - BOOTSEG
*/
xorw %cx,%cx
movw %cx,%es /* access segment 0 */
movw $0x78, %bx /* 0:bx is parameter table address */
pushw %ds /* save ds */
/* 0:bx is parameter table address */
ldsw %es:(%bx),%si /* loads ds and si */
movw %ax,%es /* ax is BOOTSECT (loaded above) */
movb $6, %cl /* copy 12 bytes */
cld
pushw %di /* keep a copy for later */
rep
movsw /* ds:si is source, es:di is dest */
popw %di
movb $36,%es:4(%di)
movw %cx,%ds /* access segment 0 */
xchgw %di,(%bx)
movw %es,%si
xchgw %si,2(%bx)
popw %ds /* restore ds */
movw %di, dpoff /* save old parameters */
movw %si, dpseg /* to restore just before finishing */
pushw %ds
popw %es /* reload es */
/* Note that es is already set up. Also cx is 0 from rep movsw above. */
xorb %ah,%ah /* reset FDC */
xorb %dl,%dl
int $0x13
/* 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.
*/
movw $disksizes, %si /* table of sizes to try */
probe_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 after boot sector */
/* (512 bytes from origin, es = cs) */
movw $0x0201, %ax /* service 2, 1 sector */
int $0x13
jc probe_loop /* try next value */
got_sectors:
movw $msg1end-msg1, %cx
movw $msg1, %si
call print_str
/* ok, we've written the Loading... message, now we want to load the system */
movw $SYSSEG, %ax
movw %ax,%es /* segment of SYSSEG<<4 */
pushw %es
call read_it
/* This 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.
*/
movw $0x3f2, %dx
xorb %al,%al
outb %al,%dx
call print_nl
pop %es /* = SYSSEG */
/* Restore original disk parameters */
movw $0x78, %bx
movw dpoff, %di
movw dpseg, %si
xorw %ax,%ax
movw %ax,%ds
movw %di,(%bx)
movw %si,2(%bx)
/* Everything now loaded. %es = SYSSEG, so %es:0000 points to
* start of loaded image.
*/
/* Jump to loaded copy */
ljmp $SYSSEG, $start_runtime
endseg: .word SYSSEG
.section ".zinfo.fixup", "a", @progbits /* Compressor fixups */
.ascii "ADDW"
.long endseg
.long 16
.long 0
.previous
/* This routine loads the system at address SYSSEG<<4, making sure no 64kB
* boundaries are crossed. We try to load it as fast as possible, loading whole
* tracks whenever we can.
*
* in: es - starting address segment (normally SYSSEG)
*/
read_it:
movw $0,sread /* load whole image including prefix */
movw %es,%ax
testw $0x0fff, %ax
die: jne die /* es must be at 64kB boundary */
xorw %bx,%bx /* bx is starting address within segment */
rp_read:
movw %es,%ax
movw %bx,%dx
movb $4, %cl
shrw %cl,%dx /* bx is always divisible by 16 */
addw %dx,%ax
cmpw endseg, %ax /* have we loaded all yet? */
jb ok1_read
ret
ok1_read:
movw sectors, %ax
subw sread, %ax
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
movw %ax,%cx
addw sread, %ax
cmpw sectors, %ax
jne ok3_read
movw $1, %ax
subw head, %ax
jne ok4_read
incw track
ok4_read:
movw %ax, head
xorw %ax,%ax
ok3_read:
movw %ax, sread
shlw $9, %cx
addw %cx,%bx
jnc rp_read
movw %es,%ax
addb $0x10, %ah
movw %ax,%es
xorw %bx,%bx
jmp rp_read
read_track:
pusha
pushw %ax
pushw %bx
pushw %bp /* just in case the BIOS is buggy */
movw $0x0e2e, %ax /* 0x2e = . */
movw $0x0007, %bx
int $0x10
popw %bp
popw %bx
popw %ax
movw track, %dx
movw sread, %cx
incw %cx
movb %dl,%ch
movw head, %dx
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
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:
call print_nl /* nl for readability */
movw $5, %cx /* error code + 4 registers */
movw %sp,%bp
print_loop:
pushw %cx /* save count left */
cmpb $5, %cl
jae no_reg /* see if register name is needed */
movw $0x0007, %bx /* page 0, attribute 7 (normal) */
movw $0xe05+0x41-1, %ax
subb %cl,%al
int $0x10
movb $0x58, %al /* 'X' */
int $0x10
movb $0x3A, %al /* ':' */
int $0x10
no_reg:
addw $2, %bp /* next register */
call print_hex /* print it */
movb $0x20, %al /* print a space */
int $0x10
popw %cx
loop print_loop
call print_nl /* nl for readability */
ret
print_str:
movw $0x0007, %bx /* page 0, attribute 7 (normal) */
movb $0x0e, %ah /* write char, tty mode */
prloop:
lodsb
int $0x10
loop prloop
ret
print_nl:
movw $0x0007, %bx /* page 0, attribute 7 (normal) */
movw $0xe0d, %ax /* CR */
int $0x10
movb $0xa, %al /* LF */
int $0x10
ret
/* print_hex prints the word pointed to by ss:bp in hexadecimal. */
print_hex:
movw (%bp),%dx /* load word into dx */
movb $4, %cl
movb $0x0e, %ah /* write char, tty mode */
movw $0x0007, %bx /* page 0, attribute 7 (normal) */
call print_digit
call print_digit
call print_digit
/* fall through */
print_digit:
rol %cl,%dx /* rotate so that lowest 4 bits are used */
movb $0x0f, %al /* mask for nybble */
andb %dl,%al
addb $0x90, %al /* convert al to ascii hex (four instructions) */
daa
adcb $0x40, %al
daa
int $0x10
ret
sread: .word 0 /* sectors read of current track */
head: .word 0 /* current head */
track: .word 0 /* current track */
sectors:
.word 0
dpseg: .word 0
dpoff: .word 0
disksizes:
.byte 36,18,15,9
msg1:
.ascii "Loading ROM image"
msg1end:
.org 510, 0
.word 0xAA55
start_runtime:
/* Install iPXE */
call install
/* Set up real-mode stack */
movw %bx, %ss
movw $_estack16, %sp
/* Jump to .text16 segment */
pushw %ax
pushw $1f
lret
.section ".text16", "awx", @progbits
1:
/* Run iPXE */
virtcall main
/* Uninstall iPXE */
call uninstall
/* Boot next device */
int $0x18
