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|
!
! This is a bootblock to load an execute a standalone program from an
! MSDOS filesystem on a floppy disk.
!
! The program is divided into two parts, the first 512 bytes contains a
! loader to fetch one block from a file called 'BOOTFILE.SYS' from the
! root directory of the disk. The second 512, which is stored in this file,
! loads the executable using functions supplied by the first.
!
! The 2nd part is loaded as if it's a floppy boot block and can be used to
! store, for instance, a LILO boot block to let LILO boot from an MSDOS
! floppy.
!
! The second part is NOT yet complete!
!
ORGADDR=$0500
use16
! Absolute position macro, fails if code before it is too big.
macro locn
if *-start>?1
fail! *-start>?1
endif
.blkb ?1 + start-*
mend
org ORGADDR
start:
include sysboot.s
! Data into the temp area, 30 clear bytes
org floppy_temp
root_count: .blkw 1
old_bpb: .blkw 2
bios_disk: .blkb 12
locn(codestart-start)
cld ! Assume _nothing_!
mov bx,#$7C00 ! Pointer to start of BB.
xor ax,ax ! Segs all to zero
mov ds,ax
mov es,ax
mov ss,ax
mov sp,bx ! SP Just below BB
mov cx,#$100 ! Move 256 words
mov si,bx ! From default BB
mov di,#ORGADDR ! To the correct address.
rep
movsw
jmpi cont,#0 ! Set CS:IP correct.
cont:
sti ! Let the interrupts back in.
! Need to fix BPB for fd0 to correct sectors (Like linux bootblock does)
mov di,#bios_disk
mov bp,#0x78
! 0:bx is parameter table address
push ds
lds si,[bp]
! ds:si is source
mov cx,#6
! copy 12 bytes
push di
rep
movsw
pop di
pop ds
! New BPB is 0:di
mov [bp],di
mov 2[bp],ax
mov al,[dos_spt]
mov 4[di],al
! For each sector in root dir
! For each dir entry
! If entry name is == boot_name
! then load and call sector
! First dir = dos_fatlen*dos_nfat+dos_resv
mov ax,[dos_fatlen]
mov dl,[dos_nfat]
xor dh,dh
mul dx
add ax,[dos_resv]
mov di,ax
! DI is sector number of first root dir sector.
mov ax,[dos_nroot]
mov [root_count],ax
add ax,#15
mov cl,#4
shr ax,cl
add ax,di
mov bp,ax ! bp is first data sector.
nextsect:
call linsect
mov ax,#$0201
int $13
jc floppy_error
! ... foreach dir entry
mov si,bx
nextentry:
! test entry name
push si
push di
mov di,#boot_name
mov cx,#11
rep
cmpsb
pop di
pop si
je got_entry
add si,#32
cmp si,#512
jnz nextentry
inc di
sub [root_count],#16
jp nextsect
jmp no_system
got_entry:
mov ax,[si+26] ! Cluster number of start of file
test ax,ax
jz no_system ! Make sure we have a block.
mov di,ax ! Save the cluster number we are loading.
call linclust
mov ax,#$0201
int $13
jc floppy_error
mov bx,#7
mov ax,#$0E3E
int $10
jmpi $7C00,0 ! No magics, just go.
! Convert a cluster number into a CHS in CX:DX
linclust:
sub ax,#2
mov dl,[dos_clust]
xor dh,dh
mul dx
add ax,bp
mov di,ax
! JMP linsect ...
!
! This function converts a linear sector number in DI
! into a CHS representation in CX:DX
!
linsect:
xor dx,dx
mov ax,di
div [dos_spt]
inc dx
mov cl,dl ! Sector num
xor dx,dx ! Drive 0
shr ax,#1 ! Assume dos_heads == 2
adc dh,#0 ! Head num
mov ch,al ! Cylinder
ret
no_system:
floppy_error:
mov si,#error_msg
no_boot: ! SI now has pointer to error message
lodsb
cmp al,#0
jz EOS
mov bx,#7
mov ah,#$E ! Can't use $13 cause that's AT+ only!
int $10
jmp no_boot
EOS:
xor ax,ax
int $16
int $19 ! This should be OK as we haven't touched anything.
jmpi $0,$FFFF ! Wam! Try or die!
error_msg:
.asciz "\r\nError during initial boot\r\nPress a key:"
export boot_name
boot_name:
.ascii "BOOTFILESYS"
name_end:
! NNNNNNNNEEE
!
!-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
!
! Part 2, loaded like a boot block by part 1
! ... Here we will allow some assumptions (cause we've set them)
fat_table = ORGADDR+$400 ! Where to load the fat
locn(512)
part2_addr:
push di
mov cx,#$100 ! Move 256 words
mov si,#$7C00 ! From default BB
mov di,#part2_addr ! To the correct address.
rep
movsw
jmpi cont2,#0 ! Set CS:IP correct.
cont2:
! 1) Load FAT
! This assumes all of FAT1 is on the first track, this is normal
mov ax,[dos_fatlen]
mov ah,#2
mov bx,#fat_table
mov cx,[dos_resv] ! Fat starts past bootblock
inc cx
xor dx,dx ! Head zero
int $13
!jc floppy_error
mov bx,#$7C00
pop di ! Cluster to start load.
! load whole cluster
next_cluster:
call next_fat
cmp di,#0
jnz next_cluster
br maincode
next_fat:
!mov ax,di
!mov bx,ax
!shr ax,#1
!add bx,ax
! The end ... place a marker.
locn(1023)
.byte $FF
maincode:
|
