Hi again,
news about boot sector properties:
- our FAT32 boot sectors always trust DL, so the code in SYS
which says "if A:/B: set 0, else set 0xff" should be changed
to "if A:/B: set 0, else set 0x80" to make the boot sector
look nicer to disk tools, right? And makebootfat and my tool
sys-freedos-linux should mention that options like -E number
or --drive=number have no effect on FAT32, correct?
- I wrote a patch to make the FAT32 LBA boot sector use the
DS:SI partition table entry pointer for the partition offset,
please help by proof-reading it... See below.
- our boot sectors do not need any geometry in LBA mode but
they never use LBA for A: because it "might be a buggy
cdrom boot floppy emulation". I suggest to allow LBA even
for A: for the FAT16 boot sector, would that be okay?
- our FAT12, FAT16 and FAT32 CHS boot sectors work on 8086,
and FAT32 LBA needs 386. SYS uses LBA for FAT32 if it can
do so on the PC on which you run SYS for the target drive.
I suggest to add a command line option to force CHS or LBA.
Please suggest a syntax.
- I suggest to change the way in which SYS writes the
FAT12 / FAT16 drive number: instead of using 0 A:/B:
and 255 (auto) else, use 0 A:/B: and 128 (hdd) else,
and change the boot sectors to trust DL from whoever
calls the boot sector (MBR, BIOS, boot menu) unless
some SYS command line makes SYS modify code in the
sector to ignore DL, possibly for buggy BIOS compat.
SYS could default to "ignore if A:/B:, else trust",
then you need 2 options (force ignore, force trust).
Comments and syntax suggestions please...
- the FAT12 / FAT16 boot sectors are too stuffed to
check int13.8 for real geometry, but we could say
that LBA is often supported - then we get to my
suggestion "allow LBA even for A: if FAT16"... :-).
When you use LBA, wrong CHS geometry is irrelevant.
The above would fix the following:
- mtools / mkdosfs can fail to set the partition
offset (number of "hidden sectors") but a MBR
usually passes the right partition table entry
pointer - my patch makes FAT32 LBA boot easier
- CHS geometries can easily mismatch between when
you format a drive and when you use a drive to
boot from it - the "use LBA boot sector" option
for FAT32 for SYS and the "use LBA even for A:
if FAT16" options would make it easier to use
LBA to boot which has no CHS risks. See above.
- it was not intuitive that FAT12 / FAT16 used
"drive 255 means automatic" while FAT32 always
used automatic, so I suggest to always use the
automatic choice unless a SYS option forces use
of a given drive number, even in FAT12 / FAT16
(maybe not for FAT12 A:, see above)
Eric
Patch for boot32lb.asm for "use DS:SI partition table entry":
--- boot32lb.asm (revision 1365)
+++ boot32lb.asm (working copy)
@@ -107,27 +107,39 @@
; ENTRY
;-----------------------------------------------------------------------
-real_start: cld
- cli
+real_start: cld ; input: DS:SI pointer to partition table
+ cli ; entry in RAM, DL BIOS boot drive number
sub ax, ax
+ mov bx, ds ; preserve DS of DS:SI in BX
mov ds, ax
mov bp, 0x7c00
mov ax, 0x1FE0
mov es, ax
- mov si, bp
mov di, bp
+ ; mov si, bp
+ xchg si, bp ; preserve SI in BP but break BP
mov cx, 0x0100
rep movsw ; move boot code to the
0x1FE0:0x0000
jmp word 0x1FE0:cont
-loadseg_off dw 0, LOADSEG
+%define loadseg_off (loadseg_seg-2)
+ ; dw 0 ; the extra "mov bx,ds" gets us here
+loadseg_seg dw LOADSEG ; sys.c hardcodes offset of this!
; -------------
-cont: mov ds, ax
+cont: mov ds, bx ; restore DS of DS:SI
+ mov si, bp ; restore SI of DS:SI
+ mov bp, 0x7c00 ; fix BP, we need it
+ test byte [si], 0x7f ; is ds:[si] 0 or 0x80?
mov ss, ax ; stack and BP-relative moves up, too
lea sp, [bp-0x20]
+ jnz no_partdata ; was ds:[si] 0 or 0x80?
+ push dword [si+8] ; partition offset
+ pop dword [nHidden] ; relative to SS:BP :-)
+no_partdata: mov ds, ax ; drop caller DS:SI now
+ mov word [loadsegoff_60],cx ; set to 0
sti
mov [drive], dl ; BIOS passes drive number in DL
-------------------------------------------------------------------------
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