Page 1 of 2
Running Mikeos on a *real* 8086
Posted: Thu Apr 19, 2012 12:53 pm
by Burik
Hey there, this is my first post here. I am the proud owner of an Olivetti M24 [8086 CPU, 640k RAM, 20Mb HDD] and I'd like to run Mikeos on it. Too bad its 3.5" drive can't read anything above 720K floppies, hence I have to modify the bootloader according to 720K specs. Original bootloader as follows:
Code: Select all
*snip*
; Values are those used by IBM for 1.44 MB, 3.5" diskette
23
24 OEMLabel db "MIKEBOOT" ; Disk label
25 BytesPerSector dw 512 ; Bytes per sector
26 SectorsPerCluster db 1 ; Sectors per cluster
27 ReservedForBoot dw 1 ; Reserved sectors for boot record
28 NumberOfFats db 2 ; Number of copies of the FAT
29 RootDirEntries dw 224 ; Number of entries in root dir
30 ; (224 * 32 = 7168 = 14 sectors to read)
31 LogicalSectors dw 2880 ; Number of logical sectors
32 MediumByte db 0F0h ; Medium descriptor byte
33 SectorsPerFat dw 9 ; Sectors per FAT
34 SectorsPerTrack dw 18 ; Sectors per track (36/cylinder)
35 Sides dw 2 ; Number of sides/heads
36 HiddenSectors dd 0 ; Number of hidden sectors
37 LargeSectors dd 0 ; Number of LBA sectors
38 DriveNo dw 0 ; Drive No: 0
39 Signature db 41 ; Drive signature: 41 for floppy
40 VolumeID dd 00000000h ; Volume ID: any number
41 VolumeLabel db "MIKEOS "; Volume Label: any 11 chars
42 FileSystem db "FAT12 " ; File system type: don't change!
43
*snip*
Too bad I'um unable to find the correct values for a 720K floppy.. I tried 1440 for LogicalSectors, 9 for SectorsPerTrack and F9 for MediumByte and I've basically destroyed the FAT
Any help?
Re: Running Mikeos on a *real* 8086
Posted: Thu Apr 19, 2012 1:46 pm
by bubach
Those values you mention is right, but a 720kb floppy with FAT12 also uses 1kb cluster values, while 1.44mb floppies use 512byte. This might break MikeOS FAT-driver if it's not done right to account for different cluster sizes. Try setting "SectorsPerCluster db 2". It will however take some serious FAT hacking in a hex editor to convert the values inside the actual FAT for the files needed at boot. Maybe you could find software to deal with 720kb floppies, or is the 1.44mb drives incompatible with older floppy sizes?
Re: Running Mikeos on a *real* 8086
Posted: Thu Apr 19, 2012 3:55 pm
by Burik
Thanks for your reply, I gave "SectorsPerCluster db 2" a try with no luck, the bootdisk can't boot neither the M24 or my main PC. On my Linux box equipped with a 1.44mb drive I'm able to mount the 720K disk and list its contents, the disk itself looks fine. On the M24 running MSDOS5.0 I can't even read the disk. I'm going to take a look at the FAT driver and see if I can accomodate it to the new cluster size.
Re: Running Mikeos on a *real* 8086
Posted: Thu Apr 19, 2012 4:43 pm
by Rudster816
I cant find any part of the OS that uses 32 bit registers, but according to the download page, Mikeos requires a 386 with 1MB of RAM.
http://mikeos.berlios.de/#downloads
Re: Running Mikeos on a *real* 8086
Posted: Thu Apr 19, 2012 7:00 pm
by Senqo
It requires at least a 386 because the FS and GS segment registers are used.
Re: Running Mikeos on a *real* 8086
Posted: Thu Apr 19, 2012 7:35 pm
by bubach
wow, thats stupid. i mean it's a real mode OS ffs
how hard could it be to live without fs and gs? make do.
Re: Running Mikeos on a *real* 8086
Posted: Fri Apr 20, 2012 11:33 am
by Burik
haha, I guess this settle things once and for all, I admit I didn't even read the kernel source, thanks guys.
Code: Select all
*snip*
123 mov fs, ax ; live entirely in 64K
124 mov gs, ax
*snip*
Re: Running Mikeos on a *real* 8086
Posted: Mon Apr 23, 2012 6:47 am
by M-Saunders
Delete those two lines and try it. I've never had access to anything pre-386 to try it on, hence why I said that was a minimum, just to be sure. But take out those two lines referring to GS and FS and see how it works.
M
Re: Running Mikeos on a *real* 8086
Posted: Tue Apr 24, 2012 3:37 am
by amd64pager
What about access to
this?
Re: Running Mikeos on a *real* 8086
Posted: Tue Apr 24, 2012 1:59 pm
by Kazinsal
There are also lines of code in areas such as the FAT12 driver that (if I remember correctly) reference 32-bit registers, usually to clear EAX.
What you could do is use the "CPU 8086" NASM directive to instruct the parser to error out on any post-8086 instructions. Then you'll know where everything potentially-offending is.
Re: Running Mikeos on a *real* 8086
Posted: Tue Apr 24, 2012 3:59 pm
by Burik
M-Saunders wrote:Delete those two lines and try it. I've never had access to anything pre-386 to try it on, hence why I said that was a minimum, just to be sure. But take out those two lines referring to GS and FS and see how it works.
M
Will do. I've little spare time atm but I'll surely keep you updated.
And Blacklight, thanks for the tip.
Re: Running Mikeos on a *real* 8086
Posted: Sun Apr 29, 2012 4:57 am
by Burik
I am as stuck as a fly on flypaper
According to the always trustworthy PC Hardware Book, the disk description table for a 720Kb, 3.5" disk should be as follows:
Code: Select all
OEMLabel db "MIKEBOOT"
BytesPerSector dw 512
SectorsPerCluster db 2 ; 1 for 1.44Mb disk
ReservedForBoot dw 1
NumberOfFats db 2
RootDirEntries dw 112 ; 224 for 1.44Mb disk
LogicalSectors dw 1440 ; 2880 for 1.44Mb disk
MediumByte db 0F9h ; 0F0h for 1.44Mb disk
SectorsPerFat dw 9
SectorsPerTrack dw 9 ; 18 for 1.44Mb disk
Sides dw 2
HiddenSectors dd 0
LargeSectors dd 0
DriveNo dw 0
Signature db 41
VolumeID dd 00000000h
VolumeLabel db "MIKEOS "
FileSystem db "FAT12 "
I also changed the floppy_ok routine according to the new values:
Code: Select all
floppy_ok:
mov ax, 19
call l2hts
mov si, buffer
mov bx, ds
mov es, bx
mov bx, si
mov ah, 2
mov al, 7 ; Read 7 sectors [14 for 1.44Mb disk]
pusha
And I'm building the 720K image this way:
mkdosfs -C -F 12 -r 112 -s 2 -S 512 -v disk_images/mikeos720.flp 720
Too bad the kernel.bin check in the next_root_entry routine fails miserably with the "kernel.bin not found" message, way before the FAT is read into RAM. I can't for the life of me figure out what I'm doing wrong.
Re: Running Mikeos on a *real* 8086
Posted: Fri May 04, 2012 1:48 pm
by JuEeHa
If I remember correctly 8086 didn't have pusha/popa opcodes. Try manualy pushing all registers.
Re: Running Mikeos on a *real* 8086
Posted: Fri May 04, 2012 4:12 pm
by Burik
OMG you're damn right, I didn't even think about that. Too bad manually pushing and popping the registers increases the boot sector size to over 600 bytes, I'll try to cut out the non-essential stuff and see how it goes.
Re: Running Mikeos on a *real* 8086
Posted: Fri May 04, 2012 7:23 pm
by ACcurrent
Intel manual states that they were added in 286. (Volume 3A 17-5)