Naushit Sakarvadia wrote:
>
> > > 1) Can this be done with GNU compilers and
> > > if so, which is the best one to use?
> >
> > Yes and no. GCC only builds 32-bit code, and x86 machines boot into 16-bit
> > mode. However, as86 (or more recently, gas) can assemble the 16-bit code
> > you need to switch to protected mode.
>
> yes that's pitty for <386 computer but another GNU varient called BCC
> whichcompiles 16 bit code. which is being used by ELKS guys..( for hint).You can
> use this compiler to to generate Object files....(all the best)
Of course you can use the GCC tools to create a bootstrap.
I have enclosed some code which worked. (I just might not have the
correct version at hand.)
> >
> > > 2) What compiler directives (or linker commands) do I need to know
> > > to set things like program origin, data space etc.
> >
You might use a seperate linker file boot.ld using ld -Tboot.ld ... (a
lot missing here) and your
boot.ld might then look like this:
SECTIONS {
.bootstrap 0xfffffff0 : { real.o(__bootstrap); }
.realstart 0xfffffe00 : { real.o(__realstart); }
.text 0xfffc0000 : { *(.text) _etext = .; }
.data .... <missing>
}
> > You'll need to sit down and acquaint yourself with the ld info page for
> > that.
>
Definitly.
------
This is the bootstrap code I mentioned above (it might not be the
correct version)
/* this is the entry point for the CPU right after RESET */
/* __bootstrap must be mapped to 0xfffffff0 */
.section __bootstrap,"a"
.code16
.byte 0x67 /* as far as I can remember, this was necessary */
jmp _realstart
/* this is the second part executed in real mode */
.global _realstart
.section __realstart,"a"
_realstart: /* entry point. processor's reset vector
should point
here */
.code16
/* Switch to protected mode with flat 4 GB address spaces (one for
code, one for data) */
movl $gdt_desc,%ebx
andl $0xffff,%ebx
.byte 0x66
lgdt %cs:(%ebx)
movl %cr0,%eax # enter protected mode
xor %ax,%ax
inc %ax
lmsw %ax
jmp _flush
_flush:
ljmp $SEGMENT_SYSTEM_CODE,$_sysinit # required far jump to
protected
mode code
/* this is the gdt which is only used to jump to protected mode */
.align 8
gdt_desc:
.word 0x0020 # gdt limit=32, 4 GDT entries
.long gdt
.align 8
gdt:
.word 0,0,0,0 # dummy
.word 0,0,0,0 # unused
.word 0xFFFF # 4Gb - (0x100000*0x1000 = 4Gb)
.word 0x0000 # base address=0
.word 0x9A00 # code read/exec
.word 0x00CF # granularity=4096, 386 (+5th nibble of limit)
.word 0xFFFF # 4Gb - (0x100000*0x1000 = 4Gb)
.word 0x0000 # base address=0
.word 0x9200 # data read/write
.word 0x00CF # granularity=4096, 386 (+5th nibble of limit)
--
Christian Zankel <[EMAIL PROTECTED]>
