[9fans] loading plan 9 directly from linuxbios

[ron minnich <rminnich@lanl.gov>]

I'm working to get plan 9 to load directly from linuxbios. The first
problem I've hit is that Plan 9 assumes that the gdt table is set up by
9load in a friendly fashion. Because linuxbios has to accomodate so many
different OSes with directly conflicting GDT setups, it's not really
practical to try to make linuxbios have a plan 9 friendly gdt. I'd like to
have plan 9 just set up its own gdt correctly.

I'm trying to get this to work but am still at the 'system wedges up'
stage. here is my (doubtless incorrect) code. Can somebody remind me what
the MSW is.

Also what is a reasonable way to do this:
	$PADDR($tgdtptr(SB))
or is that impossible?


TEXT _start0x00100020(SB),$0
        CLI            /* make sure interrupts are off */
        /* for linuxbios. */
        /* now set up the gdt so we have sane plan 9 style gdts. */
        MOVL    $tgdtptr(SB), AX
        ANDL    $~KZERO, AX
        MOVL    (AX), GDTR
/*      MOVL    $tgdtptr(SB),GDTR /**/
        MOVW    $1, AX
        MOVW AX,MSW
/*
 *      clear prefetch queue (weird code to avoid optimizations)
 */
        DELAY

/*
 *      set all segs
 */
        MOVW    $(1<<3),AX      /**/
        MOVW    AX,DS
        MOVW    AX,SS
        MOVW    AX,ES
        MOVW    AX,FS
        MOVW    AX,GS

/*      JMP     $(2<<3):$mode32bit(SB) /**/
         BYTE   $0xEA
         LONG   $mode32bit-KZERO(SB)
         WORD   $(2<<3)

TEXT    mode32bit(SB),$0
	/* end linuxbios stuff */
        MOVL    $PADDR(CPU0PDB), DI             /* clear 4 pages for the
tables
etc. */
        XORL    AX, AX
        MOVL    $(4*BY2PG), CX
        SHRL    $2, CX

etc.
etc.



The GDT is this:

/* for linuxbios mods. *
/*
 *  gdt to get us to 32-bit/segmented/unpaged mode
 */
TEXT    tgdt(SB),$0

        /* null descriptor */
        LONG    $0
        LONG    $0

        /* data segment descriptor for 4 gigabytes (PL 0) */
        LONG    $(0xFFFF)
        LONG    $(SEGG|SEGB|(0xF<<16)|SEGP|SEGPL(0)|SEGDATA|SEGW)

        /* exec segment descriptor for 4 gigabytes (PL 0) */
        LONG    $(0xFFFF)
        LONG    $(SEGG|SEGD|(0xF<<16)|SEGP|SEGPL(0)|SEGEXEC|SEGR)

/*
 *  pointer to initial gdt
 */
TEXT    tgdtptr(SB),$0

        WORD    $(3*8)
        LONG    $tgdt-KZERO(SB)



I suspect the tgdtptr is bogus ... probably an 0x8000xxxx address.

Any hints welcome.

ron