freebios2 mod to reset code.

[ron minnich]

Now that we are using romcc, and all functions are inline, it is getting
increasingly painful to deal with linuxbios images smaller than 64k. This
problem happens as we need a 16-bit jump to the start of the linuxbios
code, which limits us to jumping to 0xffff0000. But if linuxbios is bigger 
than that, we need to be able to jmp to 0xfffe0000, which is not possible 
with the current setup. 

We can grow this if we do the following: 

make reset a rel jump to 0xfffffff0 - (sizeof entry16). Then put entry 16
right before 0xfffffff0. Entry16 code turns on 32-bit mode and then jumps
to the real start of the linuxbios image -- which can now be anywhere in
the flash image. I could use this now, as could others.

here is sample code which I am trying. It now assembles and inspection of 
the object reveals that it is certainly close to correct.

Here's where I wish I had a really good emulator for linux -- checking 
this out would be a whole lot easier.

The test reset16 code:

.section .reset
	.code16

.globl rgdtptr16
.globl reset_start
.type reset_start, @function

reset_start: 
	cli

	xorl	%eax, %eax
	movl	%eax, %cr3    /* Invalidate TLB*/
	invd
	movw	%cs, %ax
	shlw	$4, %ax
	movw	$gdtptr16_offset, %bx
	subw	%ax, %bx
	data32  lgdt %cs:(%bx)

	movl	%cr0, %eax
	andl	$0x7FFAFFD1, %eax /* PG,AM,WP,NE,TS,EM,MP = 0 */
	orl	$0x60000001, %eax /* CD, NW, PE = 1 */
	movl	%eax, %cr0

	/* Now that we are in protected mode jump to a 32 bit code 
segment. */
	data32	ljmp	$ROM_CODE_SEG, $__protected_start

/** The gdt has a 4 Gb code segment at 0x10, and a 4 GB data segment
 * at 0x18; these are Linux-compatible. 
 */

.align	4
.globl rgdtptr16
rgdtptr16:
	.word	gdt_end - gdt -1 /* compute the table limit */
	.long	gdt		 /* we know the offset */

.globl	reset_vector
.align 4
reset_vector:
	jmp reset_start

The net effect is this: 16-bit reset jmps to a very small startup code 
right before 0xfffffff0 that enables full 32-bit mode. We can then jmp 
anywhere we want -- to 0xfffe0000 for example -- which we can't do now. 
This will alleviate the problem of linuxbios images growing to > 64K which 
is a consequence of romcc right now. This will allow us to live with 
really big linuxbios images until romcc makes tighter code.

Comments?  Errors in the code? 

thanks

ron