Re: [Etherboot-developers] Q: does xstart work?

[Eric W B. <ebi...@ln...>]

Eric W Biederman <ebi...@ln...> writes:

> ke...@us... (Ken Yap) writes:
> 
> > >>	ADDR32	LJMPI(_execaddr-_start)
> > 
> > Incidentally if you feel motivated to convert this bit of data
> > segment-modifying code to a push onto the stack followed by a ret,
> > please do.
> 
> I will or something similiar, accessing the 32bit data segment
> from 16bit is a fairly bad thing to do.


O.k. I have all of the routines converted.
xstart16 still needs a little work to get the bootp data structure below
1MB.  But othewise it should work fine.

The open issues are:
- How to copy/move bootp low for 16bit code.
- How to properly allocate the real mode stack.
    I am thinking I can just use the initial stack and push
    the code on there.

For reference the trampoline _real_call pushes on the real mode stack
is just 80 bytes.  80 bytes shouldn't overflow even a small stack.
The worst cases is meme820 which allocates a 280 byte buffer for the
memory map.

Eric


	.globl	xstart16
xstart16:
	pushl	%ebp
	movl	%esp,%ebp
	pushl	%ebx
	pushl	%esi
	pushl	%edi
	movl	8(%ebp),%edx
	movl	12(%ebp),%ebx

	/* FIXME handle the bootp record */
	movl	16(%ebp),%ecx	/* bootp record (32bit pointer) */
	shll	$12,%ecx	/* convert to segment:offset form */
	shrw	$12,%cx

	pushl	$ 10f
	pushl	$ 20f - 10f
	call	_real_call
	.section ".text16"
10:	.code16
	popw	%ax		/* get the return ip addr */
	pushl	%ecx		/* bootp record */
	pushl	%ebx		/* file header */
	pushw	%cs		/* Setup the far return address */
	pushw	%ax		
	pushl	%edx		/* Setup the far address to call */
	lret			/* Back into the routine I'm calling */
20:	.code32
	.previous
	
	popl	%edi
	popl	%esi
	popl	%ebx
	popl	%ebp
	ret