[syslinux] comboot+PXE stack

[H. Peter Anvin]

Pawel Oleksik wrote:
> 
>>In addition, and I forgot this: real-mode utilities can only access the 
>>bottom 1 MB of memory, which is not where your COM32 program runs.  You 
>>have, however, a "bounce buffer" in low memory, guaranteed to be at 
> 
> Thanks God (and you, Peter, of course) that com32 can be used!
> But on the other hand, it seems to me that Murali's menu system is 
> prepared to work with com16.  How can I combine it with com32?
> 
> If I had any other problems I'll ask on the list again.
> 

One of my goals with the new libcom32 is to port Murali's menu system. 
Unfortunately I haven't gotten there yet.

If you're in a *com16* program, you can just invoke the interrupt vector 
directly, like this:

asm volatile("int $0x22" : "=a" (eax) : "a" (0x0009), "b" (0x0071));

You don't need to set ES because it is already set to your segment.

However, to copy data which is not in your segment, you need a special 
copy routine, like this:

static inline void memcpy_fromseg(void *ptr, uint16_t seg, uint16_t off, 
size_t len)
{
	uint32_t esi = (uint32_t)ptr;
	uint32_t edi = off;
	uint32_t ecx = len;

	asm volatile("movw %0,%%fs ; cld ; fs ; rep ; movsb"
			: "+S" (esi), "+D" (edi), "+c" (ecx)
			: "abd" (seg)
			: "memory");
}