Date: Wed, 24 Oct 2001 08:59:30 +0800 From: "XuYifeng" <xuyifeng@peoplemail.com.cn> To: <freebsd-questions@freebsd.org> Subject: A stupid question about pmap Message-ID: <001b01c15c27$2332db60$ef01a8c0@davidwnt>
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Hi,
I am learning pmap code in FreeBSD 4.4-stable, and confused by some code,
for example, in function get_ptbase, the code is:
--------
static unsigned *
get_ptbase(pmap)
pmap_t pmap;
{
unsigned frame = (unsigned) pmap->pm_pdir[PTDPTDI] & PG_FRAME;
/* are we current address space or kernel? */
if (pmap == kernel_pmap || frame == (((unsigned) PTDpde) & PG_FRAME)){
return (unsigned *) PTmap;
}
/* otherwise, we are alternate address space */
if (frame != (((unsigned) APTDpde) & PG_FRAME)) {
APTDpde = (pd_entry_t) (frame | PG_RW | PG_V);
#if defined(SMP)
/* The page directory is not shared between CPUs */
cpu_invltlb();
#else
invltlb();
#endif
}
return (unsigned *) APTmap;
}
-----
I know pm_pdir[PTDPTDI] contains page directory page's physical address,
while PTDpde is a virtual address to access page directory's page directory
entry which is recursivly mapped on virtual address space.
question is why could (pmap->pm_pdir[PTDPTDI] & PG_FRAME) and (PTDpde & PG_FRAME)
be same thing? a physical address = a virtual address?
I am really confused by this.
Any help will be appreciated,
---
XuYifeng
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