[PATCH] Re: BOOT_CS

Coywolf Qi Hunt
2004-02-23T04:44:03Z

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Jamie Lokier wrote:

> Coywolf Qi Hunt wrote:
>
>>>(the jump and indirect branch aren't guaranteed to have the
>>>proper effects, although technically neither should be required due to
>>>the %cr0 write):

???

>>
>>
>>IMHO, why bother to re-reload %cs again?
>>
>>In setup.S, %cs is reloaded already. The enable paging code maps the
>>address identically, so %cs already contains the proper selector.
>
>
> It's to flush the instruction prefetch queue: that's one of the side
> effects of ljmp.

Re-loading %cs and flushing prefetch queue are two different things.

>
> I recall an Intel manual that said ljmp is required when switching
> between real and protected modes, to flush the prefetch queue.

Not necessarily ljmp, imho

>
> Unfortunately I don't remember what that manual said about just setting PG.
>
> I'd guess that current generation CPUs don't care about ljmp when
> changing modes, but older ones certainly do.
>
> -- Jamie
>

FYI, intel's example code located in STARTUP.ASM Listing arround line
180, chapter 9, IA-32 Intel Architecture Software Developer's Manual,
Volume 3: System Programming Guide

Please consider my patch for this issue.

Coywolf

--
Coywolf Qi Hunt
Admin of http://GreatCN.org and http://LoveCN.org

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--- head.S.orig 2004-02-18 11:57:16.000000000 +0800
+++ head.S 2004-02-23 12:35:24.000000000 +0800
@@ -115,10 +115,8 @@
movl %cr0,%eax
orl $0x80000000,%eax
movl %eax,%cr0 /* ..and set paging (PG) bit */
- jmp 1f /* flush the prefetch-queue */
-1:
- movl $1f,%eax
- jmp *%eax /* make sure eip is relocated */
+ pushl $1f /* flush the prefetch-queue */
+ ret /* and normalize $eip */
1:
/* Set up the stack pointer */
lss stack_start,%esp