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Groups > alt.os.assembly > #14
| From | Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> |
|---|---|
| Newsgroups | alt.lang.asm, alt.os.assembly, alt.os.development |
| Subject | Re: Little Big Mode |
| Date | 2016-06-14 17:14 -0400 |
| Organization | Aioe.org NNTP Server |
| Message-ID | <20160614171413.3a4d60cb@_> (permalink) |
| References | <nj3hgq$1nvk$1@gioia.aioe.org> <nj436b$gmj$1@dont-email.me> <njocm8$1u8n$1@gioia.aioe.org> |
Cross-posted to 3 groups.
On Tue, 14 Jun 2016 09:45:07 +0200 "wolfgang kern" <nowhere@never.at> wrote: > After checking I can say that all (386+) Intel/AMD CPUs work > identical in TrueRealMode, that mean segments are limited to 64Kb > (aka 16bit). There aren't single instructions to override this > limits, but we got the well known trick to have Unreal Mode: > > 1. check if A20 is ON (only 1 MB available if OFF) > 2. enter PM32 and set segment limits, ie: base 0 and full 4GB range. By "enter PM32" do you mean set CR0.PE? ... Did you mean "enter PM" here instead of "enter PM32"? As I understand it, neither PM16 or PM32 is activated until the far jump, but access to hidden PM features is enabled via setting CR0.PE. > 3. switch to PM16 (just far jump) for a 16-bit code-segment limit *). Is a far jump to activate PM (either PM16 or PM32) necessary for this? My understanding was that once CR0.PE was set, then you could change limits and return to RM16, i.e., no far is jump required to activate PM, or for unreal mode. I've not attempted this. That is just based on what I've read. As a side note, normally, we minimize the instructions between setting CR0.PE and executing a PM far jump instruction. PM activates (either PM16 or PM32) only with a far jump after PM is enabled via CR0.PE being set, correct? Do BIOS calls use far jumps frequently? So, I'm wondering how long you can execute code in RM16 after enabling PM via CR0.PE. Do BIOS calls use far jumps frequently? Rod Pemberton
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Little Big Mode "Mike Gonta" <mikegonta@gmail.com> - 2016-06-06 05:58 -0400
Re: Little Big Mode Cecil Bayona <cbayona@cbayona.com> - 2016-06-06 10:00 -0500
Re: Little Big Mode "wolfgang kern" <nowhere@never.at> - 2016-06-14 09:45 +0200
Re: Little Big Mode Cecil Bayona <cbayona@cbayona.com> - 2016-06-14 10:08 -0500
Re: Little Big Mode "wolfgang kern" <nowhere@never.at> - 2016-06-14 21:18 +0200
Re: Little Big Mode Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> - 2016-06-14 17:14 -0400
Re: Little Big Mode "wolfgang kern" <nowhere@never.at> - 2016-06-15 10:00 +0200
Re: Little Big Mode JJ <jj4public@vfemail.net> - 2016-06-07 13:21 +0700
Re: Little Big Mode CN <qmbmnp3799@pacbell.net> - 2016-06-15 03:08 -0700
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