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Hi people,

Sorry for so many questions but I'm very confused .

When I set up a GDT Table, there is a limit and base.
Base have a maximum for 4GB, but why is the limit only 16bits?
another question is, if I make the BASE 1MB, the BASE contain 0x10000 but why not ox1024?
I'm a little bit confused, do you guys know a good tut ore something?

Thanks Peter

It's again the hexadecimal system - 0x100000 = 1* 16^5 = 1,048,576 (dec) = 1 MB

Thanks , do you either why the limit is only 24 bits and de base 32?

because the 286 (for which it was designed) only had 24 address lines(therefore 24bits)

iirc: if you set limit to max it will mean 4GB: get the intel manuals they describe it better than i ever could and a lot of other things youre going to need to know

also: you wont get very far without knowing HEX -- learn it BEFORE continuing it is very useful -- plus its what everyone else uses so you will be at a huge disadvantage if you dont know it

btw: anytime you see 0x before a number it means that it is in HEX (that is zero not o)


hope this helps you out

Thanks for your reply ,

I know the HEX and the o whas a spelling mistake,
Where can I find the intell manual, I'm Dutch and only 14 years old so its hard to understand all those hard things but thanks.
1 question....

When I make a segment model in PMODE, The maximum amount of one segment model is the maximum of the LIMIT, that is 16MB.


Is that right?

Thanks

Hi Peter,
you can either download the reference manual from Intel (http://www.intel.com/design/pentium4/ma ... ex_new.htm) or browse though it in this online version (my.tele2.ee/mtx/i386/manual.htm - pretty usefull if you want to quote something on a message board). Anyway, the reference manual might be usefull to look thinks up but it can't replace a good tutorial or a messageboard.

When I make a segment model in PMODE, The maximum amount of one segment model is the maximum of the LIMIT, that is 16MB.

A segment in pmode can span the whole 4GB address-space. First of all it should be said that 20bits are used to describe the limit (my.tele2.ee/mtx/i386/chp06-03.htm#06-03 - have a look at the ASCII art). These 20bits can, of course, only descibe 1MB. In order to support limits larger than 1MB a small trick is used. If a special attribute bit in the descriptor is set (I think it's the 'G'-bit) the value in the limit-field is in 4kb (2^12) pages. The imit is basically multiplied with 4096.

20bits + 12bits = 32 bits

examples:
G-bit = 0
limit = 0x100
-> segment limit is 0x100 (=245bytes - sorry)

G-bit = 1
limit 0x100
-> segment limit is 0x100*0x1000 = 0x100000 (=1MB)

regards,
gaf

sorry for the mis-information its been a long time since i studied the GDT and i didnt have my manuals with me

Thanks alot! ,

I was on a holiday so I couldn't reply.