noob: 32 bit vs 64 bit?

[thinking]

hiho@ll

i have a very noob question about 32bit vs. 64bit architecture
it's not really a gentoo but i already posted somewhere else, but i didn't get the answer i was searching for (i know maybe the wrong question)
http://www.linuxquestions.org/questions/showthread.php?s=&threadid=354948&highlight=32bit+64bit

for those who don't want read the thread above, the same thread here:
shortly i'll explain what i know and think about the topic:
a cpu has registers to work with
on 32bit these registers are a maximum of 32bit in size
on 64bit they are 64bit in size
so they can store larger values which means they can mostly (for example) be faster or use better hardware (larger harddisk, more ram)

why i ask the question:
if you compile a prog for example a C prog which uses integer
the int value on a 32bit architecture will be 32bit in size on 64bit it's 64bit
ok
now what if i write a prog where i know i don't need to store greater values than 32bit

if i compile the prog on a 32bit architecture, will it run on 64bit?
if not, why? i think the cpu registers are just bigger, why shouldn't work 32bit code on a 64 architecture[/url]?

maybe it's simple and my direction of thinking is right
but i don't think so, that's why i'm asking

thx@ll

[alistair]

32bit programs will run on amd64 (with bios set to allow 32bit mode). For exampe u can quite happily install gentoo-x86 on a 64 bit machine. But u should note that u can't link a 32bit app to a 64 bit library or vise versa.

4bit(i think) flags determine whether the cpu is in 32 or 64 bit mode, Ill let more informed others go into the details of this.

[thinking]

hmm
does this mean if the cpu is in 64bit mode you can't do 32bit calculations like 32bit cpu?
i know i can do the same calculation because 64bit is bigger! but the cpu in 64bit mode has to read more altough i only want 32bit calculations, not?

?? why i can't link 32bit code to 64bit? (i think that's exactly my problem of understanding the topic)
well, i'm thinking to much as a software developer

[alistair]

if your in 64 mode then u can't execute 32 instructions. The kernel switchs operating modes for u though so u dont need to worry. (dont ask me how). U can happily execute 64 and 32 bit programs concurrently.

As for the linking. just imagine the mess that would be caused if some 64bit code passed a long (ie 64bit's) to 32bit code.

sadly i can't find a very interesting doc on the 64bit kernel

[thinking]

hmm
thx
i think i understand the problem now

[andrew_j_w]

One important point to remember is that for AMD64 int is still only 32 bits, it's only long that is 64 bit. The biggest problems comes when programmers try to cast a pointer (64 bits) into an int (32 bits).

HTH,
Andrew

[babo]

so basicly if I understand correctly in order to make a program that runs on x86 to run on amd64 you need to change int's to long's?
PS: I am pretty dumb about programing, but am still interested

[scharkalvin]

Just as the 386 family (that's 386,586,pentium,PII,PIII.P4,athlon,etc) have a real-8086 emulation mode which
enables you to run DOS programs inside of a 32 bit OS (such as windows-NT) the AMD64 cpu's have an
emulation mode that enables you to run 32 bit programs inside of a 64bit OS. However the buck stops there.
You CAN'T run the real-86 emulation mode inside the 32 bit emulation mode, or inside the 64 bit mode.
This means that the DOS-emu package will NOT run with a 64bit OS. I don't know how many people need
to run DOS-emu, but if you do you CAN'T run it on an AMD-64 system UNLESS you run it on a 32 bit
kernel. So in this case you will need a dual boot system, chroot is NOT enough!

[Biased turkey]

[stonie]

[alistair]

woops... why did i think that? maybe i should play with my bios a little more frequently

[Nemonux]

@scharkalvin: Interesting what you say about 8086 emulation. I'll get an AMD64 processor soon and until now I sometimes use dos-box to play some old games. The dualboot workaround even means that I have to provide all needed applications as 32bit binaries, is that right? So I would need a secound root file tree, just like the chroot stuff? I wanted to try to avoid that. Looks like the old gamse have to stay on my notebook.
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[Matt6543]

I know that this thread is over and done, but I feel I have to comment on the main question......
32bit can not run on 64bit. I'll do a comparision with 16bit and 32bit on a x86 (Intel/AMD) code directly.

If I use this (pointless) code:
mov ax , 0x123
add ax , 0x321
mov bx , ax
(so it makes register ax and bx = 444)

Compiled in 16 bit code is:
B8 23 01 05 21 03 89 C3

If its run in 32 bit mode - the processor will think the code is this...
B8 23 01 05 21 03 89 C3
mov eax , 0x21050123
add [ebx+edi] , ecx (or some thing like that)
There for it did not even finish the first operation correctly and the second is almost rubbish



Matt

[meteozwh]

That's right. And another difference that I found no one ever mentioned here is the amd64 ABI(Application Binary Interface) is totally different from x86 ABI.

The amd64 arch have more registers than x86 and as from AMD64, functions use register to pass the parameters while in the x86 mode, functions use the stack to pass the parameters. That's a very important issue to concern.

[Akkara]

[poly_poly-man]

If you're programming in c, basically totally ignore how many bits there are... the only difference is made to long and pointers.

for normal use, there's not a really big difference. For normal programming, there's not a really big difference. You only start hitting strangeness while using more than about 1gb ram (starts to slow down on 32-bit - over about 3 or 4 gb, you slow down even more for PAE), or programming in assembly (strange rules about what you can or can't push, etc.)
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