Foone🏳️⚧️
you breakpoint at the function, and see the following 32 bits at the top of the stack.
7B0E9721
what's the linear address of the return value? Well, obviously, it's
2197:0E7B!
you just gotta remember to reverse the bytes but not the nibbles, that'd be silly. Little Endian makes perfect sense.
Foone🏳️⚧️
btw the colon in that 32bit address means I lied. That's not really the address. The address is, obviously, 227EB
Foone🏳️⚧️
because the colon means this is 16-bit segmented x86! we're in Real Mode!
and real mode is nonsense of a higher order
Foone🏳️⚧️
@kboyd of course
Kevin Boyd
@foone that's on Baker Street, right?
Foone🏳️⚧️
You've got two 16-bit integers, called the segment (or segment selector, or just selector) and your offset.
And it'd make sense that those are just two chunks of a larger 32bit linear address, but they're not. They're two 16-bit chunks of a 20-bit address.
Foone🏳️⚧️
and that'd make sense if only the lower 4 bits of the segment were used.
but all bits are used. They just add the segment and the offset together, after bitshifting the segment over by 4.
So the linear address of AAAA:BBBB is AAAA*16+BBBB.
Foone🏳️⚧️
this does mean that you can make multiple pointers to the same address that have different bit patterns, yes.
Foone🏳️⚧️
so you can have two pointers to the same object but ptr1 != ptr2. This makes programming in real mode Extra Fun!
Foone🏳️⚧️
I dunno. I don't even want to go into Near vs Far pointers and how you have implicit segments (so if you see MOV AX, [1234], you know it gets a value from address 1234, but you don't know WHICH 1234 they mean)
but I had to turn 7B0E9721 into 2197:0E7B in my head and I had a sudden realization of HOW FUCKING STUPID THIS WAS
Foone🏳️⚧️
I LIKE USING MODERN DEBUGGERS WHERE POINTERS REMEMBER HOW TO POINT TO THINGS WITHOUT BEING IN PARTS I HAVE TO MANUALLY PUT TOGETHER
John Carlsen 🇺🇸🇳🇱🇪🇺
Did you perhaps mean *ptr1 != *ptr2 ?
Foone🏳️⚧️
@johnlogic No, it's the pointer addresses that are different, not their content
Foone🏳️⚧️
I've currently got a dos game open in both Ghidra and DOSBox and I'm going to 2978:0E7B and it's different in DOSBox, cause it's off by 64 bytes.
why? I don't know. DOS hates me
Foone🏳️⚧️
why? BECAUSE SEGMENTED POINTERS ARE MADE OF HATRED
Foone🏳️⚧️
@realMagnesium12 generally "you don't" but if you have to, you'd have to calculate the linear addresses and compare those
Magnesium
@foone But how do you actually compare two segmented pointers "the save way?"
Foone🏳️⚧️
@root42 yeah, it's DS. what's DS set to at this time, though? that's why it's tricky
root42
@foone if there is no prefix it should be DS:[1234], or…?
ask
@foone my favorite thing about this is the 1981 IBM PC had literally 4 rows of ram chips each making up 64k which is exactly one segment size.
So if you space out your segments by 64k then the segment register is literally just selecting which of the 4 rows of chips to use.
You can also expand it with an additional 10 non-overlapping segments for 14 total (the rest of the address space is ROM).
Foone🏳️⚧️
@ask Heh, that's a neat way to look at it!
０７６萌ＳＮＳ is a social network, courtesy of ０７６. It runs on GNU social, version 2.0.2-beta0, available under the GNU Affero General Public License.
All ０７６萌ＳＮＳ content and data are available under the Creative Commons Attribution 3.0 license.