CONVERTER 32/64.DLL DIPTRACE CODE

But if one party doesnвЂ™t even have the capability of talking about that thing, translating between the two will be quite difficult indeed. ItвЂ™s one thing when two people have two different words to describe the same thing.

CONVERTER 32/64.DLL DIPTRACE CODE

ItвЂ™s not like you can build a temporary address window, because 32-bit flat code doesnвЂ™t know about these temporary address windows they abandoned selectors, remember? If there is some memory in the 64-bit address space at offset 0x000006fb`01234567, 32-bit code will be unable to access it. And the 64-bit address space is four billion times larger than the 32-bit address space. The size of the pointer has changed, which means that converting a 32-bit structure to a 64-bit structure and vice versa changes the size of the structure. Neither of these two observations holds true for 32-bit to 64-bit thunking. ItвЂ™s just more cumbersome from the 16-bit side since you have to build a temporary address window.

This means that any memory that 32-bit code can access 16-bit code can also access. ItвЂ™s just that the 16-bit address space can only see the underlying linear address space in windows of 64KB, whereas the 32-bit address space can see it all at once. Next, notice that the 16-bit address space is still fully capable of referring to every byte in the 32-bit address space, since they are both windows into the same underlying linear address space. This means that data structures containing pointers do not change size between their 16-bit and 32-bit counterparts. The value 0x0123:0x467 requires 32 bits, and wow, so too does a 32-bit pointer. Now letвЂ™s look at the consequences of this for thunking.ДЇirst, notice that a full-sized 16-bit pointer and a 32-bit flat pointer are the same size. ItвЂ™s not that there are no selectors itвЂ™s just that there is effectively only one selector, so thereвЂ™s no need to say it all the time. And then I can just declare that all addresses will be in this form and nobody would have to bother specifying which selector to use since it is implied.вЂќДЄnd if you said this, then you invented the Win32 addressing scheme. ThereвЂ™s no need to chop it up into 64KB chunks like I had to back in the 16-bit days. With this selector, I can access the entire 32-bit linear address space. (Accessing the bytes past 0xFFFF required a 32-bit offset, of course.) Now, if you were clever, you could say вЂњWell, let me create a selector and set its base to 0x00000000 and its limit to 0xFFFFFFFF. With the introduction of the 80386, the maximum limit for a selector was raised from 0xFFFF to 0xFFFFFFFF. For example, the entry for selector 0x0123 might say вЂњ0x0123 is a read-only data selector which begins at linear address 0x00524200 and has a limit of 0x7FFF.вЂќ This means that the address 0x0123:n refers to the byte whose linear address is 0x00524200 + n, provided that n в‰¤ 0x7FFF. This means вЂњThe byte at offset 0x4567 relative to the selector 0x0123.вЂќ Each selector had a corresponding entry in one of the descriptor tables which describes things like what type of selector it is (can it be used to read data? write data? execute code?), but whatвЂ™s important here is that it also contained a base address and a limit. For example, a typical address might be 0x0123:0x4567. In 16-bit Windows, addresses were specified in the form of a selector (often mistakenly called a вЂњsegmentвЂќ) and an offset.

Okay, I suspect most people havenвЂ™t had to deal with selectors (and thatвЂ™s probably a good thing). The terms 16 and 32 refer to the size of the offset relative to the selector. Why canвЂ™t we do the same thing to allow 32-bit code to call 64-bit code?Д«oth 16-bit and 32-bit Windows lived in a 32-bit linear address space. It was possible to use generic thunks in 16-bit code to allow it to call into 32-bit code.

Converter 32/64.dll diptrace