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> DOS > MS COFF 16bit code, no jmp possible |
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okasvi 18 Aug 2006, 19:46
Code: mov eax, *  |
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18 Aug 2006, 19:46 |
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Tomasz Grysztar 18 Aug 2006, 20:00
COFF is a 32-bit format, it doesn't have 16-bit relocations.
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18 Aug 2006, 20:00 |
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mysli 18 Aug 2006, 20:03
what do you mean by
mov eax, * ?? Actually mov eax,0 was just an example, no matter where I point the jmp to, I always get this "illegal use of symbol", btw loop doesn't work either. So to me it appears FASM has under this conditions (MS COFF + use16) some problems evaluating jump (loop => nothing else as jmp) locations. MYSLI |
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18 Aug 2006, 20:03 |
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mysli 18 Aug 2006, 20:08
Ok, COFF handles only 32bit relocations, so there is no way to overcome this problem, I must continue to code my jumps with "db 0EAh, ..." ?
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18 Aug 2006, 20:08 |
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Tomasz Grysztar 18 Aug 2006, 20:14
In fact I would consider COFF not applicable at all for such purposes.
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18 Aug 2006, 20:14 |
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mysli 18 Aug 2006, 21:21
The thing is, I wrote a linker which links MS COFFs, the reason is I use VC++ to write 32bit code which can be executed in protected mode, since I don't want to run this code under Windows, I needed a small loader running under DOS which brings me into protected mode and calls the 32bit code generated by VC++. I was pretty happy to find out that fasm is able to generate a MS COFF file, so my linker was right away able to interpret this file and makes an executable out of all my OBJs running under DOS. I managed this only with this few hickups I described above. Yes, you may be right, MS COFF for my loader might not be the right format, but it was pretty obvious for my needs. I think I will just live with my workarounds, anyway the loader will not be modified that often.
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18 Aug 2006, 21:21 |
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okasvi 18 Aug 2006, 22:37
mysli wrote: The thing is, I wrote a linker which links MS COFFs, the reason is I use VC++ to write 32bit code which can be executed in protected mode, since I don't want to run this code under Windows, I needed a small loader running under DOS which brings me into protected mode and calls the 32bit code generated by VC++. I was pretty happy to find out that fasm is able to generate a MS COFF file, so my linker was right away able to interpret this file and makes an executable out of all my OBJs running under DOS. I managed this only with this few hickups I described above. Yes, you may be right, MS COFF for my loader might not be the right format, but it was pretty obvious for my needs. I think I will just live with my workarounds, anyway the loader will not be modified that often. you could always make macro for jmp under use16 _________________ When We Ride On Our Enemies support reverse smileys |: |
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18 Aug 2006, 22:37 |
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mysli 20 Aug 2006, 11:27
That's what I finally did.
BTW, it's not a problem of 32bit relocations, I would say it's a general issue with the assembler. The jumps I'm talking about are all IP relative, there are no absolute addresses which would be relocated by any linker. What I now did is: foo: any code .... db 0E9h ;IP relative JMP -> same as jmp foo db foo - $ So there is no need of any relocation since the assembler calculates the IP relative distance to the destination address. This is the same for any conditional jump, they don't work either, although to me there is not really a reason imaginable why not the assembler calculates this relative jumps. Tomasz? Regards, Mysli |
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20 Aug 2006, 11:27 |
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Tomasz Grysztar 20 Aug 2006, 12:14
If you do "jmp dword foo" (even "jmp short dword foo") it's going to work correctly. However with "use16" the jumps are "word" ones by default, and those jumps modify IP only (clearing the high 16 bits of EIP). Since the "foo" is a 32-bit relocatable value that in most cases will probably not fit in 16 bits, such instruction cannot be ensured to work properly and thus assembler gives you "invalid use of symbol" error.
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20 Aug 2006, 12:14 |
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mysli 21 Aug 2006, 11:05
Why do you consider "foo" in my example as a relocatable value, to me it just represents a value of the IP. A relocatable value to me is a variable where I don't know its address during design of the code, and which depents on the absolute location in memory during execution, but I think this is not the case for the label "foo", its address is independent of the absolute location in memory and therefore no relocation is needed. "foo" will also not show up in the relocation table of the COFF later on.
How do you do this for "binaries", how are jumps evaluated there? Would there be the chance to force the assembler to take only 16bit of the EIP into account, i.e. jmp word foo or jc word foo or what ever. Then it would be up to the programmer to use the right wording to generate the right code. One more question, in general does the "use16" directive also effect the default width of the (E)IP or just the usage of the prefixes 66h/67h? Regards, Mysli |
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21 Aug 2006, 11:05 |
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Tomasz Grysztar 21 Aug 2006, 11:16
mysli wrote: Why do you consider "foo" in my example as a relocatable value, to me it just represents a value of the IP. A relocatable value to me is a variable where I don't know its address during design of the code, and which depents on the absolute location in memory during execution, but I think this is not the case for the label "foo", its address is independent of the absolute location in memory and therefore no relocation is needed. "foo" will also not show up in the relocation table of the COFF later on. All the data and code in the linkable objects is relocatable and thus all the labels are relocatable. After linking the "foo" may happen to be 1200h, but it may happen to be 7000000h also. mysli wrote: How do you do this for "binaries", how are jumps evaluated there? Since those formats (and also ELF executables, PE without fixups or MZ) are not linkable and thus use absolute ('known') addresses, which then are nothing but ordinary numbers, you can use them in any way as you would use any given number. Thus even the things like: Code: jmp (foo*foo)/bar are possible with absolute labels (if only you'd find some use for it). mysli wrote: Would there be the chance to force the assembler to take only 16bit of the As I said, the COFF format doesn't have 16-bit relocations - which is exactly what is needed to force the LINKER to make it work. The OMF objects would work for it but fasm doesn't support that format. mysli wrote: One more question, in general does the "use16" directive also effect the default width of the (E)IP or just the usage of the prefixes 66h/67h? There is no such thing as "default width of the (E)IP" in fasm. The addresses are actually 64-bit internally, but as long as the label fits in 32 bits, the "jmp dword" to that label can be generated, etc. With the relocatable formats there is the default size (usually 32 bits) for the address value that can be relocated and thus addresses are assumed to fit in those boundaries (unless the offset value is already larger than 32 bits) and it's linker's job to ensure that after relocating the values are still in correct range. |
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21 Aug 2006, 11:16 |
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