Can I get back an exe file from an .Asm formatted file disassembled with IDA?

Question

I actually have a .asm formatted file. This file was certainly obtained by disassembling an original exe file with IDA and then save as an .asm file. Now I have this .asm and I would like to get back the original exe, that I have lost. Unfortunately I cannot upload this .asm file in IDA pro and assemble it. So my question is "how to assemble it again to make an exe as original one ? Thank for your help. Bye

Answer 1

As far as assembling .asm file is concerned (assuming you are on windows platform) there are various assembler such a NASM,MASM,FLAT assembler etc. and various others in compilers suite such as GNU GCC Mingw.

But considering above case, as .asm file is generated by IDA, directly assembling it in any assembler program will give you large number of errors. Asm file generated by IDA is easy to understand but difficult to assemble.

However, if the size of asm file is small say 1-3Kb a little effort and knowledge of syntax specific to assembler tool will yield the result provided every instruction was disassembled successfully by IDA. But if the size of file is large then it is a difficult task.

Please also note that sometime IDA assembly can be erroneous such as positive sp detected in that case you have to correct/fix the stack size in assembly file manually.

Moreover, sometimes IDA cannot recognize code and it is left as data in the disassembly which will get reflected in your .asm file, this will generate error when you assemble your file.

Finally, assembling IDA generated .asm file is bad idea, it's better to go through IDA pseudo code and generate a corresponding function.