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66 lines
1.9 KiB
66 lines
1.9 KiB
*Concepts you may want to Google beforehand: C, object code, linker, disassemble*
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**Goal: Learn to write the same low-level code as we did with assembler, but in C**
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Compile
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-------
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Let's see how the C compiler compiles our code and compare it to the machine code
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generated with the assembler.
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We will start writing a simple program which contains a function, `function.c`.
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Open the file and examine it.
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To compile system-independent code, we need the flag `-ffreestanding`, so compile
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`function.c` in this fashion:
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`i386-elf-gcc -ffreestanding -c function.c -o function.o`
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Let's examine the machine code generated by the compiler:
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`i386-elf-objdump -d function.o`
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Now that is something we recognize, isn't it?
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Link
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----
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Finally, to produce a binary file, we will use the linker. An important part of this
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step is to learn how high level languages call function labels. Which is the offset
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where our function will be placed in memory? We don't actually know. For this
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example, we'll place the offset at `0x0` and use the `binary` format which
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generates machine code without any labels and/or metadata
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`i386-elf-ld -o function.bin -Ttext 0x0 --oformat binary function.o`
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*Note: a warning may appear when linking, disregard it*
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Now examine both "binary" files, `function.o` and `function.bin` using `xxd`. You
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will see that the `.bin` file is machine code, while the `.o` file has a lot
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of debugging information, labels, etc.
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Decompile
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---------
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As a curiosity, we will examine the machine code.
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`ndisasm -b 32 function.bin`
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More
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----
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I encourage you to write more small programs, which feature:
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- Local variables `localvars.c`
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- Function calls `functioncalls.c`
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- Pointers `pointers.c`
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Then compile and disassemble them, and examine the resulting machine code. Follow
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the os-guide.pdf for explanations. Try to answer this question: why does the
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disassemblement of `pointers.c` not resemble what you would expect? Where is
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the ASCII `0x48656c6c6f` for "Hello"?
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