First look at Assembly instructions

To follow this guide, you will need to use the instructions.asm file located in the guides/instructions/support directory.

Diving right into the demo, we can see one of the most important instructions that helps us, programmers, work with the stack and that is push. We discussed what the push instruction does in the reading section. Considering its call, we can understand that it takes the 0 value(as a DWORD, a number stored on 4 bytes) and moves it onto the "top" of the stack.

That push is followed by a new instruction:

popf

IMPORTANT: The popf instruction is used for setting, depending on how many bytes we pop from the stack(in our case, 4 bytes), the EFLAGS register(setting the entire register when popping 4 bytes and only the 2 lower bytes of the register when popping 2 bytes). You can read more about the popf instruction here and here.

Having in mind what the popf instruction does, try to guess what would adding the following line of code at line 15 and the mystery_label label at the line(of the current file, before adding the instruction) 53 would make the program do.

jnc mystery_label

Moving on, we can see that the 0 value is set to the eax register using the mov instruction. Can you give example of another two ways of setting the value in eax to 0 without using mov ?

HINT: Think about the logical operators.

Next, by using the test instruction we can set the flags based on the output of the logical and between eax and itself.

After resetting the flags, we store 0xffffffff in the ebx register(which is actually the largest number it can store before setting the carry flag) and then use the test instruction yet again. Similarly, what do you think adding the following line of code after the test instruction would produce ?

jnz mystery_label

We reset the flags once again and now we take a look at working with the smaller portions of the eax register. Can you guess the output of the following command, put right under the add al, bl instruction ? What about the flags ? Which flag has been set ?

PRINTF32 `%d\n\x0`, eax

Similarly, try to answer the same questions from above, but considering the next portions of the code.

After thoroughly inspecting this example, you should have a vague idea about how setting the flags works.