global directive is NASM specific. It is for exporting symbols in your code to where it points in the object code generated. Here you mark _start symbol global so its name is added in the object code (a.o). The linker (ld) can read that symbol in the object code and its value so it knows … Read more
I recently wrote some General Bootloader Tips in a Stackoverflow answer that may be of some use to you. Likely Tip #1 applies to your problem: When the BIOS jumps to your code you can’t rely on CS,DS,ES,SS,SP registers having valid or expected values. They should be set up appropriately when your bootloader starts. You … Read more
I can deduce you are using NASM (or NASM compatible) assembler. I don’t know what OS you are using to build the bootloader, but I’ll assume Linux or Windows. Other environments would be somewhat similar. You should split your bootloader into two parts to make this easier. One is the bootloader, and the second being … Read more
You included stuff at the top of your bootloader, where it will executes first. Instead include extra functions where they aren’t in the main path of execution and are only reached by call. This should work, placing the %include directives where it’s safe to put extra function or data, just like if you were writing … Read more
It can be done from assembly, but it isn’t easy. You can’t use int 21h, that’s a DOS system call and it isn’t available under Linux. To get characters from the terminal under UNIX-like operating systems (such as Linux), you read from STDIN (file number 0). Normally, the read system call will block until the … Read more
NOSPLIT: Similarly, NASM will split [eax*2] into [eax+eax] because that allows the offset field to be absent and space to be saved; in fact, it will also split [eax*2+offset] into [eax+eax+offset]. You can combat this behaviour by the use of the NOSPLIT keyword: [nosplit eax*2] will force [eax*2+0] to be generated literally. [nosplit eax*1] also … Read more
The code must be in a page with execute permission. By default, stack and read-write static data (like non-const globals) are in pages mapped without exec permission, for security reasons. The simplest way is to compile with gcc -z execstack, which links your program such that stack and global variables (static storage) get mapped in … Read more
The problem is with your stack usage. First, the ABI docs mandate rsp be 16 byte aligned before a call. Since a call will push an 8 byte return address on the stack, you need to adjust rsp by a multiple of 16 plus 8 to get back to 16-byte alignment. The 16 * n … Read more
Each character is only a single byte, but you probably want to add it to a larger result. Might as well go for 32 bits… (can hobble your routine to 16 bits if you really want to) mov edx, num3entered ; our string atoi: xor eax, eax ; zero a “result so far” .top: movzx … Read more
You can’t ret from start; it isn’t a function and there’s no return address on the stack. The stack pointer points at argc on process entry. As n.m. said in the comments, the issue is that you aren’t exiting the program, so execution runs off into garbage code and you get a segfault. What you … Read more