In order for a program to run successfully, it needs two things: an entry symbol, and a return code that represents that success. The following program provides those two things.
.global _start _start: MOV X0, #0 MOV X8, #93 SVC 0 |
Compile it using the simple Makefile from the previous article.
The symbol _start is a special symbol expected by the linker. If you try to rename like this…….
.global _ADAM _ADAM: MOV X0, #0 MOV X8, #93 SVC 0 |
…you get the following error:
$ make other as -o other.o other.s ld -o other other.o ld: warning: cannot find entry symbol _start; defaulting to 0000000000400078 |
Which will still compile and run.
If you leave off the three assembly instructions at the end, you do not return 0 indicating the program success.
.global _start _start: |
That gives you this error:
]$ ./bad_return -bash: ./bad_return: cannot execute binary file: Exec format errorTo be honest, there are no instruction in the code, and I think that is a different problem. If we skip executing the function to send the return code:
.global _start _start: MOV X0, #0 MOV X8, #93 |
That gives this error when run:
$ ./bad_return Illegal instruction (core dumped) |
Hi Adam,
I’ve stumbled across your interesting series about assembly programming and I wanted to do first steps in assembly programming.
But when I try to assemble the first program first.s
.global _start
_start:
MOV X0, #0
MOV X8, #93
SVC 0
according to your Makefile, I get the following error message
as -o first.o first.s
first.s: Assembler messages:
first.s:4: Error: expecting operand after ‘,’; got nothing
first.s:5: Error: expecting operand after ‘,’; got nothing
first.s:6: Error: no such instruction: `svc 0′
What should I do?
I am working on AARCH64…also known as ARM64. It is a different set of instructions than on x86_64. When I compile on my Dell Laptop, I get the same error as you see.
You have two choices. You can try to get the AARCH code I have shown above working, or you can switch to something based on x86_64. If you want to do AARCH64, either cross compile and run an emulator, or get a Raspberri Pi.