System Calls

System calls (or syscalls) are basic operations that your device’s system does for you. This includes things such as reading/writing from a file and taking user input.

System calls work exactly the same as functions, except instead of passing the functions name, you need to put its corosponding code into rax.

For instance, to print to the terminal:

mov rax, 1      ; code to determine which syscall to use
mov rdi, 1      ; code to tell it to print to the terminal
mov rsi, msg    ; address of string to print
mov rdx, 10     ; length of string
syscall	        ; calling the syscall

SYS_WRITE (i.e.t. cout)

mov rax, SYS_WRITE 	; or mov rax, 1 if consts not defined
mov rdi, STDOUT 	; console
mov rsi, msg 		; address of string to print
mov rdx, 10 		; length of string
syscall			    ; calling the syscall

The length does not have to be the actual length, for example, if your msg is “Hello World,” then mov rdx, 5 will only print “Hello”
Returns a positive number if printed
Returns a negative number if errored

SYS_IN i.e.t. cin

(kinda, although you need to specify max length of input)

mov rax, SYS_READ	; or mov rax, 0 if consts not defined
mov rdi, STDIN 		; or mov rdi, 0 if consts not defined
mov rsi, msg 		; address of string to save to
mov rdx, 10 		; length of input
syscall			    ; calling the syscall

Returns a positive number of the number of characters actually read
Returns a negative number if errored

SYS_OPEN

Opens a file
Returns a positive number where the file is located
Returns a negative number if errored
Compare to 0 to see if its a good file or not

mov rax, SYS_OPEN	; or mov rax, 2 if consts not defined
mov rdi, filename	; null terminated address of file
mov rsi, file_status; how to read the file
mov rdx, 10 		; length of input
syscall			    ; calling the syscall

Types of file status that we will use in class:
0 - read only
1 - write only
2 - simultaneous writing

SYS_CREATE

Same as SYS_OPEN, but will create the file if it does not exist
Numerical code: 85 instead of 2
Same return value and everything else
SYS_CLOSE
Closes a file

mov rax, SYS_CLOSE	; or mov rax, 3 if consts not defined
mov rdi, qword[fileDescriptor]
syscall			; calling the syscall

Returns a negative number if errored

There are many more system calls but these are the only ones you need to know right now.