helloworld.s

helloworld.s

/*
minimul required to print hello world.
also can compile with 
cc helloworld.s 
and exec with ./a.out

or use the linking
as helloworld.s -o helloworld.o
ld helloworld.o -o helloworld -l System -syslibroot `xcrun -sdk macosx --show-sdk-path` -e _main -arch arm64
*/

.global _main

.align 2       // important for apple silicon it is always 2!!

_main:
    mov x0, #1                     // File descriptor (stdout)
    adr x1, helloworld             // Pointer to message
    mov x2, #12      	           // Length of message
    mov x16, #4                    // write system call number
    svc 0                          // exec the system call 

    mov x0, #0                      // Exit code
    mov x16, #1                     // System call number (sys_exit)
    svc 0                           // exec the system call and exit


helloworld:    .ascii "hello world\n"

helloworld2.s

/*
this is the bare minimum code to print hello world in assembly language
even shorter using ret for return. 
 */

.global _main

.align 2        // important for apple silicon it is always 2!!

_main:

// this is equivalent to write(1, "hello world\n", 12)
// write is a system call that writes to a file descriptor
// x0 x1 x2 are the parameters to the system call
    mov x0, #1					// File descriptor (stdout)
    adr x1, helloworld				// Pointer to message
    mov x2, #12					// Length of message

// also a supervisor call is needed to execute the system call
// need register x16 to hold the system call number
    mov x16, #4					// write system call number
    svc 0						// exec the system call 

// the system jumps to the address stored in x30
// if my program doesnt have subroutine then I can 
// use ret to return to the caller. otherwise I need to store
// the x30 I get when first entering the program and load it 
// before caling ret
	ret


helloworld:    .ascii "hello world\n"

if_statement

/*

adapted for mac M1
*/
    .global _main					// Expose 'main' to the linker.
    .text							// Specify what comes next is code.
	.align 2						// Align the next instruction on a 4 byte boundary.

/*	Perry Kivolowitz
	Assembly Language Programming Made Not So Scary
	
	if06.s - demonstrating an IF statement.

	This program will compare 5 to 10. If 5 is larger
	(it isn't), the program will print TRUE. If 10 is
	larger than 5 (it is), the program will print
	FALSE.
*/

_main:
    str     x30, [sp, -16]!			// Preserve x30 due to function call.
    mov     x1, 10					// b gets 10
    mov     x0, 5					// a gets 5

    cmp     x0, x1					// a - b ... 5 - 10 makes negative 5.
    ble     1f						// If b >= a, thake the branch. ble -> branch less equal
    adr     x0, T					// a is greather than b - load the
									// address of TRUE string into x0
    b       2f						// Skip over prepping FALSE

1:  adr    x0, F					// x0 contains a pointer to the 
									// correct string either way.
									
2:  bl 		_puts					// Print the string with a new line.

	ldr     x30, [sp], 16			// Restore x30 so we can return.
    mov     x0, xzr					// Indicate success to the caller.
    ret								// Return back to the caller.

F:  .asciz "FALSE"					// Null terminated C string.
T:  .asciz "TRUE"					// Null terminated C string.
    //.data							// What comes next is data.

    .end							// Tells the assembler that it
									// should emit an error if any
									// more data or instructions
									// come after.

/*
.asciz declares a string and automatically appends a null character (\0) 
at the end of the string. 
This is useful when working with C functions, 
which expect strings to be null-terminated.
.ascii declares a string exactly as it is. 
It does not append a null character at the end of the string.
 */

for Mac I need to add the
adr x0, F instead of ldr x0, =T as in Linux