thello.s: update & fix comments, etc.

- add attribution (mostly stolen from https://polprog.net/blog/netbsdasmprog/)
- replace syscall args comment with the actual one from syscall.c
- rename $hello to $hello_str

(in response to first part of issue #2)

thello.s

@@ -1,29 +1,40 @@
-# SYSCALL ARGS
-# rdi rsi rdx rcx r8 r9
-
+	#
+	#	thello.s: "Hello world!" in x86_64 assembly language
+	#
+	# this example was mostly stolen from:  https://polprog.net/blog/netbsdasmprog/
+	#
 .section	.text
 
 	.globl _start
 
 _start:
 	andq $-16, %rsp		# clear the 4 least significant bits of stack pointer to align it
 
+	# from src/sys/arch/x86/x86/syscall.c:
+	#	ifdef __x86_64__
+	#	/*
+	#	 * The first 6 syscall args are passed in rdi, rsi, rdx, r10, r8 and r9
+	#	 * (rcx gets copied to r10 in the libc stub because the syscall
+	#	 * instruction overwrites %cx) and are together in the trap frame
+	#	 * with space following for 4 more entries.
+	#	 */
+	#
 	mov $4, %rax		# SYS_write
 	mov $1, %rdi
-	mov $hello, %rsi
+	mov $hello_str, %rsi
 	mov hello_len, %dl	# Note: does not clear upper bytes. Use movzxb (move zero extend) for that
 	syscall
 
 	mov $1, %rax		# SYS_exit
 	xor %rdi, %rdi
 	syscall
 
-.section .data
+.section	.data
 
-hello:
+hello_str:
 	.ascii "Hello world!\n"
 hello_len:
-	.byte . - hello
+	.byte . - hello_str
 
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