Subject: Fibonacci Sequence Program

Hey!

	I just thought I would point it out that the person here made a common mistake in the file fibonacci.s
 While common, it is in fact wrong. The following code segment is incorrect:

	# dump output

	li      r0, 4				; syscall number ( system write )
	li      r3, 1				; first arg stdout
	lis     r4, ha16(output) 	; first half of string
	addi    r4, r4, lo16(output) ; second half of string
	li      r5, len			; message length
	sc					; call kernel
	**	

	# return fib number
	xor.    r3, r3, r3      		; zero out register 3
	mr      r3, r11

	The place I have marked with a (**) is incorrect due to the system call error handling mechanism. 
What happens, is that if an error occurs execution resumes at the next line (PC++), but if an error does 
not occur execution resumes at (PC+2). This means that in effect the xor. r3,r3,r3 instruction is never 
called, ineffectual though it is anyway. As a result, what I would do is replace the (**) with nop [which
 translates to ori r0,r0,0] because this program does not care about handling errors. Or it could 
set r3 to -1, or something similar that would indicate error.

	I just thought I would point this out because this error occurs quite often in assembly language
 programming, and I thought I would clarify this mechanism.

Hope that was helpful!

Martin Mroz