; loop_64.asm  simple using rcx and loop
;              for(i=9; i>0; i++) A[i] = 0;
; Assemble:	  nasm -f elf64  loop_64.asm
; Link:		  gcc -m64 -o loop_64  loop_64.o
; Run:		  ./loop_64 > loop_64.out
; Output:	  cat hello_64.out
	
        extern	printf		; the C function, to be called

        section .data		; Data section, initialized variables
fmt:    db "A[%ld]=0", 10, 0  ; The printf format, "\n",'0'

	section .bss
A:	resq	10	        ; A[0] .. A[9] in C  A[0] unused
sav:	resq	1		; in case printf clobbers rcx
	
        section .text           ; Code section.
        global main		; the standard gcc entry point
main:				; the program label for the entry point
        push    rbp		; set up stack frame, must be aligned
	
	mov	rcx,9
loop1:	mov	qword [A+rcx*8],0 ; A[i] = 0

	mov	[sav],rcx       ; printf clobbers rcx  debug printout 
	mov	rdi,fmt         ; address of format, standard register rdi
	mov	rsi,rcx         ; address of first data, standard register rsi
	mov	rax,0		; no float or double in xmm
        call    printf		; Call C function
	mov	rcx,[sav]
	
	loop	loop1		; decrement rcx, jump if rcx > 0 zero

	                        ; [A+rcx*8-8]  A[8] .. A[0]
	pop	rbp		; restore stack

	mov	rax,0		; normal, no error, return value
	ret			; return