; syscall1_64.asm   demonstrate system, kernel, calls
; Compile:	nasm -f elf64 syscall1_64.asm
; Link		gcc -m64 -o syscall1_64 syscall1_64.o
; Run:		./syscall1_64
;

	section	.data
msg:	db  "syscall1_64.asm running",10	; the string to print, 10=crlf
len:	equ $-msg     ; "$" means here, len is a value, not an address
msg2:	db  "syscall1_64.asm finished",10
len2:	equ $-msg2
msg3:	db  "syscall1_64.asm opened",10
len3:	equ $-msg3
msg4:	db  "syscall1_64.asm read",10
len4:	equ $-msg4
msg5:	db  "syscall1_64.asm open fail",10
len5:	equ $-msg5
msg6:	db  "syscall1_64.asm another open fail",10
len6:	equ $-msg6
msg7:	db  "syscall1_64.asm read fail",10
len7:	equ $-msg7

name:	db  "syscall1_64.asm",0 	; "C" string
fd:	dd  0			; file descriptor, int
flags:	dd  0			; hopefully read-only
	section .bss
line:	resb  8193		; read/write buffer 16 sectors of 512
lenbuf:	resd     1		; number of bytes read

	extern  open
	global	main
	section .text
main:
        push    rbp		; set up stack frame

; header msg
	mov	rdx,len		; arg3, length of string to print
	mov	rcx,msg		; arg2, pointer to string
	mov	rbx,1		; arg1, where to write, screen
	mov	rax,4		; write command to int 80 hex
	int	0x80		; interrupt 80 hex, call kernel

open1:	
	mov	rdx,0		; mode
	mov	rcx,0		; flags, 'r' equivalent O_RDONLY
	mov	rbx,name	; file name to open
	mov	rax,5		; open command to int 80 hex
	int	0x80		; interrupt 80 hex, call kernel
        mov	[fd],rax	; save fd
	cmp	rax,2		; test for fail
	jg	read		; file open

; file open failed msg5
	mov	rdx,len5	; arg3, length of string to print
	mov	rcx,msg5	; arg2, pointer to string
	mov	rbx,1		; arg1, where to write, screen
	mov	rax,4		; write command to int 80 hex
	int	0x80		; interrupt 80 hex, call kernel

read:	
; file opened msg3
	mov	rdx,len3	; arg3, length of string to print
	mov	rcx,msg3	; arg2, pointer to string
	mov	rbx,1		; arg1, where to write, screen
	mov	rax,4		; write command to int 80 hex
	int	0x80		; interrupt 80 hex, call kernel

doread:	
	mov	rdx,8192	; max to read
	mov	rcx,line	; buffer
	mov	rbx,[fd]	; fd
	mov	rax,3		; read command to int 80 hex
	int	0x80		; interrupt 80 hex, call kernel
	mov	[lenbuf],rax	; number of characters read
	cmp	rax,0		; test for fail
	jg	readok		; some read

; read failed msg7
	mov	rdx,len7	; arg3, length of string to print
	mov	rcx,msg7	; arg2, pointer to string
	mov	rbx,1		; arg1, where to write, screen
	mov	rax,4		; write command to int 80 hex
	int	0x80		; interrupt 80 hex, call kernel
	jmp	fail		; nothing read

; file read msg4
readok:	
	mov	rdx,len4	; arg3, length of string to print
	mov	rcx,msg4	; arg2, pointer to string
	mov	rbx,1		; arg1, where to write, screen
	mov	rax,4		; write command to int 80 hex
	int	0x80		; interrupt 80 hex, call kernel

write:	
	mov	rdx,[lenbuf]	; length of string to print
	mov	rcx,line	; pointer to string
	mov	rbx,1		; where to write, screen
	mov	rax,4		; write command to int 80 hex
	int	0x80		; interrupt 80 hex, call kernel

fail:	
; finished msg2
	mov	rdx,len2	; arg3, length of string to print
	mov	rcx,msg2	; arg2, pointer to string
	mov	rbx,1		; arg1, where to write, screen
	mov	rax,4		; write command to int 80 hex
	int	0x80		; interrupt 80 hex, call kernel
	
	mov	rbx,0		; exit code, 0=normal
	mov	rax,1		; exit command to kernel
	int	0x80		; interrupt 80 hex, call kernel