CSC231 Flags Settings Examples

--D. Thiebaut (talk) 20:54, 3 November 2014 (EST)

---

;;; -------------------------------------------------------------------
;;; flagbits.asm
;;; D. Thiebaut
;;; 
;;; a program with several compare instructions and the resulting
;;; flags generated by the ALU.
;;; This program was debugged by ddd and the flags resulting from 
;;; the execution of each instruction was then added as a comment
;;; on the same line as the instruction.
;;; The flags are 
;;; CF: carry
;;; PF: parity (1 if the number of 1s in the result is even)
;;; AF: auxiliary carry (carry into the MSB)
;;; SF: sign (1 if result is negative, 0 otherwise)
;;; ZF: zero (1 if result is 0, 1 otherwise)
;;; IF: interrupt flag (used for critical sections)
;;; OF: overflow (1 if the result overflowed assuming signed numbers)
;;; ID: direction bit (for string operations)
;;; -------------------------------------------------------------------


EXIT    equ             1
        
      	;; ------------------------------------------------------------
	;; data areas
	;; ------------------------------------------------------------

	section	.data

        
	;; ------------------------------------------------------------
	;; code area
	;; ------------------------------------------------------------

	section	.text
	global	_start

_start: nop
        nop                     ; immediate   Flag values
                                ; value       AFTER the instruction
                                ;-----------  ----------------------
        mov     al, 0x43        ; 67
        cmp     al, 0x43        ;                PF ZF IF ID
        cmp     al, 0x42        ; 66                IF ID
        cmp     al, 0x44        ; 68          CF PF AF SF IF ID
        cmp     al, 0xff        ; 255 or -1   CF PF AF IF ID
        cmp     al, 0x81        ; 129 or -127 CF SF IF OF ID

        mov     al, 0xfe        ; 254, or -2 
        cmp     al, 0xff        ; 255  or -1  CF PF AF SF IF ID
        cmp     al, 0xfd        ; 253  or -3  IF ID
        cmp     al, 0x81        ; 129 or -127 PF IF ID
        cmp     al, 0x7f        ; 127         AF IF OF ID

	;; exit()

	mov	eax,EXIT
	mov	ebx,0
	int	0x80		; final system call