The comparison is the same for signed and unsigned numbers, but the conditional jumps are different.

Comparing numbers

Danger

• Sequential
• Repetition
• Selection

The following will teach you how to do a GOTO. I must emphasis that you use a "Jump" only to implement Repetition and Selection, even though you have the power to do things differently!

General Jumps

Since we want the assembler to compute things for us, we will give the target as a label. (You can do it manually, however, if you go back and change the program later, it might change the distance of the jump and if you forget to make the change, you just broke your program! Therefore, always let the assembler worry about it.) Because of the distance limitation, we will call this a shortlabel. Actually, it could be an address, which is simply a number.

Note: Valid characters in labels are letters, numbers, _, $, #, @, ~, ., and ?. The only characters which may be used as the first character of an identifier are letters

Jumps do not alter the flags.

There is an unconditional jump, or one that is always preformed. The unconditional jump is not not limited to a short jump, and properly coded, can jump anywhere in memory!

The format is:

  jmp label               ; unconditional jump
  jconditional shortlabel ; conditional jump

Flag-based jumps

Signed Conditional Jumps

Selection

Suppose we want to do something in C like:

if (ax < bx)
{
     X = -1;
}
else
{
X = 1;
}

	cmp 	ax, bx  	
	jl  	axLess          ; go to 'axLess' if ax < bx
	mov 	word [X], 1     ; This is the 'else part 
	jmp 	Both            ; skip the 'then' part 
axLess: 
	mov 	word [X], -1    ; This is the 'then' part
Both:

        cmp 	ax, bx  	
	jge 	notLess         ; use a jump with the opposite condition
	jmp 	axLess          ; now do a long jump 
notLess:
  	mov 	word [X], 1     ; This is the 'else part 
	jmp 	Both            ; skip the 'then' part 
axLess:
  	mov 	word [X], -1    ; This is the 'then' part 
Both:  			        ; Everyone back together here 

An Example Program

C

#include 

int main ( void )
{

  short  X;
  short  in1, in2;

  printf( "Please enter two signed 16-bit integers\n" );
  printf( "Enter an integer: " );
  scanf( "%d", &in1 );

  printf( "Enter an integer: " );
  scanf( "%d", &in2 );


  if ( in1 < in2 )
    {
      X = -1;
    }
  else
    {
      X = 1;
    }

  printf( "X = %d\n", X );

  return 0;

}

Assembly Language Version Of The Same Program

section .data
instruct db	'Please enter two signed 16-bit integers', 10, 0
instr1   db     'Enter an integer: ',0
mystring db	10, 'X = %d', 10, 0
instring db     '%d',0

section .bss
X	resw	1
in1	resd	1
in2	resd	1
section .text
    global main                       ;must be declared for linker (ld)

main:  
                               ;tell linker entry point
extern	printf
extern  scanf
;; put your code here

getinput:
	push	instruct
	call	printf
	add	esp, 4

	push	instr1
	call	printf
	add	esp, 4

	push	in1
	push	instring
	call	scanf
	add	esp, 8

getinput2:
	push	instr1
	call	printf
	add	esp,4
	push	in2
	push	instring
	call	scanf
	add	esp, 8

	mov	eax, [in1]
	mov	ebx, [in2]

chkinput:
        cmp 	ax, bx  	
	jge 	notLess         ; use a jump with the opposite condition
	jmp 	axLess          ; now do a long jump 
notLess:
  	mov 	word [X], 1     ; This is the 'else part 
	jmp 	Both            ; skip the 'then' part 
axLess:
  	mov 	word [X], -1    ; This is the 'then' part 
Both:  			        ; Everyone back together here 

	mov	ax, [X]
	cwde			; Since we are printing using %d, we need to convert X
				; to 32 bits.
	
	push	eax
	push	mystring
	call	printf
	add	esp, 8

;; The final part of the program must be a call to the operating system to exit
;;; the program.
        mov     ebx,0   ;successful termination of program
        mov     eax,1   ;system call number (sys_exit)
        int     0x80    ;call kernel

;; Notice the file just ends.

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©2004, Gary L. Burt