1. Computer Architecture
x86 Programming
Computer Architecture

2. * Where k is an int variable
CMP Instruction
CMP Instruction compares two operands
Internally comparison is performed via subtraction
One operand has to be a register or constant (8, 16, or 32 bits)
Other operand can be a variable (memory address) or constant
Sets flags as the SUB instruction
Changes CF, OF, SF, PF, and ZF
Operands are not modified.
Instruction
Result
cmpb %BL, %AL
FLAGS(%AL - %BL)
cmpl $8, %EAX
FLAGS(%EAX – 8)
cmpl k, %EAX
FLAGS(%eax – k)
cmpl $8, k
FALGS(k – 8)
* Where k is an int variable

3. JMPcc Instruction
JMPcc is a family of instruction where ‘cc’ stands for condition code
Performs a jump or a branch if a condition is met
Conditions are flag values set by earlier calls to CMP
There are 100s of flavors of JMPcc instruction
For different combination of conditions (FLAG values)
For different JMP sizes
8, 16, or 32-bit offset values to be added to IP
We will cover a few in class. Refer to manual for details.

4. JMPcc Instruction CMP a, b A compare must be performed first!
Flags Used
JMP Taken IF
JE label1
ZF=1
a == b
JNE label1
ZF=0
a != b
JG label1
ZF=0, SF=CF
a > b
JL label1
SF!=CF
a < b
JLE label1
ZF=1, SF!=CF
a <= b
JGE label1
SF=OF
a >= b

5. Example Usage Convert the following Java code to assembly
public static main(String[] args) {
int a = 10, b = 20;
int c = 0;
if (a > b) {
c = a + b; }
else {
c = a – b;
.text /* Every thing below goes in Code Segment */
.global _start /* Global entry point into program */
_start: /* Indicate where program starts */
movl a, %eax /* eax = a */
cmpl b, %eax /* Set Flags */
JG ifPart /* a > b */
subl b, %eax /* eax -= b */
jmp endIF /* Done with else */
ifPart:
addl b, %eax /* eax += b */
endIF:
movl %eax, c /* c = eax */

6. Example Usage (Contd.) Convert the following Java code to assembly
public static main(String[] args) {
int a = 10, b = 20;
int c = 0;
if (a > b) {
c = a + b; }
else {
c = a – b;
/* Continued from previous slide*/
/* exit*/
movl $1, %eax
movl $0, %ebx
int $0x80
.data /* Everything below goes in data segment */
a: .int 10
b: .int 20
c: .int 0