1. Carnegie Mellon
Ithaca College
Machine-Level Programming IV Control Comp 21000: Introduction to Computer Organization & Systems Systems book chapter 3*

2. Today Control: Condition codes Conditional branches Loops
Ithaca College
Today
Control: Condition codes
Conditional branches
Loops
Switch Statements

3. Jumping A jump instruction causes execution to jump to a new address
Really just puts the argument of the jump into the EIP
Jump destinations are usually indicated by a label
A label marks a place in code
1 xorq %rax, %rax
2 jmp .L1
3 movq (%rax), %rdx
4 .L1:
popq %rdx
The instruction jmp .L1 causes program to skip line 3
jmp is an unconditional jump instruction
Note that we wouldn’t really use the code this way!

4. Jumping or A jump instruction has two types of arguments
Direct jumps. Target is part of the instruction
Use a label
Indirect jumps. Target read from a register or memory.
Use a ‘*’ followed by an operand specifier
jmp *%eax or
jmp *(%eax)
Any addressing mode can be used
Note that jmp is an unconditional jump instruction

5. Jumping Other jump instructions are conditional
Either jump or continue executing at next instruction in the code
Depends on some combination of the condition codes
Conditional jumps can only be direct
Assembler changes labels into actual memory addresses
See section of the book
Not very important now; will be very important in chapter 7

6. Jumping jX Instructions
Ithaca College
Jumping
jX Instructions
Jump to different part of code depending on condition codes jX
Condition
Description
jmp 1
Unconditional je ZF
Equal / Zero
jne
~ZF
Not Equal / Not Zero js SF
Negative
jns
~SF
Nonnegative jg
~(SF^OF)&~ZF
Greater (Signed)
jge
~(SF^OF)
Greater or Equal (Signed) jl
(SF^OF)
Less (Signed)
jle
(SF^OF)|ZF
Less or Equal (Signed) ja
~CF&~ZF
Above (unsigned) jb CF
Below (unsigned)

7. Reading condition codes (cont)
Assume a is in %rdx and b in %rax
cmpq %rax, %rdx
jl SUM
Goal: jump if a is less than b
First instruction sets the condition code.
cmpq computes a – b
a < b then a – b < 0
Second instruction checks to see if the compq proved that a – b < 0
Checks (SF^OF)
Goal: jump if a < b or a – b < 0
jl LABEL ; if (SF^OF) %RIP  LABEL

8. Reading condition codes (cont)
Assume a is in %rdx and b in %rax
cmpq %rax, %rdx
jl SUM
How does this work?
cmpq computes a – b
If a < b then a – b < 0
If TRUE, then a – b will be negative AND there will be no overflow
If there is positive overflow (a – b is large), we have a – b < 0 but OF is set
If there is negative overflow (a – b is very small), we have a – b > 0 but OF is set
In either case the sign flag will indicate the opposite of the sign of the true difference. Hence, use exclusive-or of the OF and SF
jl LABEL ; if (SF^OF) %RIP  LABEL CF ZF SF OF
Condition codes
SF ^ OF

9. jl jump less (Signed) SF^OF
cmpq %cl, %al
Jl LABEL
%cl and %al are 8 bits
%al holds 50 %cl holds – = SF = OF = OF ^ SF is 50 < 20 Or 50 – 20 < 0
%al holds 50 %cl holds – = SF = OF = OF ^ SF is 50 < 70 Or 50 – 70 < 0 1 1
True
False

10. jl jump less (Signed) SF^OF
cmpq %cl, %al
Jl LABEL
%cl and %al are 8 bits
%al holds –50 %cl holds –20 –50 – – = SF = OF = OF ^ SF is –50 < –20 Or 50 – 20 < 0
%al holds –20 %cl holds –50 –20 – – = SF = OF = OF ^ SF is 50 < 70 Or 50 – 70 < 0 1 1
False
True

11. jl jump less (Signed) SF^OF
cmpq %cl, %al
Jl LABEL
%cl and %al are 8 bits
%al holds 127 %cl holds – 5 127– – 5 = = SF = OF = OF ^ SF is 127< – 5 Or 127– – 5 < 0
%al holds – 128 %cl holds + 5 – 128– 5 = = SF = OF = OF ^ SF is -128< 5 Or – 128– 5 < 0 1 1 1
underflow
overflow 1
True
False

12. Condition Codes (Explicit Setting: Test)
Explicit Setting by Test instruction
testl/testq Src2, Src1 testl b,a like computing a&b without setting destination
Sets condition codes based on value of Src1 & Src2
Useful to have one of the operands be a mask
CF set to 0
ZF set when a&b == 0
SF set when a&b < 0
OF set to 0
Note: typically the same operand is repeated to test whether it is negative, zero, or positive:
testl %eax, %eax sets the condition codes depending
on the value in %eax
Note 2: there are also testw and testb instructions.
Remember that bitwise operations are more efficient than arithmetic