This document discusses different types of dependencies that can occur in programs: - Data dependencies occur when an instruction refers to data from a previous instruction. There are three types: true data dependencies where an instruction depends on a previous result; output dependencies where two instructions write to the same register; and anti-dependencies where an instruction depends on data that could be overwritten. - Control dependencies occur when the execution of one instruction depends on the outcome of another instruction, such as in an if-then statement. - Resource conflicts occur when two instructions need the same hardware resource at the same time, such as a functional unit or register, stalling execution even if the instructions do not have a data or control dependency.

3. DEPENDENCIES
o In this section we shall consider:
 Data Dependence.
 Control Dependence.

4. DATA DEPENDENCIES
A data dependency is a situation in which
a program statement (instruction) refers to the data
of a preceding statement.

5. TYPES OF DATA DEPENDENCIES
Three cases are exist…
 True Data Dependence (flow dependence –
read after write)
 Output Dependence (write after write)
 Anti Dependence (write after read)

6. TRUE DATA DEPENDENCY
 True data dependency is where one instruction depends
on the final outcome of a previous instruction.
 Also known as flow dependency or read after write
dependency
 Consider the code:
ADD r1,r2
(r1 =r1+r2;)
MOV r3,r1
(r3 = r1;)
 Can fetch and decode second instruction in parallel
with first
 Can NOT execute second instruction until first is
finished

7. CONTINUE . . .
MOV r1,[mem]
MOV r3,r1
MOV r2,5
(Load r1 from memory)
(r3 = r1;)
(r2 = 5;)
 The superscalar machine would execute the first and
third instructions in parallel, yet have to wait anyway for
the first instruction to finish before executing the second
 This holds up MULTIPLE pipelines

8. OUTPUT DEPENDENCY
This type of dependency occurs when two instructions both
write a result.
If an instruction depends on the intermediate result, problems
could occur
Also known as write-write dependency
R3 = R3 + R5;
(I1)
R4 = R3 + 1;
(I2)
R3 = R5 + 1;
(I3)
R7 = R3 + R4;
(I4)
I2 depends on result of I1 and I4 depends on result of I3 –
true data dependency
If I3 completes before I1, result from I1 will be written lost –
output (write-write) dependency

9. ANTIDEPENDENCY(WRITE AFTER READ)
 Called Antidependency because it is the exact opposite of
data dependency
 Data dependency: instruction 2 depends on data from
instruction 1
 Antidependency: instruction 1 depends on data that could
be destroyed by instruction 2

10. CONTROL DEPENDENCY
S1. if (a == b)
S2. a = a + b
S3. b = a + b

11. RESOURCE CONFLICT
 Two or more instructions requiring
access to the same resource at the same
time
-- e.g. functional units, registers, bus
 Similar to true data dependency, but it is
possible to duplicate resources

12. COMPARISON OF TRUE DATA, PROCEDURAL,
AND RESOURCE CONFLICT DEPENDENCIES