Lesson 3--Intro to ISA

Question 1

3 types of instructions

Answer 1

1. Data or memory access (load, store, etc.)
2. Control flow (branch)
3. Arithmetic or logic (add, subtract, etc.)

Question 2

In ISA design what is an Operation?

Answer 2

fundamental RISC-like minimal unit of work

Question 3

True or False: VLIW exposes a scheduler in the compiler

Answer 3

True. Superscalar hides this.

Question 4

What’s the difference between Registers and Memory

Answer 4

Memory is off chip, not specialized, slow

Registers are On chip, connected to the logic of the processor, fast

Question 5

VLIW Design Principles

Answer 5

• -instructions consist of parallel operations. Compiler must guarantee safe parallel operations.
• -Simplification of HW to improve energy efficiency.
• -Latency and functional unit are exposed.

Question 6

True or False: VLIW’s advantages come largely from having an intelligent compiler that can schedule many instructions simultaneously.

Answer 6

True

Question 7

True or False: VLIW-compatible code is easily portable to hardware otehr than the chip it is designed for

Answer 7

False

Question 8

True or False: VLIW has some problems with the inflexibility of its compiler-first design

Answer 8

True

Question 9

Role of VLIW compilers

Answer 9

1. Gather independent operations and bundle them into sections.
2. Resolve all structural and resource hazards.
3. If the VLIW is clustered, the compiler assigns operations to the clusters and generates cross cluster register references.

Question 10

Horizontal Decisions

Answer 10

Decisions that need to occur when operations sharing the same variable occur within an instruction word

Question 11

Vertical Decisions

Answer 11

Decisions across pipelined instructions. Such as load and stores that have latencies.

Question 12

Fixed overhead encoding

Answer 12

Prepend mask bits to a VLIW instruction to identify “what goes where”
Mask0 = 10100111 this means put instructions in the ‘1’s slots and NOPs in the ‘0’ slots

Question 13

Distributed encoding advantages

Answer 13

• -Explicitly insert stop bit for delimiter
• -Distributed and incremental encoding cost.
• -No need to compute next PC.

Question 14

Distributed encoding disadvantages

Answer 14

• -More complex.

- -Bits are embedded as part of the instruction, so have to get them back to NOPs.

Question 15

Fixed overhead encoding advantages

Answer 15

–Simple scheme, just need to track the instructions and the mask.

Question 16

Fixed overhead encoding disadvantages

Answer 16

–Space overhead for the longest possible VLIW instruction. Must always encode length of mask.

Question 17

Template-based encoding advantages

Answer 17

• -Low overhead

- -No penalty for short instructions.

Question 18

Template-based encoding disadvantages

Answer 18

Limited number of templates

Question 19

In ISA design what is an Instruction?

Answer 19

Fundamental unit of encoding. Refers to a parallel set of operations

Question 20

What is a Bundle?

Answer 20

Memory aligned encoding unit designed to occupy one word of memory.
In VEX a syllable is a bundle.