COMP 206: Computer Architecture and Implementation

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COMP 206Computer Architecture and Implementation

• Montek Singh
• Wed., Sep 10, 2003
• Topic Instruction Set Design

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Organization of an Instruction
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Classification by Operands

• Important machines that are difficult to classify
• Intel 80x86
• variable instruction size 1-17 bytes
• memory can be destination
• uses implied registers
• Motorola 680x0
• Instruction size 2, 4, 6, 8, 10 bytes
• Two address format only (2, 2)

(m,n) means m memory operands n total
operands
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Instruction Set Design Objective 1

• Code size (code density)
• Depends on
• size of MM/cache
• access time of cache (on-chip/off-chip)
• CPU-MM bandwidth
• Frequently used (written down) instructions
  should be short
• Implies variable-length instructions

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Instruction Set Design Objective 2

• Execution speed (performance)
• Only frequently executed instructions should be
  included in the instruction set
• Infrequently executed instructions slow down the
  others
• Complex and long instructions tend to be used
  infrequently
• Defining hardware-software interface
• Frequently executed instructions should be fast
• Pipelining should be made as easy as possible
• Overlapped execution lowers CPI value
• Single instruction length, simple instruction
  formats, and few addressing modes for easy
  decoding
• Three (register) address instructions decouple
  CPU and memory, and also do not destroy their
  operands (reducing memory accesses)

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Instruction Set Design Objective 3

• Size and complexity of hardware (ALU, CU)
• Implementing infrequently executed instructions
  ties down hardware that is rarely used, and could
  be used for some other purpose with greater
  advantage
• Some instructions should not be included in the
  instruction set

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Instruction Set Design Objective 4

• Instruction set as a programming language
• Needs of a human programmer (less important
  today)
• Several desirable properties of instruction sets
  have been recognized and described, such as
  orthogonality (each operand can be specified
  independently of the others) and consistency
  (being able to predict the remainder of an
  architecture given partial knowledge of the
  system)
• Needs of an optimizing compiler
• Simple instructions are more suitable for code
  optimizations
• Optimizing compilers try to find the shortest or
  fastest code sequence that implements the
  semantics of a HLL program. To make code
  reorganization tractable, an instruction set is
  needed that makes
• the size of each instruction easy to calculate
• the execution time of each instruction easy to
  calculate
• the interactions between instructions easy to
  figure out.
• ISA features such as complex addressing modes,
  variable length instructions, special-purpose
  registers provide too many ways of doing the same
  thing and lead to combinatorial explosion

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Addressing Modes
R the register file M the memory address
space d the size of the data item being
accessed (1, 2, 4, 8 bytes)

• We cant directly refer to data values, only
  their addresses
• Except for immediate operands
• Register deferred and direct addressing modes can
  be synthesized from displacement addressing mode

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Registers versus Cache

• Similarities
• Both are small, fast, and expensive (flip-flops)
• Both are used to increase execution speed of CPU
• Both operate based on locality of reference
• Differences
• Registers are visible in ISA caches are not
  (except for instructions for invalidation,
  prefetch, or flushing)
• Number of registers is fixed by instruction
  format size of cache is easily changeable
• Registers have higher BW 3 words/cycle, and are
  random-access caches have lower BW 1
  word/cycle, and are associative
• Register access time is fixed cache access time
  is statistical
• Register allocation is explicit by compiler
  cache allocation is automatic
• Registers require fewer bits to address caches
  require full memory addresses
• Registers create no I/O problems caches do

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Organization of Registers

• One general-purpose set (all interchangeable,
  typeless)
• One general-purpose set (a few with dedicated
  uses)
• PDP-11 eight 16-bit registers (R6 stack
  pointer, R7 PC)
• VAX 11/780 sixteen 32-bit registers (four
  special-purpose, R14 stack pointer, R15 PC)
• Two sets
• Motorola 68000 eight 32-bit data, eight 32-bit
  address
• IBM 370 sixteen 32-bit integer, four 64-bit FP
• DLX, MIPS 31 32-bit integer, 32 32-bit FP
• Three sets
• CDC 6600 eight 18-bit integer, eight 18-bit
  address, eight 60-bit FP
• Many registers with dedicated use
• Intel 80x86

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Notations for Information Representation
On holy wars and a plea for peace, Danny Cohen,
IEEE Computer 14(10), pages 49-54, Oct 1981
Q How do we number these various units of
information in a consistent manner?
9 6 2 1 7 6 6
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Why Is Numbering Important?

• English text is written left-to-right and the
  characters are numbered left-to-right
• Numbers can be numbered in two different ways
• Memory locations are numbered (addresses)
• Consequences of numbering
• Data is stored in memory according to byte
  numbering (the lower-numbered byte goes into a
  byte in memory with a smaller address)
• Data is sent through a bit-serial communication
  channel according to bit numbering (bit 0 goes
  first, followed by bit 1, etc.)
• When displaying computer representation for
  humans
• Numbers are written in the usual way (MSD on
  left, LSD on right)
• Text is written in such a way as to match the
  numbering of numbers

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Consequences of Numbering
Machine A Big Endian Numbering
Machine B Little Endian Numbering
n o t g n i h s a W 1 9 9 5 f e
d c b a 9 8 7 6 5 4 3 2 1 0
1 9 9 5 W a s h i n g t o n 0
1 2 3 4 5 6 7 8 9 a b c d e f
n o t g n i h s a W 5 9 9 1 f e
d c b a 9 8 7 6 5 4 3 2 1 0
Fix Complicate protocol and treat numbers and
character strings differently, which has to be
done in software (by attaching descriptors to
data items)
1 9 9 5 W a s h i n g t o n f e
d c b a 9 8 7 6 5 4 3 2 1 0
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Odds and Ends about Numbering

• The Little Endian notation is compatible with
  mathematical conventions of positional notation
• The Little Endian notation has the disadvantage
  that is displays English text in reverse
• To overcome this, manuals for Little Endian
  machines usually display character strings
  vertically
• Example machines
• Little Endian PDP-11, VAX, 80x86
• Big Endian IBM 370, MIPS, DLX, SPARC
• Mixed Motorola 68000, Z8000
• Big Endian byte ordering
• Little Endian bit ordering

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Alignment of Words in Memory

• CPU accesses a 32-bit word of data starting at
  byte address xx00
• Such an address (multiple of 32b/8b/B 4B)
  is called word-aligned
• Memory controller is simple and fast, data
  available in one cycle
• CPU accesses a 32-bit word of data starting at
  byte address 01111
• Byte addresses are 01111, 10000, 10001, 10010
  (misaligned address)
• Doubles the access time of word
• Requiring aligned addresses results in simpler
  memory controller and faster execution
• Costs some loss of storage, and adds complexity
  in code generators

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Sub-Word Accesses
CPU Register File (32 bits)

• Byte operand in register is usually the rightmost
  byte of register
• Byte may come from any of the four memory banks
• Needs routing/permuting hardware
• Either at memory side of bus (justified bus)
• Byte always travels on rightmost quarter of bus
• Or on CPU side (unjustified bus)
• Bus lanes are extensions of memory bank lanes
• Source of complications in either case

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Control Transfer Instructions

• Terminology
• BTA (Branch Target Address) The destination
  address of the branch
• The BTA is static if it is always the same during
  execution
• The BTA is dynamic if it can vary during a single
  execution of a program (procedure return, O-O
  dynamic dispatch, switch statements are major
  examples)
• Branch is taken if next instruction to be
  executed is at address BTA
• Branch is not taken if next instruction to be
  executed is the one following the branch
  instruction (fall-through)
• Branch outcome whether the branch is taken or
  not taken
• Forward branch BTA gt (PC), where (PC) is the
  address of the branch instruction
• Backward branch BTA lt (PC)
• An unconditional branch is always taken

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Code Generation Examples for Branches
while (a lt b) a b-- x
if (x gt 0) y z else y -z
blez r7, L18 addu r3, r3, r4 j L33 L18 subu r3,
r3, r4 L33
j L33 L34 addu r5, r5, 1 addu r6, r6, -1 addu
r7, r7, 1 L33 slt r2, r5, r6 bne r2, r0, L34
Register r3 contains y Register r4 contains
z Register r5 contains a Register r6 contains
b Register r7 contains x
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Classification of Branches
Classifying branches into these four groups
permits us to compute some of the dynamic
frequencies if some others have been measured.
Rule of thumb Backward branches tend to be
taken, forward branches tend not to be taken.
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Computing Branch Frequencies
Assume that 75 of all branches are forward, and
that 55 of all branches are taken. If 80 of
all backward branches are taken, what is the
probability that a taken branch is a forward
branch?
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Evaluating Branch Conditions

• Typical set of condition codes (e.g., Motorola
  680x0)
• NegativeResult, ZeroResult, ArithmeticOverflow,
  CarryOut
• Many RISC machines do not use condition codes
  (e.g., MIPS, Alpha)
• Magnitude comparisons are done with explicit
  COMPARE instructions that put their results into
  named registers
• Overflow and carry-out have to be inferred by
  software
• Some instructions have two variants one traps on
  overflow, the other does not