Chapter 11
Instruction Sets
Addressing Modes and Formats
2
Team Members
 Jose Alvarez
 Daniel Monsalve
 Marlon Calero
 Alfredo Guerrero
 Oskar Pio
 Andres Manyoma
3
Addressing Modes
 An addressing mode is the method by which an
instruction references memory
 Types of addressing modes:

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




Immediate
Direct
Indirect
Register
Register Indirect
Displacement (Indexed)
Stack
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Immediate Addressing
 Simplest form of addressing
 Actual data is stored in instruction
 Value of the operand is stored within instruction
 Data sizes vary by processor and by instruction
 No memory reference (other than instruction fetch)
required to obtain operand
 Size of number limited to size of address field
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Immediate Addressing Diagram
Instruction
Opcode
Operand
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Direct Addressing
 Instruction points to location of stored value, but value
itself is out of memory
 Single memory reference to access data
 No additional calculations needed to work out effective
address
 Frequently used for global variables in high level
language
 Limited address space
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Direct Addressing Diagram
Instruction
Opcode
Address A
Memory
Operand
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Indirect Addressing
 Memory pointed to by address field contains the address
of the operand
 Large address space
 2n where n = word length
 Multiple memory accesses to find operand
 Hence slower
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Indirect Addressing Diagram
Instruction
Opcode
Address A
Memory
Pointer to operand
Operand
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Register Addressing
 Specified instruction contains required operand
 Limited number of registers
 Small address field needed
 Leads to shorter instructions, but faster fetch cycle
 No memory access
 Fast execution
 Limited address space
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Register Addressing Diagram
Instruction
Opcode
Register Address R
Registers
Operand
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Register Indirect Addressing
 Very similar to indirect addressing
 Only one main difference between the two
 Algorithms:
 Register Indirect: EA = (R)
 Indirect: EA = (A)
 Advantages/disadvantages also similar to those of
indirect addressing:
 Advantage: Large address space (2n)
 Disadvantage: Extra memory reference
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Register Indirect Addressing Diagram
Instruction
Opcode
Register Address R
Memory
Registers
Pointer to Operand
Operand
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Displacement Addressing
 Very powerful addressing mode
 Combines capabilities of direct and register indirect
addressing ( EA = A + (R) )
 Three common uses for displacement addressing
 Relative addressing
 Base-register addressing
 Indexing
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Displacement Addressing (continued)
 Relative addressing
 Implicitly referred to in PC. If most memory references
near to instruction being executed, use of relative
addressing saves address bits in instruction
 Base-register addressing
 Convenient means of implementing segmentation
 Indexed addressing
 Address field references a main memory address;
referenced register contains positive displacement from
that address
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Displacement Addressing Diagram
Instruction
Opcode
Register R
Address A
Memory
Registers
Pointer to Operand
+
Operand
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Stack Addressing
 Linear array of locations
 Other names: Pushdown list, last-in-first-out queue
 Stack is a reverse block of locations
 Has a pointer associated with it
 Stack locations in memory are register indirect
addressed
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Stack Addressing Diagram
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x86 Addressing Modes (continued)
 8 addressing modes available









Immediate
Register operand
Displacement
Base
Base with displacement
Scaled index with displacement
Base with index and displacement
Base scaled index with displacement
Relative
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x86 Addressing Mode Calculation
ARM Addressing Modes
 Arm has a rich set of addressing modes.
 Typically they are classified with respect to the type of
instruction.
 Load/Store Addressing
 Data Processing Instruction Addressing
 Branch Instructions
 Load/Store Multiple Addressing
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ARM Addressing Modes
Load/Store
 Loads and stores Instructions are the only instructions that
reference memory, always done indirectly through a base
register plus offset.
 3 alternatives are :
 Offset
 Preindex
 Postindex
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ARM Data Processing Instruction
Addressing & Branch Instructions
 Data Processing
 Or mixture of register and immediate addressing
 Branch
 Immediate
 Instruction contains 24 bit value
 Shifted 2 bits left
 On word boundary
 Effective range +/-32MB from PC
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ARM Load/Store Multiple
Addressing
 Load/store subset of general-purpose registers
 16-bit instruction field specifies list of registers
 Sequential range of memory addresses
 Increment after, increment before, decrement after,
and decrement before
 Base register specifies main memory address
 Incrementing or decrementing starts before or after
first memory access
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Instruction Formats
 Layouts of bits in an instruction
 Includes opcode
 Includes (implicit or explicit) operand(s)
 Usually more than one instruction in an instruction set
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Instruction Length
 Affected by and affects:


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

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Memory size
Memory organization
Bus structure
CPU complexity
CPU speed
Tradeoff between powerful instruction repertoire and
saving space
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Instruction Length (continued)
 Should be equal to or multiple of memory transfer
length
 Should be multiple of character length (usually 8 bits)
 Word length of memory is “natural” unit of organization
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Allocation of Bits
 Tradeoff between number of opcodes and power of the
addressing capability
 More opcodes mean more bits in the opcode field
 In an instruction format, this reduces number of bits
available for addressing
 Interrelated factors that determine the use of
addressing bits:
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Allocation of Bits (continued)
 Number of addressing modes: can be implicit, but
sometimes one or more mode bits are needed
 Number of operands: today’s machines provide for two
operands, each requiring its own mode indicator
 Register versus memory: one operand address is
implicit and consumes no instruction bits, but causes
awkward programming and many instructions (a total of
8 to 32 user-visible registers is desirable)
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Allocation of Bits
 Number of register sets: most machines have one set
of 32 general-purpose registers to store data and
addresses
 Architectures like Pentium have more specialized sets, that
by a functional split the instruction uses fewer bits
 Address range: related to number of address bits, has
severe limitations (which is why direct addressing is
rarely used)
 With displacement addressing, the range is opened up to
the length of the address register
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Allocation of Bits
 Address granularity: a factor for addresses that
reference memory rather than registers
 In a system with 16- or 32-bit words, an address can
reference a word or a byte at the designer’s choice
 Byte addressing is convenient for character manipulation
but requires more address bits
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PDP-8 Instruction Format
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PDP-10 Instruction Format
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Variable-Length Instruction
 Make it difficult to decouple memory fetches
 Fetch part, then decide whether to fetch more, and
maybe miss in cache before instruction is complete
 Fixed length allows full instruction to be fetched in one
access
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PDP-11
 Designed to provide powerful and flexible instruction
set within constraints of 16-bit minicomputer
 Employs set of eight 16-bit general-purpose registers
 Two of these registers have additional significance
 One is used as a stack pointer for special-purpose stack
operations
 The other is used as the program counter, which contains
the address of the next instruction
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PDP-11 (continued)
 PDP-11 instructions set and addressing capability are
complex
 This increases both hardware cost and programming
complexity
 The advantage is that more efficient or compact
programs can be developed
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PDP-11 Instruction Format
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VAX
 Most architectures provide a relatively small number of
fixed instruction formats
 To avoid problems two criteria were used in designing
the VAX
 All instructions should have the “natural” number of
operands
 All operands should have the same generality in
specification
 VAX instruction begins with a 1-byte opcode, which
suffices to handle most VAX instructions
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VAX Instruction Examples
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VAX Instruction Formats
 The remainder of the instructions consists of up to six
operand specifiers
 Is, at minimum, a 1-byte format in which the leftmost 4
bits are the address mode specifier
 The only exception to this rule is the literal mode,
which is signaled by the pattern 00 in the leftmost 2
bits, leaving space for a 6-bit literal
 Because of this exception, a total of 12 different
addressing modes can be specified
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VAX Instruction Formats
 An example uses an 8-, 16-, or 32-bit displacement. An
indexed mode of addressing may be used. In this case,
the first byte of the operand specifier consists of the 4bit addressing mode code of 0100 and a 4-bit index
register identifier.
 The remainder of operand specifier consists of base
address specifier, which may be one or more bytes.
 The VAX instruction set provides for a wide variety of
operations and addressing modes. This gives the
programmer a very powerful and flexible tool for
developing programs.
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Questions
1) How many x86 addressing modes were presented?
Ans: 9- Imme, Reg operand, Displacement, Base,
Base with Displacement, Scaled Index with
Displacement, Base with index and Displacement,
Base with scaled Index and Displacement, Relative.
2) What are the most common addressing modes?
Ans: Immediate, direct, indirect, register, register
indirect, displacement, stack
3) How many ARM Addressing modes are there?
Load/Store addressing, Data Process Inst Addr,
Branch Inst, Load/Store multiple Addr.
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Questions
4) How many factors can go into determining the use of
determining addressing bits?
Ans: # of Addr. Modes, # of Operands, Register vs
Mem, # of reg sets, Address Range, Address
Granularity
5) What is the fastest Addressing mode?
Ans: Immediate Opt code + Operand
6) What is an advantage of using Direct addressing mode?
Ans: Simplicity Opt code + lower address space.
7) What addressing mode combines the capabilities of
direct & reg indirect addr?
Ans: Displacement
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Questions
8) Which addressing mode it’s also known as pushdown list
addressing mode.
Ans: Stack Addressing mode.
9) Which of the addressing modes is the simplest?
Ans: immediate addressing mode
10) More opcodes mean more ___ in the opcode field?
Ans: Bits
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Thank You
[APPLAUSE]