Chapter 11
Instruction Sets:
Addressing Modes and Formats
Gabriel Baron
Sydney Chow
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Immediate
Direct
Indirect
Register
Register Indirect
Displacement (Indexed)
Stack
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Simplest form of addressing
Operand value is present in instruction
Operand = A
e.g. ADD 5
Add 5 to contents of accumulator
 5 is operand
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Advantages
No memory reference to fetch data
 Fast
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Disadvantages
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Size restriction to the size of address field
Instruction
Opcode
Operand
Example: LDAA
Opcode = 86
Operand = 05
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Simple form of addressing
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Address field contains address of operand
Effective address (EA) = address field (A)
e.g. ADD A
Add contents of cell A to accumulator
 Look in memory at address A for operand
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Single memory reference to access data
No additional calculations to work out effective
address
Disadvantage
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Limited address space
Instruction
Opcode
Address A
Memory
Operand
Example: LDAA
Opcode = 96
A= E5
Address A = 00 E5
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Memory cell pointed to by address field that
contains the address of (pointer to) the operand
EA = (A) [parentheses means contents of]
Look in A, find address (A) and look there for
operand
e.g. ADD (A)
 Add contents of cell pointed to by contents of A to
accumulator
Advantages
 For word length N, space 2n now available
 Large address space
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Disadvantages
 Requires two memory references to fetch operand
 One to get its address
 Second to get its value
Slower
 Multiple memory accesses to find operand
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Instruction
Opcode
Address A
Memory
Pointer to operand
Example:
Opcode = XX
A = 11C5
11C5 = Operand
Operand
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Address field refers to a register rather than a main
memory address
EA = R
Advantages
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Very small address field needed
 Shorter instructions
 Faster instruction fetch
No memory reference required
 Very fast execution
 Multiple registers helps performance
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 Requires good assembly programming or compiler writing
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Disadvantage
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Very limited address space
Instruction
Opcode
Register Address R
Registers
Operand
Example:
Opcode = XX
Register R = Operand
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EA = (R)
Operand is in memory cell pointed to by
contents of register R
Advantages
 For word length N, space 2n now available
 Large address space
 One fewer memory access than indirect addressing
Disadvantages
 Requires two memory references to fetch operand
 Multiple memory accesses to find operand
 Slower
Instruction
Opcode
Register Address R
Memory
Registers
Pointer to Operand
Operand
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Combines the capabilities of direct addressing and register
indirect addressing
EA = A + (R)
Advantages
 Flexibility
Disadvantages
 Complexity
Requirements
 Address field hold two values
 A = base value
 R = register that holds displacement
 or vice versa
Instruction
Opcode Register R Address A
Memory
Registers
Pointer to Operand
+
Operand
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Linear array of locations (last-in-first-out)
Operand is (implicitly) on top of stack
EA = top of stack
Advantages
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Disadvantage
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No memory reference
Limited applicability
e.g.
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ADD Pop top two items from stack
and add
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The memory is divided into portions that maybe addressed by a
single index register without changing a 16-bit segment selector.
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A segment is always 64 KB in size using a 16-bit offset.
The X86 Addressing modes are as follows:
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Register
Memory
Displacement Only
Indirect
Indexed
Based Indexed
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Layout of bits in an instruction
Includes opcode
Includes (implicit or explicit) operand(s)
Usually more than one instruction format in an
instruction set
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The most basic design issue to be faced
Affected by and affects:
Memory size
 Memory organization
 Bus structure
 CPU complexity
 CPU speed
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Trade off between powerful instruction
repertoire and saving space
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Number of addressing modes
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Sometimes an addressing mode can be indicated
implicitly
 i.e. certain opcodes might always call for indexing, so
the addressing modes must be explicit and one or more
mode bits will be needed
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Number of operands
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Fewer addresses can make for longer, more
awkward programs
Register versus memory
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The more that registers can be used or operand
references, the fewer bits are needed
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Number of register sets
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Address range
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These registers can be used to store data and can be
used to store addresses for displacement addressing
For addresses that reference memory, the range of
addresses that can be referenced is related to the
number of address bits
Address granularity
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In a system with 16- or 32- bit words, an address can
reference a word or byte at the designer’s choice
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One of the simplest instruction designs for a
general purpose
Each memory reference consist of 7 bits plus
two 1-bit modifiers
The memory is divided into fixed-length pages
of 27 = 128 words
Supports indirect addressing, displacement
addressing and indexing
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Designed to be a large-scale time-shared
system, with an emphasis on making the
system program easy
Some design principals
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Orthogonality
 Two variables are independent of each other
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Completeness
 Each arithmetic data type should have a complete and
identical set of operations
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Direct addressing
 Base plus displacement addressing, usually avoided in
favor of direct addressing
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Provides a powerful and flexible instruction set
within the constraints of a 16-bit
microcomputer
Employs a set of eight 16-bit general-purpose
registers
Increases hardware cost and programming
complexity because of addressing capability
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More efficient or compact programs can be deeloped
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Begins with a 1-byte opcode
On hexadecimal codes
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Remainder of instructions consist of up to six
operand specifiers
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Actual opcode being specified in the second byte
Minimum 1-byte format which leftmost 4 bits are the
address mode specifier
Provides for a wide variety of operations and
addressing modes
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Types of addressing modes are:
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Immediate, Direct, Indirect, Register, Register-Indirect, Displacement , Stack …
Some of the Key Design issues for instruction
formats are:
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Instruction length, allocation of bits, PDP-8, PDP-10,
PDP-11, VAX.
1.
What are four addressing modes?
Ans: Immediate, Direct, Indirect, Stack.
2.
What is simplest addressing mode?
Ans: Immediate Addressing Mode.
3.
What is the disadvantage of the direct addressing
mode?
Ans: Limited Address Space.
4.
What is indirect addressing?
Ans: The operand is stored in an address, which is stored in an
memory cell.
5.
Which addressing mode refers to a register instead of a
memory address?
Ans: Register Addressing Mode
6.
How does the register indirect addressing mode access
the operand?
Ans: The operand is store in an address that is pointed to by a
memory cell.
7.
What two values does the address field hold in the
Displacement Addressing mode?
Ans: Base value, Register value
8.
What order is data sorted in the stack addressing
mode?
Ans: Last in First Out
9.
What are four X86 Addressing Modes?
Ans: Register, memory, displacement, indirect.
10.
Which instruction design is the simplest for general
purpose?
Ans: PDP-8
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X86 Addressing Modes
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