Chapter 13

Instruction Sets: Addressing Modes and Formats

 The address field or fields in a typical

instruction format are relatively small.

 In order to reference a large range of

locations in main memory or, for some
systems, virtual memory, a variety of
addressing techniques has been employed.

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 The most common addressing techniques are:
◦ Immediate
◦ Direct
◦ Indirect
◦ Register
◦ Register indirect
◦ Displacement
◦ Stack

Notations
 A=contents of an
address field in the
instruction
 R=contents of an
address field in the
instruction that refers to
a register
 EA=actual (effective)
address of the location
containing the
referenced operand
 (X) =contents of memory
location X or register X

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 Virtually all computer architectures provide

more than one of these addressing modes.

 One or more bits in the instruction format can

be used as a mode field.

 The value of the mode field determines which

addressing mode is to be used by the
processor.

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 In a system without virtual memory, the
effective address (EA) will be either a main
memory address or a register.

 In a virtual memory system, the effective

address (EA) is a virtual address or a register.

 The actual mapping to a physical address is a

function of the memory management unit
(MMU).

 Simplest form of addressing

 Operand=A

 This mode can be used to define and use

constants or set initial values of variables
◦ Typically the number will be stored in twos
complement form
◦ The leftmost bit of the operand field is used as a
sign bit

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 No memory reference other than the
instruction fetch is required to obtain the
operand, thus saving one memory or cache
cycle in the instruction cycle.

 The size of the number is restricted to the

size of the address field, which, in most
instruction sets, is small compared with the
word length.

 Simple form of addressing

 EA=A
 It requires only one memory reference and no
special calculation.
 The length of the address field is usually less
than the word length, thus limiting the
address range.
 This technique was common in earlier
generations of computers.

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 In indirect addressing, address field refers to
the address of a word in memory, which in
turn contains a full-length address of the
operand.

 EA = (A)

 Instruction execution requires two memory

references to fetch the operand
◦ One to get its address and a second to get its value

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 Similar to direct addressing. The only

difference is that the address field refers to a
register rather than a main memory address.
 EA=R
 Only a small address field is needed in the
instruction and no time-consuming memory
references are required.
 But the address space is very limited.

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 Similar to indirect addressing

 EA = (R)

 Address space limitation of the address field

is overcome by having that field refer to a
word-length location containing an address.

 Uses one less memory reference than indirect

addressing.

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 Combines the capabilities of direct

addressing and register indirect addressing
 EA = A + (R)
 Requires that the instruction have two
address fields, at least one of which is explicit
◦ The value contained in one address field (A) is used
directly
◦ The other address field refers to a register whose
contents are added to A to produce the effective
address

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Most common uses of displacement
addressing:
◦ Relative addressing
◦ Base-register addressing
◦ Indexing

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Relative Addressing
 The implicitly referenced register is the
program counter (PC)
◦ The next instruction address is added to the
address field to produce the EA
◦ Typically the address field is treated as a twos
complement number for this operation
◦ Thus the effective address is a displacement relative
to the address of the instruction
 Saves address bits in the instruction if most
memory references are relatively near to the
instruction being executed

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Base-Register Addressing
 The referenced register contains a main memory
address and the address field contains a
displacement from that address
 The register reference may be explicit or implicit
 Convenient means of implementing
segmentation
 In some implementations a single segment base
register is employed and is used implicitly
 In others the programmer may choose a register
to hold the base address of a segment and the
instruction must reference it explicitly

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Indexing
 The address field references a main memory
address and the referenced register contains
a positive displacement from that address.

 An important use is to provide an efficient

mechanism for performing iterative
operations.

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Indexing
 Autoindexing
◦ Automatically increment or decrement the index register
after each reference to it
◦ EA = A + (R)
◦ (R)  (R) + 1
 Postindexing
◦ Indexing is performed after the indirection
◦ EA = (A) + (R)
 Preindexing
◦ Indexing is performed before the indirection
◦ EA = (A + (R))

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 Remember that a stack is a linear array of

locations works as a last-in-first-out queue.
 Items are appended to the top of the stack so
that the block is partially filled at any time.
 Associated with the stack is a pointer whose
value is the address of the top of the stack
◦ The stack pointer is maintained in a register
◦ Thus references to stack locations in memory are
in fact register indirect addresses
 The machine instructions need not include a
memory reference but implicitly operate on
the top of the stack.

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 An instruction format defines the layout of

the bits of an instruction.
 It must include an opcode and, implicitly or
explicitly, indicate the addressing mode for
each operand.
 For most instruction sets more than one
instruction format is used.
 The design of an instruction format is a
complex art.

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 The most basic design issue is the instruction
length.
 The decision affects, and is affected by:
◦ Memory size
◦ Memory organization
◦ Bus structure
◦ Processor complexity
◦ Processor speed
 The most obvious trade-off here is between
the desire for a powerful instruction
repertoire and a need to save space.

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 Another design issue is how to allocate the

bits in that format.
 The following factors determines the use of
the addressing bits.
◦ Number of addressing modes
◦ Number of operands
◦ Register versus memory
◦ Number of register sets
◦ Address range
◦ Address granularity

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 The PDP-10 has a 36-bit word length and a
36-bit instruction length.
 The opcode occupies 9 bits, allowing up to
512 operations. In fact, a total of 365
different instructions are defined.
 Most instructions have two addresses, one of
which is one of 16 general-purpose registers.
Thus, this operand reference occupies 4 bits.

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 The other operand reference starts with an

18-bit memory address field. This can be
used as an immediate operand or a memory
address.
 The same general-purpose registers are also
used as index registers.

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 Variations can be provided efficiently and
compactly
 Increases the complexity of the processor
 Does not remove the desirability of making
all of the instruction lengths integrally related
to word length
◦ Because the processor does not know the length of
the next instruction to be fetched a typical strategy
is to fetch a number of bytes or words equal to at
least the longest possible instruction
◦ Sometimes multiple instructions are fetched

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 Instructions are made up of:

◦ from zero to four optional instruction prefixes
◦ a 1-, 2- or 3-byte opcode
◦ an optional address specifier (which consists of the
ModR/m byte and the Scale Index byte)
◦ an optional displacement
◦ an optional immediate field

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 The instruction prefix, if present, consists of
the LOCK prefix or one of the repeat prefixes.
◦ The LOCK prefix is used to ensure exclusive use of
shared memory in multiprocessor environments.
◦ The repeat prefixes specify repeated operation of a
string, which enables the x86 to process strings
much faster than with a regular software loop.

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 Segment override explicitly specifies which

segment register an instruction should use,
overriding the default segment-register
selection generated by the x86 for that
instruction.

 An instruction has a default operand size of

16 or 32 bits, and the operand prefix
switches between 32-bit and 16-bit
operands.

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 The processor can address memory using
either 16- or 32-bit address size.

 The opcode field is 1, 2, or 3 bytes in length.

 The ModR/m byte specifies whether an

operand is in a register or in memory; if it is
in memory, then fields within the byte specify
the addressing mode to be used.

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 Certain encoding of the ModR/m byte

specifies the inclusion of the SIB byte to
specify fully the addressing mode.

 When the addressing-mode specifier

indicates that a displacement is used, an 8-,
16-, or 32-bit signed integer displacement
field is added.

 Immediate field provides the value of an 8-,

16-, or 32-bit operand.

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 A processor can understand and execute
machine instructions. Such instructions are
simply binary numbers stored in the computer.
 If a programmer wishes to program directly in
machine language, then it would be necessary to
enter the program as binary data.
 Consider the simple BASIC statement
N=I +J +K
and suppose we wish to program this statement
in machine language and to initialize I, J, and K
to 2, 3, and 4, respectively.

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 The program starts in location 101 (hexadecimal).

 Memory is reserved for the four variables starting
at location 201.
 The program consists of four instructions:
◦ Load 201 into AC
◦ Add 202 to AC
◦ Add 203 to AC
◦ Store AC into 204

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  A slight improvement is to
write the program in
hexadecimal rather than
binary notation.

 Each line contains the address

of a memory location and the
hexadecimal code of the
binary value to be stored in
that location.

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 For more improvement, we

can make use of the symbolic
name or mnemonic of each
instruction.
 Each line of input still
represents one memory
location.
 To store arbitrary data in a
location, a pseudoinstruction
with the symbol DAT is used.

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 A much better system, and one
commonly used, is to use
symbolic addresses known as
assembly language.
 Some lines have no address,
implying that the address of
that line is one more than the
address of the previous line.
 Programs written in assembly
language are translated into
machine language by an
assembler.

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