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Instruction Sets: Addressing Modes and Formats (Addressing Modes Ways…
Instruction Sets: Addressing Modes and Formats
Addressing Modes
Ways to specify an operand
in an instruction
Immediate
Operand is a specific value
Direct
Operand is the value of a variable
Indirect
Operand address is stored in another variable
Register
Operand is a specific register
Register indirect
Operand address is stored in a register
Displacement (replace)
Replace the value of a variable with an expression
Stack
Operand address is stored in stack register
Basic Addressing Modes: Xem slide 6 bài 13
Immediate Addressing
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
Advantage: 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
Disadvantage: 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
Direct Addressing
Address field contains the effective address of the operand
Effective address (EA) = address field (A)
Was common in earlier generations of computers
Requires only one memory reference and no special calculation
Limitation is that it provides only a limited address space
Indirect Addressing
Reference to the address of a word in memory which contains a full-length address of the operand
EA = (A)
Parentheses are to be interpreted as meaning contents of
Advantage: For a word length of N an address space of 2N is now available
Disadvantage: Instruction execution requires two memory references to fetch the operand
One to get its address and a second to get its value
A rarely used variant of indirect addressing is multilevel or cascaded indirect addressing
EA = ( . . . (A) . . . )
Disadvantage is that three or more memory references could be required to fetch an operand
Register Addressing
Address field refers to a register rather than a main memory address
EA = R
Advantages
Only a small address field is needed in the instruction
No time-consuming memory references are required
Disadvantage: The address space is very limited
Register Indirect Addressing
Analogous to indirect addressing
The only difference is whether the address field refers to a memory location or a register
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
Displacement Addressing
Most common uses
Relative addressing
Base-register addressing
Indexing
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 (value = A) is used directly
The other address field refers to a register whose contents are added to A to produce the effective address
Relative Addressing
The implicitly referenced register is the program counter (PC)
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
Exploits the concept of locality
Saves address bits in the instruction if most memory references are relatively near to the instruction being executed
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
Exploits the locality of memory references
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
Indexed Addressing
The address field references a main memory address and the referenced register contains a positive displacement from that address
The method of calculating the EA is the same as
for base-register addressing
An important use is to provide an efficient mechanism for
performing iterative operations
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))
Stack Addressing
A stack is
a linear array of locations
Sometimes referred to as a
pushdown list
or
last-in-first-out queue
A stack is a
reserved
block of locations
Items are appended to the top of the stack so that the block is partially filled
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
Is a form of implied addressing
The machine instructions need not include a memory reference but implicitly
operate on the top of the stack
Instruction Formats
Define the layout of the
bits
of an instruction,
in terms of its constituent fields
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
Instruction Length
Most basic design issue
Affects, and is affected by
Memory size
Memory organization
Bus structure
Processor complexity
Processor speed
Allocation of Bits
Number of addressing modes
Number of operands
Register versus memory
Number of register sets
Address range
Address granularity (cốt lõi)
Variable-Length Instructions
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
Assembly Language