Computer Architecture Study Guide 3

displacement

it is an 8 bit or 16 bit immediate value given in the instruction

The 8086 memory addressing modes provide flexible access to (8086 microprocessor)

memory... allows you to asily access variables, arrays, records, pointers, etc

In direct addressing/absolute addressing mode what is required to access the data

one memory reference operation

What is required to access data in an auto indexed incrememnt mode

one register reference one memory reference one ALU oepration

What do you need in an auto indexed decrement mode to access data

one register reference, one memory reference and one alu operation is required to access the data

An assembly language program instruction consists of two parts:

opcode operand

Adressing modes are the operation field specifies the

opertaions which needs to be performed

PC relative and base register both addressing modes are suitable for

program relocation at runtime

They key to good assembly language programming is the

pruper use of memory addressing modes

Auto decrement mode is same as auto increment mode. Both can also be used to implement a stack as

push and pop (good for last in first out data structure)

Based on the availability of effective address, indirect mode is of two kind which is:

register indirect memory indirect

Three types of CPU organization:

single accumulator organization general register organization stack organization

With addressing modes, the operation must be executed on

some data which is already stored in computer registers or in the memory

The memory address of an operand consists of two components:

starting address of memory segment effective address/offset

Register mode

the operand is placed in one 8 bit or 16 bit general purpose registers. The data is in the register that is specified by the instruction

Direct addressing/absolute addressing mode

the operands offset is given in the instruction as an 8 bit or 16 bit displacement element. in this addressing mode the 16 bit effective address of the data is part of the instruction

Register indirect mode

the operands offset is placed in any one of the registers BX,BP,SI,DI as specified in the instruction. The effective address of the data is in the base register or an index register that is specified by the instruction

Auto decrememnt mode is same as auto increment mode true or false

true

How many references are required for indirect addressing mode

two

Base register addressing mode

used to implement inter segment transfer of control (effective address is obtained by adding base register value to address field value)

PC relative addressing mode

used to implement intra segment transfer of control.

Addressing modes refers to the

way in which the operand of an instruction is specified. Specifies a rule for interpreting/modifying the address field of the instruction before the operand is actually executed

based register addressing mode is best suitable to

write position independent codes

Auto indexed decrement mode

effective address of the operand is the contents of a register specified in the instruction. before accessing the operand, the contents of the register are automatically decremented to point to the previous consecutive memory location - R1

Different addressing modes used by 8086 microprocessor

implied mode immediate addressing mode register mode register indirect mode auto indexed (increment mode) auto indexed(decrement mode direct addressing/absolute addressing mode indirect addressing mode indexed addressing mode based indexed addressing

implied mode

in implied addressing, the operand is specified in the instruction itself. the data is 8 bits or 16 bits long and data is part of the instruction.

indirect addressing mode

in this mode address field of instruction contains the address of effective address.

immediate addressing mode (symbol #)

in this mode data is present in address field of instruction. Designed like one address instruction format

memory indirect

in this mode effective address is in the memory and corresponding memory address will be maintained in the address field of an instruction (two mem reference required to access the data)

register indirect

in this mode effective address is in the register and corresponding register name will be maintained in the address field of an instruction (one refernece, one memory ref is required to access the data)

Addressing modes is used for one or both of the purposes. These can also be said as the advantages of using addressing mode:

1.To give programming versatility to the user by providing facilities as pointers to memory, counter for loop control, indexing of data, etc 2.To reduce the number of bts in the addressing field of the instruction

What are the referneces required for indirect addressing mode

1st reference to get effective address 2nd reference to access the data

Addressing mode very much depends on the type of ___ organization

CPU

Auto indexed (increment mode)

Effective address of the operand is the contents of a register specified in the instruction. After accessing the operand, the contents of this register are automatically incrememnted to point to the next consecutive memory location

An offset is determined by

adding any combination of three address elements: displacement, base and index

The way of choosing operands during program execution is dependent on

addressing modes of instruction

In pc relative addressing mode, how is the effective address obtained

by adding displacement to PC

index

content of index register SI or DI

base

contents of base register, BX or BP