8086/8088 Machine language

Instruction format, Addressing

Modes, Data addressing,

Program memory & stack

addressing mode

Software

The sequence of commands used to tell a microcomputer what to do is called a program,

Each command in a program is called an instruction

8088 understands and performs operations for 117 basic instructions

The native language of the IBM PC is the machine language of the 8088

A program written in machine language is referred to as machine code

In 8088 assembly language, each of the operations is described by alphanumeric symbols instead of 0-1s.

ADD AX, BX

(Opcode) (Destination operand) (Source operand )

Instructions

LABEL: INSTRUCTION ; COMMENTAddress identifier Does not generate any machine

code

Ex. START: MOV AX, BX ; copy BX into AX

There is a one-to-one relationship between assembly and machine language instructions

A compiled machine code implementation of a program written in a high-level language results in inefficient code

– More machine language instructions than an assembled version of an equivalent handwritten assembly language program

Two key benefits of assembly language

programming

– It takes up less memory

– It executes much faster

Applications

One of the most beneficial uses of

assembly language programming is

real-time applications.

Real time means the task required by the application must be

completed before any other input to the program that will

alter its operation can occur

For example the device service routine which controls the

operation of the floppy disk drive is a good example that is

usually written in assembly language

Assembly language not only good for

controlling hardware devices but also

performing pure software operations– Searching through a large table of data for a special string of

characters

– Code translation from ASCII to EBCDIC

– Table sort routines

– Mathematical routines

Assembly language: perform real-time operations

High-level languages: used to write those parts

that are not time critical

Converting Assembly Language Instructions to

Machine Code

An instruction can be coded with 1 to 6 bytes

Byte 1 contains three kinds of information– Opcode field (6 bits) specifies the operation (add, subtract, move)

– Register Direction Bit (D bit) Tells the register operand in REG field in byte 2

is source or destination operand

1: destination 0: source

-Data Size Bit (W bit) Specifies whether the operation will be performed on

8-bit or 16-bit data

0: 8 bits 1: 16 bits

15 ------- 10 9 8 7 ---- 6 5 ---- 3 2 ---- 0

Byte 2 has three fields

– Mode field (MOD)

– Register field (REG) used to identify the register for the first operand

– Register/memory field (R/M field)

2-bit MOD field and 3-bit

Mode Field encoding

R/M field together specify the

second operand

Register/memory (R/M) Field Encoding

Examples

MOV BL,AL (88C316)

Opcode for MOV = 100010

D = 0 (AL source operand)

W bit = 0 (8-bits)

Therefore byte 1 is 100010002=8816

• MOD = 11 (register mode)

• REG = 000 (code for AL)

• R/M = 011 (destination is BL)

Therefore Byte 2 is 110000112=C316

Examples:

MOV BL, AL = 10001000 11000011 = 88 C3h

ADD AX, [SI] = 00000011 00000100 = 03 04 h

ADD [BX] [DI] + 1234h, AX = 00000001 10000001 __ __ h =

01 81 34 12 h

Addressing Modes and

Formats

Addressing Modes

Immediate

Direct

Indirect

Register

Register Indirect

Displacement (Indexed)

Stack

Immediate Addressing

Operand is part of instruction

Operand = address field

e.g. ADD 5

◦ Add 5 to contents of accumulator

◦ 5 is operand

No memory reference to fetch data

Fast

Limited range

Immediate Addressing Diagram

OperandOpcode

Instruction

Direct Addressing

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

Single memory reference to access data

No additional calculations to work out

effective address

Limited address space

Direct Addressing Diagram

Address AOpcode

Instruction

Memory

Operand

Indirect Addressing (1)

Memory cell pointed to by address field

contains the address of (pointer to) the

operand

EA = (A)

◦ 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

Indirect Addressing (2)

Large address space

2n where n = word length

May be nested, multilevel, cascaded

◦ e.g. EA = (((A)))

Draw the diagram yourself

Multiple memory accesses to find

operand

Hence slower

Indirect Addressing Diagram

Address AOpcode

Instruction

Memory

Operand

Pointer to operand

Register Addressing (1)

Operand is held in register named in

address filed

EA = R

Limited number of registers

Very small address field needed

◦ Shorter instructions

◦ Faster instruction fetch

Register Addressing (2)

No memory access

Very fast execution

Very limited address space

Multiple registers helps performance

◦ Requires good assembly programming or

compiler writing

◦ N.B. C programming

register int a;

c.f. Direct addressing

Register Addressing Diagram

Register Address ROpcode

Instruction

Registers

Operand

Register Indirect Addressing

C.f. indirect addressing

EA = (R)

Operand is in memory cell pointed to by

contents of register R

Large address space (2n)

One fewer memory access than indirect

addressing

Register Indirect Addressing Diagram

Register Address ROpcode

Instruction

Memory

OperandPointer to Operand

Registers

Displacement Addressing

EA = A + (R)

Address field hold two values

◦ A = base value

◦ R = register that holds displacement

◦ or vice versa

Displacement Addressing Diagram

Register ROpcode

Instruction

Memory

OperandPointer to Operand

Registers

Address A

+

Relative Addressing

A version of displacement addressing

R = Program counter, PC

EA = A + (PC)

i.e. get operand from A cells from current

location pointed to by PC

c.f locality of reference & cache usage

Base-Register Addressing

A holds displacement

R holds pointer to base address

R may be explicit or implicit

e.g. segment registers in 80x86

Indexed Addressing

A = base

R = displacement

EA = A + R

Good for accessing arrays

◦ EA = A + R

◦ R++

Combinations

Postindex

EA = (A) + (R)

Preindex

EA = (A+(R))

(Draw the diagrams)

Stack Addressing

Operand is (implicitly) on top of stack

e.g.

◦ ADD Pop top two items from stack

and add

x86 Addressing Modes

Virtual or effective address is offset into segment

◦ Starting address plus offset gives linear address

◦ This goes through page translation if paging enabled

12 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

x86 Addressing Mode Calculation

ARM Addressing Modes

Load/Store

Only instructions that reference memory

Indirectly through base register plus offset

Offset

◦ Offset added to or subtracted from base register contents to form the memory address

Preindex

◦ Memory address is formed as for offset addressing

◦ Memory address also written back to base register

◦ So base register value incremented or decremented by offset value

Postindex

◦ Memory address is base register value

◦ Offset added or subtractedResult written back to base register

Base register acts as index register for preindex and postindex addressing

Offset either immediate value in instruction or another register

If register scaled register addressing available

◦ Offset register value scaled by shift operator

◦ Instruction specifies shift size

ARM

Indexing

Methods

ARM Data Processing Instruction Addressing

& Branch Instructions

Data Processing

◦ Register addressing

Value in register operands may be scaled using a

shift operator

◦ 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.

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

ARM Load/Store Multiple Addressing Diagram

Instruction Formats

Layout of bits in an instruction

Includes opcode

Includes (implicit or explicit) operand(s)

Usually more than one instruction format

in an instruction set

Instruction Length

Affected by and affects:

◦ Memory size

◦ Memory organization

◦ Bus structure

◦ CPU complexity

◦ CPU speed

Trade off between powerful instruction

repertoire and saving space

Allocation of Bits

Number of addressing modes

Number of operands

Register versus memory

Number of register sets

Address range

Address granularity

PDP-8 Instruction Format

PDP-10 Instruction Format

PDP-11 Instruction Format

VAX Instruction Examples

x86 Instruction Format