1. CENG04:Microprocessor Systems Midterm Lecture 2 & 3

2. Introduction Intel P Instruction Encoding and Decoding

Machine Language

• It is the native binary code that the microprocessor understand.

Assembler

• It is use to translate assembly instruction to a machine code.

Opcode (Operation Code)

• Selects the operation performed by the microprocessor such as MOV, ADD, INC, JMP, & etc.

• It is either one or two byte in length for most machine language instruction.

Encoding

• It is a processes representing entire assembly instruction as a binary value or hexadecimal format (human perspective)

Decoding

• It is the process of converting hexadecimal format to assembly language (machine perspective)

3.

Figure 1 The format of the 8086 80486 Instruction a) 16-bit instruction, b) 32-bit instruction

8086 80486 Instruction Format Intel P Instruction Encoding and Decoding 4.

Figure 2 8088/8086 Instruction Format

8088/8086 Instruction Format Intel P Instruction Encoding and Decoding 5.

Instructionsconsist of:

• operation (opcode) e.g. MOV

• operands (number depends on operation)

Operandsspecified using addressing modes

Addressing modemay include addressing information

• Registers

• Constant values

• Variable

Instruction Encoding Intel P Instruction Encoding and Decoding 6.

Encodingof instruction must includes:

• Opcode

• Operands

• Addressing information

Encodingis process representing entire instruction as abinary value

• Number of bytes needed depends on how much information must be encoded .

Instructions are encoded byassembler:

• .OBJfile (link, then loaded by loader)

Instructions aredecodedby processor during execution cycle

Instruction Encoding Intel P Instruction Encoding and Decoding 7.

Override Prefixes

• It is the first 2 bytes of a 32-bit instruction format

• These bytes are not always used

• Divided in Two Parts:

• • Address Size modifies the size of the address used the instruction and this byte is equal to67hif in used.

• • • If the 80386/80486 is operating as 16-bit instruction mode machine (real or protected mode) and 32-bit instruction, byte is equal to67h

• • • If the 80386/80486 is operating as 32-bit instruction mode machine (real or protected mode) and 32-bit instruction, byte is removed.

Instruction Encoding Intel P Instruction Encoding and Decoding 8.

• • Operand Size-modifies the size of the register.

• • • If the 80386/80486 is operating as 16-bit instruction mode machine (real or protected mode) and 32-bit instruction, byte is equal to66h

• • • If the 80386/80486 is operating as 32-bit instruction mode machine (real or protected mode) and 32-bit instruction, byte is removed.

• These toggle the size of the register and operand address from 16-bit to 32-bit or vice versa.

Instruction Encoding Intel P Instruction Encoding and Decoding 9.

First byte:Opcode, Direction, & Word bits

• Opcode select the operation performed by the microprocessor (use the lists of opcodes)

• Direction (D) indicates the flow of data

• • D = 1, data flow from (R/M) field to REG field

• • D = 0, if data flow from REG field to R/M field.

• Word (W) determine the size of data or register

• • W = 1, 16- or 32-bit data width (word or dword)

• • W = 0, 8-bit data width (byte)

Note:W bit appear in most of the instruction but D bit mainly appears withMOV and some other instructions.

Instruction Encoding Intel P Instruction Encoding and Decoding 10.

Second Byte:Mode, Register, & Register/Memory

• Modefield

• • This field specifies the addressing modes for selected instruction.

• • This selects the type of addressing and whether a displacement is present or with the selected instruction.

• • • MOD = 11, selects data addressing modes

• • • MOD = 00, 01, or 10, selects memory addressing modes

• REGfield

• • Field for register assignment

• R/M (Register or Memory)

• • Field for register act as a memory or memory location assignment.

How to Encode Instructions as Binary Values?Intel P Instruction Encoding and Decoding 11.

Figure 3 Table for MOD field a) 16-bit, b) 32-bit

Instruction Encoding Intel P Instruction Encoding and Decoding 12.

Figure 4 REG field for w=0 & w=1

Instruction Encoding Intel P Instruction Encoding and Decoding 13.

Figure 5 16- and 32-bit R/M field, segment register field, and scaled factor.

How to Encode Instructions as Binary Values?Intel P Instruction Encoding and Decoding 14.

Register Addressing

• It uses the R/M field to specify a register instead of memory location

Special addressing modes

• This addressing modes occurs whenever memory data are referenced by only the displacement mode of addressing for 16-bit instructions.

• • MOV [1000h], DL

• • MOV NUMB, DL

• MOD= 00 andR/M =110

• Scaled-Index Byte

• • Indicates the additional forms of scaled-index addressing.

• • Occurs when R/M = 100

Instruction Encoding Intel P Instruction Encoding and Decoding 15.

Determine the equivalent machine code of the following assembly code:

• MOV DX, AX

• MOV DX, [BX + DI + 1234h]

• ADD AX, 1023

• SUB DX, 1234h

• AND [BX + 12h], AX

• MOV EAX, [EBX + 4*ECX]

• MOV [5267h], DH

• MOV CS, AX

• MOV DS, AX

• MOV AX, [BX]

Examples (Encoding) Intel P Instruction Encoding and Decoding 16.

Given the following machine code (hex code), determine the equivalent assembly instruction for each.

• 8B923412h

• 81C17856h

• 2C26h

• 8B163412h

• 20D8h

Examples (Decoding) Intel P Instruction Encoding and Decoding