ECE291Computer Engineering II

Lecture 3

Josh Potts

University of Illinois at Urbana- Champaign

Josh Potts ECE291

Outline

• Programming with registers• Instruction components and format• Addressing modes• Sampling of addressing modes

Josh Potts ECE291

Programming ModelRegisters

Note:32 bit registers are not available on 8086, 8088, 80286

Josh Potts ECE291

Programming with RegistersGeneral-Purpose Registers

• AX(accumulator) often holds the temporary result after an arithmetic and logic operation (also addressed as EAX, AH, or AL)

• BX (base) often holds the base (offset) address of data located in the memory (also addressed as EBX, BX, BL)

• CX (count) contains the count for certain instructions such as shift count (CL) for shifts and a counter (CX or ECX) with the LOOP instruction (also addressed as ECX, CH, or CL)

• DX (data) holds

– the most significant part of the product after a 16- or 32-bit multiplication,

– the most significant part of the dividend before a division, and

– I/O port number for a variable I/O instruction (also addressed as EDX, DH, DL)

Josh Potts ECE291

Programming with Registers Pointer and Index Registers

• SP (stack pointer) used to address data in a LIFO (last-in, first-out) stack memory, most often used when – the PUSH and POP instructions are executed– a subroutine is CALLed or RETurned within a program – Don’t ever mess with this directly

• BP (base pointer) often used to address an array of data in the stack memory

• SI (source index) used to address source data indirectly for use with the string instructions

• DI (destination index) normally used to address destination data indirectly for use with the string instructions

• IP (instruction pointer) always used to address the next instruction executed by the microprocessor

Josh Potts ECE291

Programming with Registers Flag Register

• Flags indicate the condition of the microprocessor as well as its operation

• The flag bits change after many arithmetic and logic instructions execute

• Example flags,– C(carry) indicates carry after addition or a borrow after subtraction

– O(overflow) is a condition that occurs when signed numbers are added or subtracted

– Z(zero) indicates that the result of an arithmetic or logic operation is zero

– T(trap) when the trap flag is set , it enables trapping through the on-chip debugging feature

Josh Potts ECE291

Programming with Registers Segment Registers

• CS(code) defines the starting address of the section of memory-holding code(programs and procedures used by programs)

• DS(data) a section of memory that contains most data used by a program

• ES(extra) an additional data segment• SS(stack) defines the area of memory used for the stack.

– the location of the current entry point in the stack segment is determined by the stack pointer register.

– the BP register addresses data within the stack segment

• FS and GS available on 80386 and 80486 allow two additional memory segments for access by programs

Josh Potts ECE291

Machine Language

• Machine language is the native binary code that the microprocessor understands and uses as the instructions that control its operation

• Interpretation of machine’s language allows debugging or modification at the machine language level

• Microprocessor requires an assembler program, which generates machine code – the machine language instructions are too complex to generate by

hand

• Machine language instructions for the 8086-80486, vary in length from 1 to as many as 13 bytes– there are over 20000 variations of machine language instructions

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Machine Language (cont.)

• 16 -bit instruction mode – if the machine operates in the real mode the instructions for Intel

family of microprocessors are 16 -bit instructions

– this means that instructions use 16-bit offset address and 16-bit registers

• In the protected mode the D bit in the descriptor (within a look-up table of descriptors) indicates how the 80386/80486 instructions access register and memory data in protected mode– D = 0, the 80386/80486 assumes 16 bit instructions

– D = 1, the 80386/80486 assumes 32 bit instructions

• the 32-bit instruction mode assumes all offset addresses are 32 bits as well as all registers

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Instruction Components and Format

• s

Instruction Components

Opcode Mode Displacement DataImmediate value

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Addressing Modes

• Register - transfers a byte or word from the source register or memory location to the destination register or memory location

MOV BX, CX• Immediate - transfers an immediate byte or word of data into

the destination register or memory location

MOV AX, 3456h• Direct - moves a byte or word between a memory location and

a register

MOV AL, [1234h] (1234h is treated as a displacement within data segment)

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Addressing Modes(cont.)

• Register Indirect (base relative or indexed) - transfers a byte or word of data between a register and the memory location addressed by an index (DI or SI) or base register (BP or BX)

MOV AX, [BX]• Base Plus Index (base relative indexed) - transfers a byte or

word of data between a register and the memory location addressed by a base register (BP or BX) plus index (DI or SI) register

MOV DX, [BX + DI]

Josh Potts ECE291

Addressing Modes(cont.)

• Register Relative - transfers a byte or word of data between a register and the memory location addressed by an index (DI or SI) or base register (BP or BX) plus displacement

MOV AX, [BX + 1000h]

• Base Relative Plus Index (base relative indexed) - transfers a byte or word of data between a register and the memory location addressed by a base register (BP or BX) plus an index register (DI or SI)

MOV AX, [BX + SI + 100h]

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Instruction Components

Instructions have four components that specify the operation to execute, and how to treat the associated data.

D W

OPCODE

MOD REG R/M

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Instruction ComponentsOPCODE

• Opcode (one or two bytes) selects the operation (e.g., addition, subtraction, move) performed by the microprocessor

D W

OPCODE

D - direction of the data flow D = 0 data flow to R/M field from register field D = 1 data flow to the register field from R/M in the next byte of the instruction

W - data size W = 0 data size is a byte W = 1 data size is a word/double word

Josh Potts ECE291

Instruction ComponentsMOD

• MOD field specifies the addressing mode for the selected instruction and whether the displacement is present with the specified addressing mode

• If the MOD filed contains a 00, 01, or 10, the R/M field selects one of the data memory-addressing modes, e.g.,– MOV AL, [DI] (no displacement)– MOV AL, [DI + 2] (8-bit displacement)

MOD REG R/M

MOD FUNCTION

00 no displacement01 8-bit sign-extended displacement10 16-bit displacement 11 R/M is a register (register addressing

mode)

Josh Potts ECE291

Instruction ComponentsREG & R/M in Register Assignment

Register assignment for the REG and R/M fields

Code W = 0 (Byte) W = 1(Word) W =1 (Double Word)

000 AL AX EAX001 CL CX ECX010 DL DX EDX011 BL BX EBX100 AH SP ESP101 CH BP EBP110 DH SI ESI111 BH DI EDI

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Register AssignmentExample

• Consider 2 byte instruction 8BECh in the machine language program (assuming 16-bit instruction mode)

binary representation: 1000 1011 1110 1100, from this we have opcode: 100010 => MOV

D = W 1 => a word moves into the register specified in the REG field

MOD 11 => R/M field also indicates register

REG 101 => indicates register BP

R/M 100 => indicates register SP

consequently the instruction is: MOV BP, SP

Josh Potts ECE291

Use of R/M Filed in Determining Addressing Mode

• If the MOD field contains a 00, 01, or 10, the R/M field takes on a new meaning

• Examples:1. if MOD = 00 and R/M = 101

the addressing mode is [DI]

2. if MOD = 01 or 10 and R/M = 101

the addressing mode is

[DI + 33h] or LIST[DI + 22H], where 33h, LIST, 22h are arbitrary values

for displacement

Code Function

000 DS:[BX+SI]001 DS:[BX+DI]010 SS:[BP+SI]011 SS:[BP+DI]100 DS:[SI]101 DS:[DI]110 SS:[BP]111 DS:[BX]

Base plusIndex

Registerindirect

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Example

• Consider machine language instruction 8A15h

binary representation is: 1000 1010 0001 0101opcode: 100010 => MOV

D 1 => a word moves into the register specified in the REG field

W 0 => byte

MOD 00 => no displacement

REG 010 => indicates register DL

R/M 101 => indicates addressing mode [DI]

the instruction is: MOV DL, [DI]

Josh Potts ECE291

Direct Addressing Mode

• Direct Addressing mode (for 16-bit instructions) occurs whenever memory data are referenced by only the displacement mode of addressing, e.g.,

MOV [1000h], DL moves the contents of DL into data segment memory location 1000h

MOV NUMB, DL moves the contents of DL into symbolic data segment memory location NUMB

1 0 10 0 0 0 0 1 1 10 0 0 0 0

1 000000000000000

OPCODE D W MOD REG R/M

Displacement-low Displacement-high

Byte 1 Byte 2

0

Byte 3 Byte 4

MOV [1000h], DL

Whenever the instruction has only a displacement:

MOD is always 00R/M is always 110

Josh Potts ECE291

Immediate Instruction

• Consider an instruction: MOV word PTR[BX + 1000h], 1234h

1 1 00 0 1 1 1 0 1 11 0 0 0 1

1 000000000000000

OPCODE W MOD R/M

Displacement-low Displacement-high

Byte 1 Byte 2

1 010000000101100

Data-low Data-high

Byte 3 Byte 4

Byte 5 Byte 6

Moves 1234h into the word-sized memorylocation addressed by the sum of1000h, BX, and DS x 10h

WORD PTR directive indicates to the assembler that the instruction uses a word-sized memory pointer(if the instruction moves a byte of immediate data, then BYTE PTR directive is used.

The above directive are only needed when it is not clear if the operation is a byte or a word, e.g.,MOV [BX], AL clear a byte moveMOV [BX], 1 not clear, can be byte-, word, or double word-sized moveshould be for instanceMOV BYTE PTR [BX], 1

Josh Potts ECE291

Segment MOV Instructions

• The contents of a segment register are moved by MOV, PUSH, POP

• Segment registers are selected by appropriate setting of register bits (REG field)

Code Segment Register

000 ES001 CS010 SS011 DS100 FS101 GS

Note: MOV CS, ?? and POP CSare not allowed

Example: MOV BX, CS

1 0 10 0 1 0 0 0 0 11 1 0 1 1

OPCODE MOD REG R/M

REG is 001 => selects CSR/M is 011 => selects BX

Note that the opcode for this instruction is different for the prior MOV instructions

Josh Potts ECE291

Sampling of Addressing Modes

Josh Potts ECE291

Sampling of Addressing Modes

Josh Potts ECE291

Sampling of Addressing Modes