Arithmetic, Logic Instructions and Programssaqazi.com/EEE342/...Arithmetic_Logic_Instructions_and_Programs_… · Arithmetic, Logic Instructions and Programs COMSATS University Islamabad

Arithmetic, Logic

Instructions and

Programs

Dr. Sohaib Ayyaz QaziCOMSATS University Islamabad

1Lecture# 08

Microprocessor System and Interfacing

Rotate, Shift and Data Serialization


2

Shift Instructions

LSL Instruction

LSL Rd ; logical shift left

Shifted from Right to Left

0 in LSB and MSB in C flag

What it can be used for ?

LSR Instruction

LSR Rd ; logical shift right

Shifted from Left to Right

0 in MSB and LSB in C flag

What it can be used for ?

Cannot be used for Signed Division



3

Shift Instructions

ASR – Arithmetic Shift Right

ASR Rd ; shift right

Shifted from Right to Left

MSB is constant

LSB in C Flag

MSB is copied to next bit

Used for Signed Division



4

Class Example - 1

LDI R20, 0xD0 ; R20 = 1101 0000 (-48) C = 0

ASR R20 ; R20 = 1110 1000 (-24) C = 0

ASR R20 ; R20 = 1111 0100 (-12) C = 0

ASR R20 ; R20 = 1111 1010 (-6) C = 0

ASR R20 ; R20 = 1111 1101 (-3) C = 0

ASR R20 ; R20 = 1111 1110 (-1) C = 1



5

SWAP Instruction

SWAP Rd ; swap nibbles

Works on all Registers (R0 – R31)

Lower bits (D0 – D3) are Swapped with higher

bits (D4 – D7)



6

Class Exercise – 1

Assume that the R20 has the number 48. Show

how we can use ROR to divide R20 by 8

LDI R20, 0x30 ;

CLC ; Clear Carry flag

ROR R20 ; R20 = 24 (0001 1000)


ROR R20 ; R20 = 12 (0000 1100)


ROR R20 R20 = 6 (0000 0110)



7

Class Exercise – 2

Write a program equivalent to the following

instruction using ROL or ROR instructions

LDI R20, 0x72 ;

LDI R16, 4

LDI R21, 0

BEGIN:

CLC

ROL R20

ROL R21

DEC R16

BRNE BEGIN

OR R20, R21

HERE: JMP HERE

LDI R20, 0x72

SWAP R20

BCD and ASCII Conversion


8

BCD – Binary Coded Decimal

Unpacked BCD

Lower 4 bits represents a number (0 – 9)

Upper 4 bits are zero

Requires one byte of memory

Packed BCD

Single Byte Contains 2 BCD numbers

One in lower nibble and one in upper nibble

Efficient as it stores more data

BCD and ASCII Conversion


9

Home Assignment

Read about ASCII Numbers

Benefit of ASCII Conversion to Packed BCD

Over Unpacked BCD

AVR Advanced

Assembly Language

Programming


10

Introducing Some More Assembler Directives


11

Arithmetic Expressions with Constant

Define Constant values using .EQU

.EQU ALFA = 50

.EQU BETA = 40

LDI R23, ALFA

LDI R24, ((ALFA – BETA) * 2 ) + 9; ((50 – 40)*2)+9 = 29

Symbol Action

+ Addition

- Subtraction

* Multiplication

/ Division

% Modulo



12

Logic Expressions with Constant

Define Constant values using .EQU

.EQU C1 = 0x50

.EQU C2 = 0x10

.EQU C3 = 0x04

LDI R21, (C1 & C2) | C3 ; R21 = (0x10 & 0x50)|0x04 = 0x14

Symbol Action

& Bitwise AND

| Bitwise OR

^ Bitwise XOR

~ Bitwise NOT



13

Shift Operands

The shift operators to shift left and right aconstant value

LDI R16, 0b00000111 << 1 ; R16 = 0b00001110

What we have to do if we have to set Z andC flags ?

Load 0b00000011 in status register

What to do if we don’t have to use the bitnumber ?

I T H S V N Z C



14

Shift Operands

Use shift operators to complete this task

Header file of each AVR contains this

definition (M32DEF.INC)

; ***** CPU **************************

; SREG - Status Register

.equ SREG_C = 0 ; Carry Flag

.equ SREG_Z = 1 ; Zero Flag

.equ SREG_N = 2 ; Negative Flag

.equ SREG_V = 3 ; Two's Complement Overflow Flag

.equ SREG_S = 4 ; Sign Bit

.equ SREG_H = 5 ; Half Carry Flag

.equ SREG_T = 6 ; Bit Copy Storage

.equ SREG_I = 7 ; Global Interrupt Enable



15

Shift Operands

To set N flag of SREG register we will use

LDI R16, 1<<SREG_N ; R16 = 1<<2

; R16 = 0b00000100

OUT SREG, R16

Exercise:

Set H and V Flag of Status register using shift

operators

LDI R16, (1<<SREG_V) | (1<<SREG_H)

OUT SREG, R16



16

HIGH () and LOW () Functions

Give high and low byte of 16-bit value

LDI R16, HIGH (RAMEND)

OUT SPH, R16

LDI R16, LOW (RAMEND)

OUT SPL, R16

How these instructions work?

RAMEND defines the address of RAM in

M32DEF.INC

.equ RAMEND = 0x085f

Register and Direct Addressing Modes


17

CPU can access data in various ways

Data could be in a register or a memory

The ways are known as Addressing Modes

Modes are determined when it is designed, itcannot be changed by programmer

13 distinct addressing modes are there(Categorized)

Single-Register (Immediate)

Register

Direct

Register Indirect

Flash Direct

Flash Indirect



18

Single – Register (Immediate)

The operand is a register

NEG R18

COM R19

In some cases there is a constant value with

register

LDI R18, 0x10

SUBI R19, 0x6



19

Two – Register

Two registers are involved to hold the data to be

manipulated

ADD R20, R23

AND R03, R04



20

Direct Addressing Mode

Entire data memory can be accessed

LDS R19, 0x560

STS 0x40, R18

Address field is 16-bit address

Doesn’t support Immediate

addressing mode



21

I/O Direct Addressing Mode

Special mode

Can address only standard registers

IN and OUT instructions are used

IN R18, 0x16 ; PINB

OUT 0x15, R18 ; PORTC



22


OUT 0x15, R19 ; R19 to PORTC

OUT PORTC, R19 ; Same as Above



23


Remember

Address b/w $20 - $5F is assigned to IO registers

IO registers have two addresses

IO Address

Data Memory Address

Some AVRs have more than 64 IO registers known

as Extended IO memory

IO registers can have different address in different

AVRs

OUT 0x15, R19 ; PORTC = R19

STS 0x35, R19 ; PORTC = R19



24

Exercise No - 3

Write Code to send $55 to Port B using

The register Name

The IO Address

The Data Memory Address



25

Register Indirect Addressing Mode

Register is used as a pointer to the data memory

Three registers (X, Y, Z) are used

All are 16-bit registers

Made by combining two GPRS (X = R27:R26, Y =R29:R28, Z = R31:R30)

Can be referred as XL and XH (Low and High)

LD R24, X ; copies value at location X in R24

To a value from location 0x0130 to R18 use

LDI XL, 0x30

LDI XH, 0x01

LD R18, X

Class Exercise - 4


26

Write a program to bring in a byte of data serially via pin PC7 and

save it in R20 register. The byte comes in with the LSB first.

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Arithmetic, Logic Instructions and Programssaqazi.com/EEE342/...Arithmetic_Logic_Instructions_and_Programs_… · Arithmetic, Logic Instructions and Programs COMSATS University Islamabad