Module 10

Adapted By and Prepared James Tan © 2001

Objectives

• In this lecture we will be looking at :

• Addressing Mode

• Typical fetch & execute cycle in a CPU

10.1 Assembly Programming Techniques

• The specified address should be as short as possible Less time & space

• For table of data Use index method.

10.2 Addressing Mode

Direct Addressing The operand address field contains the address of the

operand.• Example:

• LOCN 03:OPCODE 1001 0110 (96) (LDA)

• LOCN 04:OPERAND 0000 0000 (00)

• Two limitations

– Need 2 bytes of code

– Not efficient for processing data arrays or tables, as it can only access a single fixed address at one time no good for scanning table of data.

Page Addressing

• Consider ADD 2007• Direct addressing can be improved by using 2

registers:– Instruction Register : storing ADD 7– Page Register (in thousands) : storing 2– Effective address = 2007

• Acc Acc + (content of loc 2007)

Further development of page addressing

• Page Zero Addressing

– Although at page 4, but page bit = 0 Address = 0038

• Current Page Addressing

– Although at page 4, but page bit = 1 Address = 4038

Address Instruction Page Bit

4010 ADD 38 0

Address Instruction Page Bit

4010 ADD 38 1

10.2.2 Indirect Addressing

• The operand address field gives the address of a

memory location which contains the address of

the operand.

• Note: M6800 series does not have indirect

• Example: ADD @ 100

• @ : indirect symbol

• Meaning: go to location 100, get its contents and

use it as an address, go to that address and add its

contents to the accumulator.

10.2.3 Immediate Addressing

• The operand address field contains the operand.

• Example: ADD #4

• # : Immediate symbol

• Meaning: Add the number 4 immediately to Acc.

• Good programming habit:

– Use Constants

M = 4

LDA M

10.2.4 Indexed Addressing

• Address of Operand =

(Index Register)+(Operand Address Field)

• Related Instructions

LDX Load the index register

INX increment the index register

DEX decrement the index register

STX store the contents of index\register in

memory

Example

• ADDA $15, X

• ADDA: add to Acc

• $15: dollar sign Hex number

• X: index register contents

• Meaning: ‘add the contents of location whose address is (1516

+ index register contents) into accumulator A’.

• Consider LDA 50, X [see page 10.8 for details]

• Resultant Address is 53

10.2.5 Relative Addressing

• Address of Operand = (PC) + (Operand Address Field).

• Example: BRANCH instruction: BRA * + 5

Effect: PC = PC + 5

5 : offset

• Some factors to consider:

Negative Branch Distance: 2’s complement

PC is incremented to point to next instruction before the

calculation of branch distance takes place,

Program Example 1: Forward Branching

Address

0 BRA LOOP

1 OFFSET

2 PC ROLA

3 PSHB

4 LOOP ADD

5

• OFFSET = 2, PC = 2

• PC = PC + OFFSET = 2 + 2 4

Program Example 2: Backward Branching

Address

0 LOOP CLR

1 ASRA

2 ROLA

3 PULA

4 NOP

5 NEGA

6 BRA LOOP

7 OFFSET

8 PC

• OFFSET = negative 8 F8 in two’s complement

• PC = 8

10.2.6 & 7 Register Direct/Indirect Addressing

• Only used in CPU which supports it

Stack Addressing

• Uses Stack Pointer (SP)

• Has Push and Pop operations

• Consider Page 10.10

• SP = 100, Acc A = 5610 Acc B = 3110

• Push A Location 100 = 56

SP = 99

Push B

• See End result at page 10.11

• Question: How about result of the following instructions?

Pop A

Pop B

10.3 Fetch & Execute Cycle

See File: CE_10B.ppt

End of Lecture