1. Module 10 Adapted By and Prepared James Tan © 2001

2. Objectives In this lecture we will be looking at : Addressing Mode Typical fetch & execute cycle in a CPU

3. 10.1 Assembly Programming Techniques The specified address should be as short as possible  Less time & space For table of data  Use index method.

4. 10.2 Addressing Mode  Direct Addressing  The operand address field contains the address of the operand. Example: LOCN 03:OPCODE1001 0110 (96) (LDA) LOCN 04:OPERAND0000 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.

5. 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)

6. 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 AddressInstructionPage Bit 4010ADD 380 AddressInstructionPage Bit 4010ADD 381

7. 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.

8. 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

9. 10.2.4 Indexed Addressing Address of Operand = (Index Register)+(Operand Address Field) Related Instructions LDXLoad the index register INXincrement the index register DEXdecrement the index register STXstore the contents of index\register in memory

10. 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 (15 16 + index register contents) into accumulator A’. Consider LDA 50, X[see page 10.8 for details] Resultant Address is 53

11. 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,

12. Program Example 1: Forward Branching Address 0BRA LOOP 1OFFSET 2  PCROLA 3PSHB 4LOOPADD 5 OFFSET = 2, PC = 2 PC = PC + OFFSET = 2 + 2  4

13. Program Example 2: Backward Branching Address 0LOOPCLR 1ASRA 2 ROLA 3PULA 4NOP 5NEGA 6BRA LOOP 7OFFSET 8  PC OFFSET = negative 8  F8 in two’s complement PC = 8

14. 10.2.6 & 7 Register Direct/Indirect Addressing Only used in CPU which supports it

15. Stack Addressing Uses Stack Pointer (SP) Has Push and Pop operations Consider Page 10.10 SP = 100,Acc A = 56 10 Acc B = 31 10 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

16. 10.3Fetch & Execute Cycle See File: CE_10B.ppt

17. End of Lecture