1. Processor Function
Topic 3

2. After studying this chapter you should be able to -
describe the structure of typical assembly language instructions using the terms op-code and operand
describe and give examples of assembly language instructions of the following types
data transfer
arithmetic
logical
shift and rotate
branch

3. B Revision Question 1 Machine code is
a common programming language which can be used on any machine
the low level language specific to a particular microprocessor
the number which identifies a particular type of computer
any high level language which can be compiled for use by a microprocessor B

4. Revision Question 2
The processor register which holds the address of the next instruction to be fetched is the
memory address register (MAR)
instruction register (IR)
program counter (PC)
memory data register (MDR) C

5. Revision Question 3
The correct sequence of steps in the fetch-execute
cycle is
fetch instruction ➜ increment PC ➜ decode instruction ➜ execute instruction
increment PC ➜ fetch instruction ➜ decode instruction ➜ execute instruction
fetch instruction ➜ decode instruction ➜ increment PC ➜ execute instruction
fetch instruction ➜ decode instruction ➜ execute instruction ➜ increment PC A

6. D Revision Question 4 Data fetched from memory arrives in
the memory address register (MAR)
the instruction register (IR)
the program counter (PC)
the memory data register (MDR) D

7. B Revision Question 5 In an 8-bit microprocessor,
the data and address buses must both be 8 bits wide
the data bus is 8 bits wide, but the address bus may be more than 8 bits wide
the address bus must be 8 bits wide, but the data bus can be any width
a single 8 bit wide system bus is used to transfer all data and addresses B

8. Machine Code Processors only understand instructions in machine code
These are difficult to read and understand
e.g

9. Machine Code Assembly Language 10101001 00000001 10001101 00000011
LDA #1
STA 1000
LDA 1000
ADC #1
STA 1001
JSR OSWRCH
RTS
Mnemonic – a short code – replaces the machine code instruction

10. Assembly Language Instructions
Op-code
Operand
What the instruction is to do
Data to be operated upon

11. Assembly Language Instructions
Op-code
Operand
Meaning
LDA #1
Load the accumulator with the value 1
STA
1DFF
Store the contents of the acc in memory location 1DFF
LDA
3C15
Load the acc. with the contents of memory location 3C15

12. Assembly Language Instructions
Op-code
Operand
LDA #1

13. Machine Code Assembly Language 10101001 00000001 10001101 00000011
LDA #1
STA 1000
LDA 1000
ADC #1
STA 1001
JSR OSWRCH
RTS

14. Instruction Formats
Not all instructions are the same length e.g processor–
8-bit Op-code
8-bit Op-code
8-bit operand
8-bit Op-code
16-bit operand

15. Instruction Formats IBM mainframe instruction set 8-bit Op-code
4-bit operand
4-bit operand
8-bit Op-code
4-bit operand
4-bit operand
12-bit operand

16. Instruction Formats
X86 mainframe instruction set

17. Assembly Language Instructions
Regardless of processor, all instructions the same format -
Op-code
Operand
What the instruction is to do
Data or address of data to be operated upon

18. Instruction types Data transfer instructions Arithmetic instructions
Logical instructions
Shift and rotate instructions
Branch

19. Data transfer Used to move data from one place to another
Between registers
From memory to processor/register
From processor to memory

20. Arithmetic Used to carry out simple arithmetic e.g. +-/x
Add contents of 2 registers
Adding 1 to contents of register

21. Logical Used to make logical comparisons
Checking whether contents of two registers are equal to each other

22. Shift and rotate
Used to manipulate the individual bits within a register
Shift left 1 bit

23. Branch
Branch instructions break the normal sequential flow of execution by changing the program counter if a specified condition is met.

24. 6502 processor
Simple processor used in BBC, Apple II and Atari

25. 6502 Microprocessor System

26. 6502 Microprocessor System
introduced in about 1975
among the first microprocessors to be used in early home computers.
included the usual Arithmetic/Logic Unit with some internal registers and a Control Unit all on the same chip.
It had an external crystal-controlled clock to generate timing signals

27. 6502 Microprocessor System
ROM was used to hold a bootstrap program to permit initial operation of the system
RAM was used to hold programs and data
The interface with the external devices was via a Programmable Input/Output unit (PIO), which communicated with the external devices using 16-bit wide I/O buses.

28. 6502 Microprocessor System
The external bus was a combination of an 8-bit-wide data bus, a 16-bit-wide address bus and some control lines that carried synchronisation signals throughout the system.

29. 6502 Microprocessor System
Hence only 8 bits of data could be moved around the system, but 16-bit addresses could be used to address memory.
Memory locations with addresses in the range 0 to (216-1) could be directly addressed
Memory was made up of individually addressable 8-bit words (bytes).

30. 6502 Microprocessor System - registers
8 bits – used for storing data
16 bits – used for storing 16 bit addresses

31. 6502 Registers 8 bit registers A X Y SR IR MDR 16-bit registers MAR PCSP

32. 6502 registers - A
A is known as the Accumulator – the main general purpose register in which data is held during most arithmetical and logical operations

33. 6502 registers – X,Y
X and Y are index registers. They are designed to hold loop counters or as gp registers

34. 6502 registers - SR
SR is the status register. It is really a set of 8 1-bit registers. Each bit operates independently, and is used to flag to record the status of the processor after each instruction

35. 6502 registers - SR
N- the negative flag – set o 1 if the result of the last operation is zero
V – the overflow flag – set to 1 if the last result operation is invalid
B – the break bit – set to 1 when a break instruction is executed
D – the decimal flag – when set to 1, the processor will operate using binary coded decimal arithmetic
I – the interrupt disable flag – when set the processor will ignore any interrupt signals
Z – the zero flag – set to 1 when the last result was zero
C – the carry flag – set to 1 if the result of an operation is a number that cannot be stored in a single 8-bit register

36. 6502 registers - IR IR is the instruction register.
It holds the instruction being decoded or executed

37. 6502 registers - MDR MDR is the Memory Data register.
It holds data which has just arrived along the data bus or is just about to be sent along the data bus

38. 6502 registers - MAR MAR is the Memory Address register.
It holds an address about to be sent out along the address bus

39. 6502 registers - PC PC is the Program Counter.
It holds the address of the next instruction to be fetched, decoded and executed

40. 6502 registers - SP SP is the Stack Pointer.
Temporary data storage. The SP holds the address of the next free location in the stack

41. Exercise Copy and complete the table Page 55 Q.7
Complete questions 8-11

42. LDA #55
Load the accumulator with the value 55
See scholar animation

43. Addressing Modes Refers to the operand There are many types e.g.
Immediate
Direct
Implied
Others such as relative, indirect, indexed

44. Immediate Addressing The operand is the actual data to be used
Example – LDA #55
Other examples
CMP #27 – compare the contents of A with the number 27
LDX #A4 – load the X register with the value A4
LDY #00 – load the Y register with the number 00
8-bit Op-code
8-bit operand

45. Direct Addressing The operand is the address to be used
Example – LDA 3C15
Other examples
JMP 2015 – jump to instruction in location 2015
ADC 2099 – add the data in location 2099 to the accumulator
8-bit Op-code
16-bit operand (address)

46. Implied Addressing No operand is required
Example – TAX – transfer the contents of A to X
Other examples
TYA – transfer the contents of the Y-register to the accumulator
8-bit Op-code

47. Other addressing modes
As well as immediate, absolute and implied addressing, the 6502 processor has many other addressing modes. These include:
relative addressing - for example BEQ 09, which means "if the zero flag was set by the last operation, branch to the instruction 9 locations forward in memory"
indexed addressing - for example LDA 3C15,Y which means "load the accumulator with the data to be found at the location 3C15+Y, where Y is the value stored in the Y register
indirect addressing - for example LDA (3C15), which means "load the accumulator with the data you find at the address which is to be fond at location 3C15"

48. Instruction types Data transfer instructions Arithmetic instructions
Logical instructions
Shift and rotate instructions
Branch instructions
Various others
Now have a look at the animations in Scholar

49. Program 1 – adding two numbers
CLC clear the carry flag, just in case
LDA 2004 fetch the number from location 2004
ADC 2005 add on the number from location 2005
STA 2006 store the result in location 2006

50. Program 2 – Multiplying by 3
CLC clear the carry flag, just in case
LDA 2004 fetch the number from location 2004
ADC 2004 add the number to itself
ADC 2004 add the number on again
STA 2005 store the result in location 2005

51. Program 3 – Multiplying two numbers
LDA fetch the number from location 2004
LDX load the X register with the number from location 2005
DEX reduce X by 1 (see note 1 below)
CLC clear the carry flag just in case
ADC add the number from location 2004 to the current value in the accumulator
DEX reduce the X register by 1
BNE -6 if X is not zero, branch back 6 locations see note 2 below)
STA store the result in location 2006

52. Exercise Scholar Page 91 Past Papers Q 12 – 19 2008 Q14 a-b
2006 Q17 b+c
2010 Q15
2011 Q11