Processor Function
Topic 3
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
Revision Question 1
Machine code is
A. a common programming language which
can be used on any machine
B. the low level language specific to a
particular microprocessor
C. the number which identifies a particular
type of computer
D. any high level language which can be
compiled for use by a microprocessor
B
Revision Question 2
The processor register which holds the
address of the next instruction to be
fetched is the
A. memory address register (MAR)
B. instruction register (IR)
C. program counter (PC)
D. memory data register (MDR)
C
Revision Question 3
The correct sequence of steps in the fetch-execute
cycle is
A. fetch instruction ➜ increment PC ➜ decode
instruction ➜ execute instruction
B. increment PC ➜ fetch instruction ➜ decode
instruction ➜ execute instruction
C. fetch instruction ➜ decode instruction ➜
increment PC ➜ execute instruction
D. fetch instruction ➜ decode instruction ➜
execute instruction ➜ increment PC
A
Revision Question 4
Data fetched from memory arrives in
A. the memory address register (MAR)
B. the instruction register (IR)
C. the program counter (PC)
D. the memory data register (MDR)
D
Revision Question 5
In an 8-bit microprocessor,
A. the data and address buses must both be
8 bits wide
B. the data bus is 8 bits wide, but the
address bus may be more than 8 bits
wide
C. the address bus must be 8 bits wide, but
the data bus can be any width
D. a single 8 bit wide system bus is used to
transfer all data and addresses
B
Machine Code
• Processors only understand
instructions in machine code
• These are difficult to read and
understand
e.g 10101001 00000001
10000101 01110000
10100101 01110000
Machine Code
Assembly Language
10101001 00000001
10001101 00000011
11101000
10001101 00000011
11101000
01101001 00000001
10001101 00000011
11101001
00100000 11101110
11111111
01100000
LDA #1
STA 1000
LDA 1000
ADC #1
STA 1001
JSR OSWRCH
RTS
Mnemonic – a short
code – replaces the
machine code
instruction
Assembly Language
Instructions
Op-code
What the
instruction is
to do
Operand
Data to be
operated
upon
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
Assembly Language
Instructions
Op-code
Operand
LDA
#1
10101001
00000001
Machine Code
Assembly Language
10101001 00000001
10001101 00000011
11101000
10001101 00000011
11101000
01101001 00000001
10001101 00000011
11101001
00100000 11101110
11111111
01100000
LDA #1
STA 1000
LDA 1000
ADC #1
STA 1001
JSR OSWRCH
RTS
Instruction Formats
• Not all instructions are the same
length e.g. 6502 processor–
8-bit Op-code
8-bit Op-code
8-bit operand
8-bit Op-code
16-bit operand
Instruction Formats
• IBM mainframe instruction set
8-bit Op-code
8-bit Op-code
4-bit operand
4-bit operand
8-bit Op-code
4-bit operand
4-bit operand
12-bit operand
Instruction Formats
• X86 mainframe instruction set
Assembly Language
Instructions
Regardless of processor, all instructions the same format -
Op-code
What the
instruction is
to do
Operand
Data or
address of
data to be
operated
upon
Instruction types
•
•
•
•
•
Data transfer instructions
Arithmetic instructions
Logical instructions
Shift and rotate instructions
Branch
Data transfer
• Used to move data from one place to
another
– Between registers
– From memory to processor/register
– From processor to memory
Arithmetic
• Used to carry out simple arithmetic
e.g. +-/x
– Add contents of 2 registers
– Adding 1 to contents of register
Logical
• Used to make logical comparisons
• Checking whether contents of two
registers are equal to each other
Shift and rotate
• Used to manipulate the individual bits
within a register
• Shift left 1 bit
Branch
• Branch instructions break the normal
sequential flow of execution by
changing the program counter if a
specified condition is met.
6502 processor
Simple processor
used in BBC,
Apple II and
Atari
6502 Microprocessor
System
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
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.
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.
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 65535 (216-1) could
be directly addressed
• Memory was made up of individually
addressable 8-bit words (bytes).
6502 Microprocessor
System - registers
16 bits – used for storing 16
bit addresses
8 bits – used for storing
data
6502 Registers
8 bit registers
• A
• X
• Y
• SR
• IR
• MDR
16-bit registers
• MAR
• PC
• SP
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
6502 registers – X,Y
• X and Y are index registers. They are
designed to hold loop counters or as
gp registers
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
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
6502 registers - IR
• IR is the instruction register.
• It holds the instruction being
decoded or executed
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
6502 registers - MAR
• MAR is the Memory Address
register.
• It holds an address about to be sent
out along the address bus
6502 registers - PC
• PC is the Program Counter.
• It holds the address of the next
instruction to be fetched, decoded
and executed
6502 registers - SP
• SP is the Stack Pointer.
• Temporary data storage. The SP
holds the address of the next free
location in the stack
Exercise
• Copy and complete the table Page 55
Q.7
• Complete questions 8-11
LDA #55
• Load the accumulator with the value
55
• See scholar animation
Addressing Modes
• Refers to the operand
• There are many types e.g.
–
–
–
–
Immediate
Direct
Implied
Others such as relative, indirect,
indexed
Immediate Addressing
• The operand is the actual data to be used
• Example – LDA #55
8-bit Op-code
8-bit operand
• 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
Direct Addressing
• The operand is the address to be
used
• Example – LDA 3C15
8-bit Op-code
• Other examples
16-bit operand (address)
– JMP 2015 – jump to instruction in
location 2015
– ADC 2099 – add the data in location
2099 to the accumulator
Implied Addressing
• No operand is required
• Example – TAX – transfer the
contents of A to X
8-bit Op-code
• Other examples
– TYA – transfer the contents of the Yregister to the accumulator
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"
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
Program 1 – adding two
numbers
CLC
LDA 2004
ADC 2005
STA 2006
clear the carry flag, just in case
fetch the number from location 2004
add on the number from location 2005
store the result in location 2006
Program 2 – Multiplying
by 3
CLC
LDA 2004
ADC 2004
ADC 2004
STA 2005
clear the carry flag, just in case
fetch the number from location 2004
add the number to itself
add the number on again
store the result in location 2005
Program 3 – Multiplying
two numbers
LDA 2004
LDX 2005
DEX
CLC
ADC 2004
DEX
BNE -6
STA 2006
fetch the number from location 2004
load the X register with the number
from location 2005
reduce X by 1 (see note 1 below)
clear the carry flag just in case
add the number from location 2004 to
the current value in the accumulator
reduce the X register by 1
if X is not zero, branch back 6 locations
see note 2 below)
store the result in location 2006
Exercise
• Scholar Page 91
– Q 12 – 19
• Past Papers
–
–
–
–
2008 Q14 a-b
2006 Q17 b+c
2010 Q15
2011 Q11