Transcript Recap – Our First Computer

Recap – Our First Computer
System Bus
To registers’
read/write and
clock inputs
W
8
A
S
B
ALU
COUT
Sequence of
control signal
combinations
8
Carry
output
F
R
8
Updating the Paper Tape Approach
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The paper tape programming method has a number
of disadvantages
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Not very quick or compact
Can’t randomly access (i.e. no jumps, loops etc.)
Requires a lot of bits of storage
Solid state program memory (a big bank of read-only
registers) solves the first two problems
The last requires the introduction of operation codes
(op-codes)
Operation Codes
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CLK
R/W
A
I/O
Control Unit
Op-Code
8
CLK
R/W
CLK
R/W
B
I/O
System
Bus
C
I/O

Useful
operations
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AB
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BA
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AC
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CA
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BC
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CB
Only 6, could be
coded into just 3
bits
Op-Codes
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The paper tape program can recreate absolutely any
permutation of control signals, ALU inputs etc.
Many permutations are either not permitted or are
just not very useful, so only a restricted set of
possible data transfers are allowed.
This means that instructions can be coded into much
shorter words, or op-codes (12 bits for PICs)
It does, however, require an extra block of logic to
decode the op-codes.
Machine Code & Assembly Language
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A sequence of op-codes forms a program
It is just a list of numbers though, ideal for the
processor – not so easy for humans
A program in this form is written in machine code
Each op-code has a mnemonic equivalent (e.g. ADD,
SUB, OR etc.)
A program written in such mnemonics can be
converted to machine code using an assembler
It is, therefore, known as assembly language
Examples
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For the Z80 processor:
Assembly
Language
Machine
Code
SUB B
10010000
OR C
10110001
LD D,E
01010011
Using Program Memory
System Bus
Program
Counter
To registers’
read/write and
clock inputs
Program
Memory
Instruction
Register
Control
Unit
8
W
8
A
S
B
ALU
COUT
Carry
output
F
R
8
Using Program Memory
System Bus
Program
Counter
To registers’
read/write and
clock inputs
Program
Memory
Instruction
Register
Control
Unit
Program Counter, PC
Stores the position (or address) in program
memory of the next instruction. It is
automatically incremented after each operation.
8
W
8
A
S
B
ALU
COUT
Carry
output
F
R
8
Using Program Memory
System Bus
Program
Counter
To registers’
read/write and
clock inputs
Program
Memory
Instruction
Register
Control
Unit
Program Memory
A block of programmable read-only memory
(ROM) containing a list of op-codes, i.e. a
program.
8
W
8
A
S
B
ALU
COUT
Carry
output
F
R
8
Using Program Memory
System Bus
Program
Counter
To registers’
read/write and
clock inputs
Program
Memory
Instruction
Register
Control
Unit
Instruction Register
A special register whose only purpose is to
temporarily store the op-code of the instruction
currently being executed.
8
W
8
A
S
B
ALU
COUT
Carry
output
F
R
8
Using Program Memory
System Bus
Program
Counter
To registers’
read/write and
clock inputs
Program
Memory
Instruction
Register
Control
Unit
Control Unit
A block of logic that translates the op-code into
the corresponding ALU select inputs and the
control signals to the registers.
8
W
8
A
S
B
ALU
COUT
Carry
output
F
R
8
More on the Program Counter
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The Program Counter is a special purpose register
used to address program memory.
After each instruction, its contents are incremented
by one.
It can also be written to using the system bus,
allowing the processor to ‘jump’ to any address in
program memory.
Its contents can also be temporarily stored on a stack
allowing easy implementation of sub-routines.
A Micro-Controller
8
PC
W
System Bus
SR
ALU
Program
Memory
I/O
General Purpose
Registers
Inputs / Outputs
Instruction
Register
Cont rol Unit
Cont rol signals to
registers, ALU, etc.
PC = Program Counter (register)
W = Working Register
SR = Status Register
The Status Register (SR)
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The ALU produces a variety of flags after most
operations, the most important are:
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Carry Flag
Zero Flag
The states of these flags are stored in the status
register, SR.
This register can be read like any other.
More usefully, some operations (usually jumps) can
be conditional on the state of one or more flags.
Using these op-codes, ‘if-then’ operations, ‘for’ loops
etc. are possible.
Input/Output Registers
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Input/Output (I/O) registers behave just like
general purpose ones…
… except that I/O registers can be written to
or read from externally via the pins of the
chip housing the micro-controller.
These registers are vital, they’re the only way
the device can communicate and do
something useful.
Programming
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Micro-controller op-codes represent very simple
operations:
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Data transfers
Arithmetic or logical operations
Flow of control
Even the most elaborate of tasks can be broken
down into a sequence of these primitive operations.
Breaking down jobs in such an elemental way is what
programming is all about.
Part One of EE1A2
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Number Systems
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You should be able to:
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Binary Arithmetic Circuits
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Convert numbers between decimal, hex and binary forms
Perform binary arithmetic operations
Adder, carry look-ahead
Addition/Subtraction Circuit
ALUs
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Design of basic logic devices capable of a several binary
arithmetic and/or logical operations.
Part One of EE1A2 (cont)
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Registers
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Memory elements storing one byte of data.
Interconnection using tri-state ports connected to a common
bus.
Control signals necessary to transfer data.
Micro-controllers
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All the basic components required to make a computer
integrated onto a single chip.
The ALU, registers, program memory and input/output
circuitry – what they all do.
Programming using assembly language - welcome to the
rest of the course !