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Chapter 4: A Simple Computer. We explore the organization of a computer which is the structure and function of the components in the computer Structure : how the components in the computer are connected together, how they communicate Function : what they do

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chapter 4 a simple computer
Chapter 4: A Simple Computer
  • We explore the organization of a computer which is
    • the structure and function of the components in the computer
      • Structure: how the components in the computer are connected together, how they communicate
      • Function: what they do
  • Specifically, we will explore the
    • CPU
    • Memory
    • I/O
    • Bus
    • A primitive instruction set (MARIE)
      • MARIE is the book’s simple computer
      • We will examine it to understand what an instruction set is, before we being our examination of the instruction set for Intel
the cpu
  • The central processing unit (or processor)
    • Is the brain of the computer
    • It does all the processing
  • What does this mean?
    • The CPU is in charge of executing the current program
      • Each program is stored in memory along with data
      • The CPU is in charge of retrieving the next instruction from memory (fetch), decoding and executing it
      • Execution usually requires the use of ALU and temporary storage in registers
      • Some instructions cause data movement (memory accesses, input, output) and some instructions change what the next instruction is (branches)
    • We divide the CPU into two areas
      • Datapath – registers and ALU (the execution unit)
      • Control unit – circuits in charge of performing the fetch-execute cycle
two kinds of registers
User registers

These store data and addresses

These are manipulated by your program instructions

Example: Add R1, R2, R3

R1  R2 + R3

Computers will have between

One and hundreds of registers

Possibly divided into data and address registers

Registers are usually the size of the computer’s word size

32 or 64 bits today, previously it had been 8 or 16 bits

Control registers

Registers that store information used by the control unit to perform the fetch-execute cycle

PC – the memory location of the next instruction

IR –the current instruction being executed

Status flags – information about the results of the last instruction executed (was there an overflow, was the result positive, zero or negative?)

Two Kinds of Registers
Consists of circuits to perform arithmetic and logic operations





Operations in the ALU set status flags (carry, overflow, positive, zero, negative)

Also, possibly, temporary registers before moving results back to register or memory

control unit
in charge of managing the fetch-execute cycle

It sends out control signals to all other devices

A control signal indicates that the device should activate or perform it’s function

For instance:

Instruction fetching requires

sending the PC value to main memory

signaling memory to read

when the datum comes back from memory, move it to the IR

increment the PC to point to the next instruction

These operations are controlled by the control unit

Now the control unit decodes the instruction signals the proper ALU circuit(s) to execute it

Control Unit
the system clock
In order to regulate when the CU issues its control signals, computers use a system clock

At each clock pulse, the control unit goes on to the next task

Register values are loaded or stored at the beginning of a clock pulse

ALU circuits activate at the beginning of a clock pulse

The System Clock
the system clock1
Clock performance is based on the number of pulses per second, or its Gigahertz rating

This is a misleading spec

The number of clock pulses (cycles) that it takes to execute one instruction differs from one computer to the next

Assume computer A takes 10 clock cycles per instruction but has a 1 Gigahertz clock speed

Assume computer B can execute 10 instructions in 11 cycles using a pipeline, but has a 250 Megahertz clock speed

Which one is faster? B even though its clock is slower!

The System Clock
comparing clocks
Comparing Clocks
  • It is difficult to computer CPU performance just by comparing clock speed
    • You must also consider how many clock cycles it takes to execute 1 instruction
    • How fast memory is
    • How fast the bus is
  • In addition, there are different clocks in the computer, the Control Unit and the whole CPU are governed by the system clock
    • There is usually a bus clock as well to regulate the usage of the slower buses
the bus
A bus is a collection of wires that allow current to flow over them

The current is the information being passed between components

There are 3 parts to a bus

data bus

for data and program instructions

control bus

control signals from the CU to the devices, and feedback lines for ack that they are ready or for interrupting the CPU

address bus

the address of the memory location or I/O device that is to perform the given operation

The Bus
the bus1
Additionally, computers may have multiple buses

Local bus

connects registers, ALU and CU together

System bus

connects CPU to main memory

Expansion or I/O bus

connects System bus to I/O devices

The Bus
more on buses
More on Buses
  • Buses connect two types of devices
    • Masters
      • Devices that can initiate requests
        • CPU
        • some I/O devices
    • Slaves
      • Devices that only respond to requests from masters
        • Memory
        • some I/O devices
more on buses1
More on Buses



  • Some buses are dedicated
    • The bus directly connects two devices (point-to-point bus)
  • Most buses connect multiple components
    • multipoint




Expansion bus

the system bus
Main memory connects to this bus through pins

The I/O subsystem connects to this bus through the expansion bus

The bus carries three types of information

The address from the CPU of the intended item to be accessed

The control information (read versus write, or status information like “are you available?”)

The data, either being sent to the device, or from the device to CPU

The System Bus
expansion bus
expansion bus
  • The expansion bus
    • is the collection of expansion slots and what gets plugged into them
    • Here we see interface cards (or expansion cards), each with the logic to interface between the CPU and the I/O device (e.g., printer, MODEM, disk drive)
who gets to use the bus
Who gets to use the bus?
  • In a point-to-point buses this is not a problem
  • In the expansion bus where multiple I/O devices may want to communicate between themselves and the CPU or memory at the same time – we need a form of Bus Arbitration
    • Daisy chain arbitration
      • Each device has a bus request line on the control bus
      • When a device wants to use the bus, it places its request and the highest priority device is selected (this is an unfair approach)
    • Centralized parallel arbitration
      • The bus itself contains an arbiter (a processor) that decides
      • The arbiter might become a bottleneck, and this is also slightly more expensive
    • Distributed arbitration
      • Devices themselves to determine who gets to use the bus, usually based on a priority scheme, possibly unfair
    • Distributed arbitration using collision detection
      • It’s a free-for-all, but if a device detects that another device is using the bus, this device waits a short amount of time before trying again
i o subsystem
I/O Subsystem
  • There are many different types of I/O devices, collectively known as the I/O Subsystem
  • Since I/O devices can vary in their speed and usage, the CPU does not directly control these devices
  • Instead, I/Omodules, orinterfaces, take the CPU commands and pass them on to their I/O devices
how to communicate with the right i o device
How to communicate with the right I/O device?
  • To communicate to the right I/O device, the CPU addresses the device through one of two forms
        • Memory-mapped I/O
        • Isolated I/O
memory mapped i o
Memory-mapped I/O
  • the interface has its own memory which are addressed as if they were part of main memory, so that some memory locations are not used, they are instead registers in the I/O interfaces
  • So each of these is given its own address
  • these addresses overlap those of memory so that a request issued to one of these memory addresses is actually a request of I/O, not memory and memory ignores the request. In such a system, the addresses are the earliest (say the first 5000 addresses).
isolated i o
Isolated I/O
  • In isolated I/O, the 5000 or so addresses are separate from memory, so that we need an extra control line to indicate if the address is a memory address or an I/O address. In memory-mapped I/O, the early addresses are shared so that, if one of these addresses is sent out, memory ignores it
memory organization
Memory Organization

Memory is organized into byte or word-sized blocks

  • Each block has a unique address
    • This can be envisioned as an array of cells
  • The CPU accesses memory by sending an address of the intended access and a control command to read or write
  • The memory module then responds to the request appropriately
    • A decoderis used to decode the binary address into a specific memory location
dividing memory across chips
Dividing Memory Across Chips
  • Main memory usually consists of more than one RAM chip
  • Each memory chip can store a certain amount of information
    • However, architects decide how memory is spread across these chips
    • For instance, do we want to have an entire byte on a single chip, or spread a byte across 2 or more chips?
    • Here, a word (16 bits) is stored in two chips in the same row
interleaving memory
Interleaving Memory
  • Main memory usually consists of more than one RAM chip
    • Hence if you buy memory to upgrade you buy a Memory Module
  • Access is more efficient when memory is organized into banks of chips with the addresses interleaved across the chips
  • in high-order interleaving, the high order address bits specify the memory bank/module
  • Using high-order interleave
    • The advantage of high-order interleave is that two different devices, working on two different areas of memory, can perform their memory accesses simultaneously
    • e.g., one device accesses address 5 and another accesses 31
  • low-order interleave
    • the low order bits of the address specify which memory bank contains the address of interest
    • Consecutive memory locations are on consecutive chips
    • The advantage of lower-order interleave is that several consecutive memory accesses can be performed simultaneously
    • For instance, fetching 4 consecutive instructions at one time
  • CPU performs the fetch-execute cycle on your program repeatedly without pause, until the program terminates
      • What happens if an I/O device needs attention?
      • What happens if your program tries to do an illegal operation?
      • What happens if you want to run 2 or more programs in a multitasking mode?
    • You cannot do this without interrupts
  • An interrupt
    • Is the interruption of the CPU so that it can switch its attention from your program to something else (an I/O device, the operating system)
the interrupt process
The Interrupt Process
  • At the end of each fetch-execute cycle, the CPU checks to see if an interrupt has arisen
    • Devices send interrupts to the CPU over the control bus
      • If the instruction causes an interrupt, the Interrupt Flag (in the status flags) is set
    • If an interrupt has arisen, the interrupt is handled as follows
      • The CPU saves what it was doing (PC and other important registers are saved to the run-time stack in memory)
      • The CPU figures out who raised the interrupt and executes an interrupt handler to handle that type of interrupt
        • interrupt handler is a set of code, stored in memory
      • Once the interrupt has been handled, the CPU restores the interrupted program by retrieving the values from the run-time stack
a simple computer
A Simple Computer
  • We now put all of these elements together into a reduced computer (MARIE)
  • Machine Architecture that is Really Intuitive &Easy
      • MARIE is too easy, it is not very realistic, so we will go beyond MARIE as well
      • We will explore MARIE’s
        • CPU (registers, ALU, structure)
        • Instruction set (the instructions, their format – how you specify the instruction, addressing modes used, data types available
        • Interrupts, I/O
        • Some simple programs in MARIE
marie s architecture
Data stored in binary, two’s complement

Stored programs  stores program data and instructions in same memory

16-bit word size with word addressing (you can only get words from memory, not bytes)

4K of main memory using 12 bit addresses, 16-bit data

MARIE’s Architecture
marie s architecture2


this is the only data register (16 bits)

PC (12 bits)

IR (16 bits)

Status flags


– stores the address to be sent to memory, 12 bits


stores the datum to be sent to memory or retrieved from memory, 16 bits

8-bit input and 8-bit output registers

MARIE’s Architecture
marie cpu
  • The structure of our CPU with the registers shown
  • MAR sends to memory, the MBR stores the data being sent to memory or retrieved from memory
  • InREG and OutREG receive data from and send data to I/O respectively
marie s interconnection
MARIE’s interconnection
  • The registers are interconnected, and connected with main memory through a common data bus.
  • Each device on the bus is identified by a unique number that is set on the control lines whenever that device is required to carry out an operation.
  • Separate connections are also provided between the accumulator and the memory buffer register, and the ALU and the accumulator and memory buffer register.
  • This permits data transfer between these devices without use of the main data bus.
marie s fetch execute cycle
MARIE’s Fetch-Execute Cycle
  • PC stores the location in memory of the next Instruction
  • 1) fetch instruction by sending the address to memory (PC to MAR to memory)
  • 2) memory sends back instruction over data bus, to MBR, move it to IR, increment PC
  • 3) Decode the instruction (look at op code, place 8-bit data address in MAR if needed
  • If operand (Memory value) required, fetch it from memory
    • operand is the part of a computer instruction which specifies what data is to be manipulated or operated on. A computer instruction describes an operation such as add or multiply X, while the operand specify on which X to operate as well as the value of X
  • 5) Execute instruction
  • 6) If necessary, process interrupt
marie s isa
  • A computer’s instruction set architecture specifies the format of its instructions and the primitive operations that the machine can perform.
  • The ISA is an interface between a computer’s hardware and its software.
  • Some ISAs include hundreds of different instructions for processing data and controlling program execution.
  • The MARIE ISA consists of only 13 instructions.
marie s instructions
MARIE’s Instructions
  • This is the format

of a MARIE instruction:

  • The fundamental MARIE instructions are:
MARIE has a major flaw in that all data must be stored in memory and the Memory addresses known at compile time
  • What about using pointers?
    • We have to add instructions that have indirect access to memory to allow for pointers, so we add AddI and JumpI
  • We also add Clearto clear the accumulator
  • We also add JnSto permit procedure calls (jump but also save the PC so we can return when done with the procedure)
marie s instructions1
MARIE’s Instructions
  • This is a bit pattern for a LOAD instruction as it would appear in the IR
  • We see that the
    • opcode is 1
    • address from which to load the data is 3
marie s micro operations
MARIE’s micro-operations
  • Each of our instructions actually consists of a sequence of smaller instructions called microoperations.
  • The exact sequence of microoperations that are carried out by an instruction can be specified using register transfer language (RTL).
  • In the MARIE RTL, we use the notation
    • M[X] to indicate the actual data value stored in memory
    • Location X
    •  to indicate the transfer of bytes to a register or memory location.
example of microoperations
Example of microoperations
  • The RTL for the LOAD instruction is
  • The RTL for the ADD instruction is



Cycle 1

Cycle 2




Cycle 1

Cycle 2

Cycle 3

example add 2 numbers
Example: Add 2 Numbers

This code will add the two numbers stored at memory location 104 and 105

Load 104 loads the AC with the value at 104 (0023)

Add 105 adds to the AC the value at 105 (FFE9)

Store 106 takes the value in the AC (000C) and moves it to location 106

Halt then stops the program

4 6 extending our instruction set
4.6 Extending Our Instruction Set
  • So far, all of the MARIE instructions that we have discussed use a direct addressing mode.
    • This means that the address of the operand is explicitly stated in the instruction.
  • It is often useful to employ a indirect addressing, where the address of the address of the operand is given in the instruction.
    • If you have ever used pointers in a program, you are already familiar with indirect addressing.
addi x instruction
ADDI X instruction
  • Add indirect specifies the address of the address of the operand
    • Use the value at location X as the actual address of the data operand to add to AC
  • The following RTL tells us what is happening at the register level







AC = 0020

ADDI 500

Result AC = 1021

j n s x
  • Another helpful programming tool is the use of subroutines.
  • The jump-and-store instruction, JNS, gives us limited subroutine functionality.
  • Store the PC at address X, and jump to address X+1
  • The details of the JNS instruction are given by the following RTL





AC  1



Does JNS permit recursive calls? النداءات التكرارية

  • Skips the next instruction according to the value of the AC
  • The RTL for the this instruction is the most complex in our instruction set

If IR[11 - 10] = 00 then

If AC < 0 then PC  PC + 1

else If IR[11 - 10] = 01 then

If AC =0 then PC  PC + 1

else If IR[11 - 10] = 10 then

If AC > 0 then PC  PC + 1

bit pattern for skipcond instruction
Bit pattern for SKIPCOND instruction
  • This is a bit pattern for a SKIPCOND instruction as it would appear in the IR
  • We see that the opcode is 8 and bits 11 and 10 are 10, meaning that the next instruction will be skipped if the value in the AC is greater than zero.
assemblers and assembly language
Assemblers and Assembly Language
  • Compare the machine code to the assembly code
    • You will find the assembly code much easier to decipher
      • Mnemonics instead of op codes
      • Variable names instead of memory locations
      • Labels (for branches) instead of memory locations
    • Assemblyis an intermediate language between theinstruction set (machine language) and the high-level language
    • The assembler is a program that takes an assembly language program and assembles it into machine language, much like the compiler compiles a high-level language program
discussion on assemblers
Discussion on Assemblers
  • Mnemonic instructions, such as LOAD 104, are easy for humans to write and understand.
  • They are impossible for computers to understand.
  • Assemblers translate instructions that are comprehensible to humans into the machine language that is comprehensible to computers
    • In assembly language, there is a one-to-one correspondence between a mnemonic instruction and its machine code
    • With compilers, this is not usually the case.
      • A= B+C
      • A=add(B,C)
      • A=B + C
discussion on assemblers1
Discussion on Assemblers
  • Assemblers create an object program file from mnemonic source code in two passes
    • First pass
      • the assembler assembles as much of the program is it can, while it builds a symbol tablethat contains memory references for all symbols in the program.
    • Second pass
      • the instructions are completed using the values from the symbol table.
example program
Example program
  • Note that we have included
    • twodirectivesHEX and DEC that specify the radix of the constants.
  • First pass, we have
    • a symbol table
    • partial instructions.
  • Second pass
    • the assembly is complete.
    • (35)10= 0023h
    • (-23)10= FFE9
4 6 extending our instruction set1
4.6 Extending Our Instruction Set

100 | LOAD Addr

101 | STORE Next

102 | LOAD Num

103 | SUBT One

104 | STORE Ctr

105 | CLEAR

106 |Loop LOAD Sum

107 | ADDI Next

108 | STORE Sum

109 | LOAD Next

10A | ADD One

10B | STORE Next

10C | LOAD Ctr

10D | SUBT One

10E | STORE Ctr

10F | SKIPCOND 000

110 | JUMP Loop

111 | HALT

112 |Addr HEX 118

113 |Next HEX 0

114 |Num DEC 5

115 |Sum DEC 0

116 |Ctr HEX 0

117 |One DEC 1

118 | DEC 10

119 | DEC 15

11A | DEC 2

11B | DEC 25

11C | DEC 30

control unit1
Control unit
  • Causes the fetch-execute cycle to be performed are implemented either in a hardwired form or by microprogramming
    • Hardwired
      • each operation including the execution of every machine instruction uses a decoder to translate the op code into control sequences (such as move PC to MAR, signal memory read, move MBR to IR, increment PC)
      • implement a program using digital logic components.
    • Microprogrammed
      • a small program is placed into read-only memory in the microcontroller
hardwired control unit
Hardwired control unit
  • For example, a hardwired control unit for our simple system would need a 4-to-16 decoder to decode the opcode of an instruction.
  • Input is the instruction from the IR,status flags and the system clock and output are the control signals to the registers and other devices
microprogrammed control unit
Microprogrammed Control Unit

The control store is a ROM that

stores all of the microprograms

One microprogram per fetch-execute

stage, and one per instruction in the

instruction set

Receive an instruction in IR

Start the microprogram, generating

the address, fetching the instruction

from the ROM and moving it to the

microinstruction buffer to be

decoded an executed

This process is much more time

consuming than the hardwired

unit, but is easier to implement

and more flexible

4 8 real world architectures
4.8 Real World Architectures
  • MARIE shares many features with modern architectures but it is not an accurate depiction of them.
  • In the following slides, we briefly examine two machine architectures.
    • Intel architecture, which is a CISC machine
    • MIPS, which is a RISC machine.
      • CISC- complex instruction set computer.
      • RISC- reduced instruction set computer.

We delve into the “RISC versus CISC” argument in Chapter 9.

intel architecture
Intel architecture
  • The classic Intel architecture, the 8086, was born in 1979.
  • It is a CISC architecture.
  • It was adopted by IBM for its famed PC, which was released in 1981.
  • The 8086 operated on 16-bit data words and supported 20-bit memory addresses.
  • The 8086 had
    • four 16-bit general-purpose registers that could be accessed by the half-word.
    • flags register
    • an instruction register
    • no built in floating-point processing.

What was the largest memory that the 8086 could address?

  • In 1985, Intel introduced the 32-bit 80386.
  • It also had no built-in floating-point unit.
  • The 80486, introduced in 1989, was an 80386 that had built-in floating-point processing and cache memory.
  • The 80386 and 80486 offered downward compatibility with the 8086 and 8088.
  • Software written for the smaller word systems was directed to use the lower 16 bits of the 32-bit registers.
  • Currently, Intel’s most advanced 32-bit microprocessor is the P4.
  • It can run as fast as 3.06 GHz. This clock rate is over 350 times faster than that of the 8086.
  • Speed enhancing features include multilevel cache and instruction pipelining.
  • Intel, along with many others, is marrying many of the ideas of RISC architectures with microprocessors that are largely CISC.
mips architecture
MIPS architecture
  • The MIPS family of CPUs has been one of the most successful in its class.
  • In 1986 the first MIPS CPU was announced.
  • It had a 32-bit word size and could address 4GB of memory.
  • Over the years, MIPS processors have been used in general purpose computers as well as in games.
  • The MIPS architecture now offers 32- and 64-bit versions.
  • MIPS was one of the first RISC microprocessors.
  • The original MIPS architecture had only 55 different instructions, as compared with the 8086 which had over 100.
  • MIPS was designed with performance in mind: It is a load/store architecture, meaning that only the load and store instructions can access memory.
  • The large number of registers in the MIPS architecture keeps bus traffic to a minimum.
In the 1970’s, memory was expansive and small in size ,so people designed computers that would pack as many action as possible in a single instruction this saved memory space ,but added complexity.

The BCD adder that we made is a CISC since it does both add and correct in one instruction.

Microprogrammed control unit

Large number of instructions (200-500)

Instructions can do more than 1 thing (that is, an instruction could carry out 2 or more actions)

Many addressing modes

Instructions vary in length and format

This was the typical form of architecture until the mid 1980s, RISC has become more popular since then although most architectures remain CISC

Hardwired control unit

Instruction lengths fixed (usually 32 bits long)

Instruction set limited (80-100 instructions)

Instructions rely mostly on registers, memory accessed only on loads and stores

Few addressing modes

Easy to pipeline for great speedup in performance

Pure RISC machines have the following features

All RISC instructions codes have the same number of bits (Typically 32 bit)

CISC instructions can vary and have no fixed length.

Because RISC have fixed instruction code, this makes it a lot easer to be pipelined, this making it faster

The RISC instruction set includes only very simple operations that can ideally be executed in a small number of clock cycles.

These instructions can then be moved through a pipeline quickly and not hold the pipeline up

In the BCD adder we would make add and correction two instructions to make it RISC machine and have it easily pipelined

RISC instructions for reading data from memory include only a single operand load instruction and a single operand storeinstruction.

Because of this RISC machines are referred to as “load and store” machines

Most CISC machines have a single instruction to load the operand from memory and then add operand to a register Note doing two things with a single instruction. Just like in our BCD adder

Two advantages of a simple load and store

machine code only takes a few bits

machine code can be quickly decoded, which makes pipelines easier to design

The RISC instruction set is designed so that compilers can efficiently translate high-level language constructer to the instruction codes for the machine, and the result is easily portable from one machine to anther.

Compiler writers took little advantage of powerful CISC instructions because they were very machine specific and then not very portable this ended up having compilers that acted like they were on a RISC machine even though they were in CISC machine because this mad it portable

A problem with pure RISC is that it takes many small instructions to do anything, which uses a lot of memory; this excessive use of memory is cold “code bloat”

Apure RISC machine also requires a greater memory bandwidth because we are constantly microprocessors are between RISC and CISC(a combination of both), in order to reduce code bloating.