Book Summary: Code

Best Friends

• Childhood Communication:
  • As 10-year-olds, the narrator and their best friend living across the street would communicate through gestures and body language at their bedroom windows even after bedtime.
  • With no electronic devices available, they had to find creative ways to continue their conversations in the dark, such as using flashlights.
• Inventing a Flashlight Communication System:
  • The narrator and their friend tried using flashlights to draw letters in the air, but this proved too imprecise.
  • They then devised a system where each letter corresponded to a specific number of flashlight blinks, but this was inefficient.
  • Eventually, they discovered Morse code, which uses a combination of short and long blinks to represent letters, numbers, and punctuation more efficiently.
• Understanding Morse Code:
  • Morse code assigns shorter codes to more commonly used letters, making it more efficient than the narrator's initial system.
  • Pauses between dots, dashes, letters, and words are crucial for proper communication in Morse code.
  • Morse code can also be vocalized using the sounds "dih" for dots and "dah" for dashes, further reducing speech to two basic elements.
• Codes and Communication:
  • The chapter introduces the concept of codes as systems for transferring information, which is essential for understanding computer languages and communication.
  • Codes can take various forms, from spoken language to sign language to Braille, each serving a specific purpose.
  • Computers also use their own codes to store and communicate different types of information, which will be explored further in the book.

Codes and Combinations

• Morse Code Invention:
  • Morse code was invented around 1837 by Samuel Morse and further developed by others.
  • Morse code is closely tied to the invention of the telegraph.
• Sending vs. Receiving Morse Code:
  • Sending Morse code is easier than receiving and translating it back into words.
  • Translating Morse code sequences into letters is challenging without memorizing the code-to-letter mapping.
• Organizing Morse Codes:
  • Grouping codes by the number of dots and dashes provides a more logical organization.
  • The number of codes doubles with each additional dot or dash, following a pattern of powers of 2.
• Treelike Diagram for Decoding:
  • A treelike diagram can visually represent the Morse code sequences and their corresponding letters.
  • Following the arrows from left to right allows decoding of a Morse code sequence.
• Expansion of Morse Code:
  • Morse code can be expanded to longer sequences of up to six dots and dashes, providing a total of 126 possible codes.
  • The number of possible codes follows the formula: 2^(number of dots and dashes).
• Morse Code as a Binary System:
  • Morse code is considered a binary system, as it consists of only two components: dots and dashes.
  • The binary nature of Morse code and similar systems is described by powers of two.

Braille and Binary Codes

• Louis Braille's System:
  • Louis Braille, a blind French teenager, developed a system of raised dots on paper that could be read by touch.
  • Braille's system used a 6-dot cell pattern to represent letters, numbers, and punctuation marks.
  • The system was clever in its use of redundancy to allow for imprecision in punching and reading the dots.
• Expanding the Braille Code:
  • Over time, the Braille code has been expanded to include contractions, abbreviations, and codes for mathematics and music.
  • The current most common system, Grade 2 Braille, uses various combinations of the 6 dots to represent a larger set of symbols.
  • The Braille system employs shift and escape codes to alter the meaning of subsequent codes, similar to the Shift key on a keyboard.
• 8-Dot Braille:
  • An 8-dot Braille system has been used for specialized applications like music, stenography, and Japanese characters.
  • The 8-dot system increases the number of unique codes to 256, allowing for a more comprehensive representation without the need for shift and escape codes.

Anatomy of a Flashlight

• The Basics:
  • A flashlight consists of batteries, a lightbulb, a switch, and a case to hold everything together.
  • Most modern flashlights use light-emitting diodes (LEDs) instead of incandescent lightbulbs.
• Incandescent Lightbulbs:
  • Inside an incandescent bulb is a filament made of tungsten that glows when electricity is applied.
  • The bulb is filled with an inert gas to prevent the tungsten from burning up.
  • The filament is connected to thin wires attached to the base and tip of the bulb.
• Electrical Circuits:
  • An electrical circuit must be a continuous loop for electricity to flow and light up the bulb.
  • The switch controls the flow of electricity by opening or closing the circuit.
• Electrons and Electricity:
  • Electricity is the movement of electrons from atom to atom.
  • Batteries generate electricity through a chemical reaction that creates an imbalance of electrons.
  • Conductors like copper allow electrons to flow easily, while insulators like rubber and plastic resist the flow of electrons.
• Voltage, Current, and Resistance:
  • Voltage is the potential for doing work, current is the flow of electrons, and resistance is the tendency to impede electron flow.
  • Ohm's law states that current is equal to voltage divided by resistance.
  • Wattage measures the power consumed, and is calculated as voltage times current.

Communicating Around Corners

• Long-Distance Flashlight Telegraph:
  • Two friends devise a long-distance communication system using flashlights and wires.
  • When one friend closes a switch, a light turns on in the other friend's room, allowing them to communicate using Morse code.
  • They experiment with reducing the number of wires by using a "common" connection between the two circuits.
• Using the Earth as a Conductor:
  • The author introduces the concept of using the Earth as a conductor, rather than using a wire, to complete the circuit.
  • This is achieved by connecting one end of the circuit to a metal pole or pipe buried in the ground, creating a "ground" or "earth" connection.
  • Using the Earth as a conductor reduces the number of wires needed between the two locations.
• Limitations of Wire-Based Communication:
  • As the distance increases, the resistance in the wires becomes a problem, reducing the current and making it difficult to power the lightbulbs.
  • Thicker wires can help, but become expensive over long distances.
  • Increasing the voltage and using higher-resistance lightbulbs are other potential solutions.
  • These problems were faced by the pioneers of the telegraph system in the 19th century, who needed to find a way to communicate over thousands of miles.

Logic with Switches

• Aristotle's Logic and Syllogisms:
  • Aristotle's Organon is the earliest extensive writing on the topic of logic, which was considered a form of philosophy.
  • Aristotle's logic was based on the syllogism - two premises leading to a conclusion.
  • Syllogisms can be complex, with unexpected conclusions not obvious from the premises.
• Boole's Boolean Algebra:
  • George Boole, a 19th-century mathematician, developed a new type of abstract algebra, later called Boolean algebra.
  • In Boolean algebra, letters represent classes or sets, not numbers, and the operations of union and intersection replace addition and multiplication.
  • Boolean algebra provides a mathematical way to represent and solve logical syllogisms.
• Switches and Boolean Logic:
  • Switches connected in series perform the logical AND operation, while switches in parallel perform the logical OR operation.
  • A circuit of switches can be used to physically represent and test Boolean expressions.
  • Boole did not witness the realization of his algebra in electrical circuits, as the incandescent lightbulb was invented after his death.

Telegraphs and Relays

• Samuel Morse's Invention:
  • Morse, an artist and early photography enthusiast, is best known for inventing the telegraph and the Morse code.
  • In 1844, Morse demonstrated the successful transmission of the biblical message "What hath God wrought!" between Washington, D.C. and Baltimore using his telegraph system.
  • The telegraph system included a "key" for sending messages and a "sounder" for receiving them by listening to the clicks.
• Challenges of Long-Distance Telegraphy:
  • As the length of telegraph wires increased, the resistance to electrical flow became a major impediment.
  • Morse's telegraph system was more tolerant of poor line conditions compared to other designs.
  • The solution to long-distance telegraphy was the introduction of a "relay" or "repeater" system, which could amplify weak incoming signals and retransmit them.
• The Relay Device:
  • The relay is a switch triggered by an incoming electrical current, which then turns on a stronger outgoing current.
  • Relays could be used to assemble many components of a computer, foreshadowing their later importance in the development of computing technology.

Relays and Gates

• Computers and Boolean Algebra:
  • Computers are a synthesis of Boolean algebra (mathematical logic) and electricity.
  • Logic gates are the crucial components that embody this melding of math and hardware.
  • Logic gates perform simple operations in Boolean logic by blocking or allowing the flow of electrical current.
• Relays and Logic Gates:
  • Relays can be used to build logic gates like AND, OR, and NOT gates.
  • Relays can be wired in series (AND gate) or parallel (OR gate) to perform logical operations.
  • The inverter gate (NOT gate) is built using the normally closed contact of a relay.
• Additional Logic Gates:
  • The NOR gate is the inverse of the OR gate, and the NAND gate is the inverse of the AND gate.
  • These gates, along with the inverter and buffer, make up the six standard logic gates.
  • The logic gates can be combined to create more complex circuits.
• De Morgan's Laws:
  • De Morgan's laws show the equivalence between AND/OR and NAND/NOR gates.
  • These laws are useful for simplifying Boolean expressions and circuits.
• Moving Forward:
  • The next project is to build a digital adding machine using the logic gates.
  • Before that, the book will cover the fundamentals of counting in the next few chapters.

Our Ten Digits

• Universality of Numbers:
  • Numbers are the most abstract codes we deal with regularly.
  • Regardless of language, numbers are written the same way globally.
  • Mathematics is considered the "universal language" due to the ubiquity of numbers.
• Origin of Number Systems:
  • Numbers were originally invented to count things like possessions and transactions.
  • Early number systems used symbols or tally marks to represent quantities.
  • Roman numerals, though still in use, are limited in mathematical operations.
• The Hindu-Arabic Number System:
  • Brought to Europe by Arab mathematicians, this system is positional, using digits that represent different quantities based on their position.
  • Crucial innovations include the lack of a special symbol for ten and the introduction of the zero, enabling more advanced mathematics.
  • The structure of Hindu-Arabic numbers is reflected in how we pronounce and write them, using place values and powers of ten.
• Advantages of the Hindu-Arabic System:
  • Allows for efficient addition, subtraction, multiplication, and division by breaking down operations into simpler steps.
  • Can be applied to non-decimal number systems, demonstrating the system's flexibility.
  • The elegance of the underlying structure is often overlooked due to its familiarity.

Alternative 10s

• Base Ten vs. Other Number Systems:
  • Our decimal number system is based on the number of fingers we have, but other number systems are possible.
  • An octal (base 8) system would be more natural for beings with 4 fingers, while a quaternary (base 4) system would suit lobsters with 2 pincers.
  • Binary (base 2) is well-suited for digital systems that only have 0 and 1 to work with.
• Converting Between Number Systems:
  • Decimal numbers can be converted to binary, octal, or quaternary by repeatedly dividing and taking the remainders.
  • Binary numbers can be expressed more concisely in octal by grouping them in threes.
  • Simple rules govern basic arithmetic operations like addition and multiplication in non-decimal systems.
• Binary Circuits and Computers:
  • Binary digits (bits) can be represented by switches, lights, and logic gates, enabling digital computers to manipulate numbers.
  • Circuits like decoders and encoders can translate between binary and other number representations.
  • The simplicity and efficiency of binary arithmetic is a key reason it has become so central to computing technology.

Bit by Bit by Bit

• The Power of Simple Signals:
  • The story of the yellow ribbon conveys a simple yes/no signal.
  • This binary approach can be represented using a single bit, where 0 means no and 1 means yes.
  • More complex information can be conveyed using multiple bits.
• Binary as the Simplest Number System:
  • Binary, with its two digits 0 and 1, is the simplest number system possible.
  • Bits, or binary digits, are the basic building blocks of information.
  • The number of possible codes with N bits is 2^N, allowing for increasingly complex information to be encoded.
• Bits in Visual Displays:
  • The parachute pattern on the Perseverance rover uses bits to encode a message.
  • Barcodes like the UPC encode information using a series of black bars and white spaces, representing bits.
  • Two-dimensional barcodes like QR codes use a grid of black and white squares to store more information.
• Encoding Information in Bits:
  • Barcodes use sophisticated encoding schemes with error-checking mechanisms.
  • QR codes can encode text, URLs, and other data using 8-bit character encoding.
  • Bits are the fundamental building blocks of digital communication, allowing complex information to be represented and transmitted.

Bytes and Hexadecimal

• Grouping Bits into Words:
  • Computer systems often group bits into a quantity called a "word" for convenience in moving and manipulating data.
  • Early computer systems used word lengths that were multiples of 6 bits, which could be easily represented in octal.
  • However, the computer industry eventually settled on 8-bit bytes as the standard, as they are a good balance between being too small and too large.
• Introducing Hexadecimal:
  • Hexadecimal is a base 16 number system used to represent the values of bytes more succinctly than binary.
  • Hexadecimal digits range from 0-9 and A-F, with each hexadecimal digit representing 4 bits.
  • Hexadecimal is commonly used to represent color values in HTML and to express long binary numbers in a more compact form.
• Converting Between Number Systems:
  • Conversion between binary, decimal, and hexadecimal can be done using division and multiplication by powers of the base.
  • Templates are provided for converting decimal numbers to hexadecimal and vice versa.
  • Hexadecimal addition can be performed using a lookup table and normal carry rules.
• Tools for Working with Hexadecimal:
  • Calculator apps often have a "Programmer" mode that allows working with binary, octal, and hexadecimal numbers.
  • Building an 8-bit binary adder, as described in a later chapter, can also help understand hexadecimal arithmetic.

From ASCII to Unicode

• ASCII and the Importance of Character Encoding:
  • ASCII is the most vital computer standard, enabling text-based communication across different computer systems.
  • ASCII is a 7-bit code representing 128 characters, including the English alphabet, numbers, and basic punctuation.
  • ASCII was dominant, but IBM developed its own 8-bit code called EBCDIC, leading to compatibility issues.
• The Limitations of ASCII:
  • ASCII is focused on English and lacks support for accented letters, non-Latin scripts, and ideographic languages like Chinese.
  • Various extended ASCII character sets were developed to accommodate more languages, but they were incompatible with each other.
• The Development of Unicode:
  • Unicode was created as a unified standard to represent all the world's written languages.
  • Unicode is a 16-bit (later 21-bit) code that can represent over 1 million characters.
  • Unicode incorporates existing standards and supports a wide range of scripts, including Latin, Cyrillic, Chinese, Japanese, and many others.
• Unicode Transformation Formats (UTFs):
  • UTF-8 is the most widely used Unicode transformation format, maintaining compatibility with ASCII.
  • UTF-8 uses variable-length encoding, where ASCII characters are represented by 1 byte, while other characters use 2, 3, or 4 bytes.
  • The variable-length encoding of UTF-8 can sometimes lead to encoding issues, which is demonstrated by the example of the email with the "⯙" characters.

Adding with Logic Gates

• Building a Binary Adding Machine:
  • Addition is the fundamental arithmetic operation for computers.
  • The adding machine will be built using simple electrical components like switches, lightbulbs, and logic gates.
  • The machine will add 8-bit binary numbers using a control panel with switches for input and lightbulbs for output.
• Using Logic Gates for Binary Addition:
  • The carry bit can be generated using an AND gate.
  • The sum bit can be generated using an XOR gate, which is a combination of OR, NAND, and AND gates.
  • A half adder circuit can add two binary digits, but a full adder is needed to handle a carry bit from the previous column.
• Building the 8-Bit Binary Adder:
  • The 8-bit adder is constructed using eight full adder circuits, with each carry output connected to the carry input of the next more significant column.
  • The least significant column's carry input is set to 0, while the final carry output lights up an additional ninth bulb.
  • The 8-bit adder can be further expanded to create a 16-bit adder by cascading two 8-bit adders.
• Understanding Binary Number Representation:
  • Binary numbers are represented with subscripts indicating the place value of each bit, starting from the least significant bit 0 to the most significant bit.
  • The decimal equivalent of a binary number can be calculated by multiplying each bit by its corresponding power of 2 and summing the results.

Is This for Real?

• Relays to Vacuum Tubes to Transistors:
  • Early digital computers were built using relays, which were later replaced by vacuum tubes.
  • Vacuum tubes had drawbacks like being expensive, power-hungry, and prone to burning out.
  • The transistor, invented in 1947, ushered in solid-state electronics and enabled smaller, more reliable, and less power-hungry computers.
• The Integrated Circuit:
  • In the late 1950s, the integrated circuit (or "chip") was invented, allowing multiple transistors and other components to be fabricated on a single piece of silicon.
  • Integrated circuits enabled further miniaturization and cost reduction of electronic devices.
  • The 7400 series of TTL (transistor-transistor logic) chips provided pre-wired logic gates that could be used to build larger digital circuits.
• The History of Computing:
  • Early mechanical and relay-based computers, like those of Babbage and Zuse, paved the way for later electronic computers.
  • The ENIAC, completed in 1945, was the first major electronic computer, using 18,000 vacuum tubes.
  • The concept of the "stored-program" computer, articulated by John von Neumann, became the basis for modern computer architecture.
• Moore's Law and Integrated Circuit Families:
  • Gordon Moore observed that the number of transistors on a chip doubled every year (later revised to every 18 months), driving rapid improvements in computing power.
  • Two major integrated circuit families emerged in the 1970s: TTL (fast but power-hungry) and CMOS (slower but more power-efficient).
  • Integrated circuits like the 7400 series provided pre-designed logic gates that could be used to build larger digital circuits.

But What About Subtraction?

• The Difference Between Addition and Subtraction:
  • Addition involves a consistent march from right to left, with each carry added to the next column.
  • Subtraction involves a messy back-and-forth process of borrowing, which requires a different mechanism.
• Avoiding Borrowing with the Nines' Complement:
  • The nines' complement of a number can be used to subtract without borrowing.
  • Subtracting a number is the same as adding its nines' complement.
  • This approach works by adding 1 and subtracting the larger number at the end.
• Subtracting Larger Numbers from Smaller Numbers:
  • When subtracting a larger number from a smaller number, the result is a negative number.
  • This can be handled by switching the order of the numbers, adding 999, and then subtracting 1000 at the end.
  • The final result is the nines' complement of the smaller number, which represents the negative value.
• Implementing Subtraction in Binary:
  • The ones' complement of a binary number can be used in a similar way to the nines' complement in decimal.
  • A circuit with XOR gates is used to invert the bits of the second number when performing subtraction.
  • An extra carry-in bit is also added to the adder circuit to complete the subtraction.
• Representing Negative Numbers with Two's Complement:
  • Two's complement is the standard way to represent positive and negative numbers in computers.
  • In two's complement, numbers that start with 1 represent negative values.
  • Positive and negative numbers can be freely added using the rules of addition, without the need for subtraction.
  • Overflow can occur when adding two numbers of the same sign, resulting in a number outside the valid range.

Feedback and Flip-Flops

• Relays and Oscillators:
  • Relays can be wired to create oscillators that alternately open and close a circuit, generating a repetitive buzzing or ringing sound.
  • The output of an oscillator quickly alternates between 0 and 1, forming a clock signal that can be used to synchronize other circuits.
  • The frequency of an oscillator is measured in hertz (Hz), indicating the number of cycles per second.
• The R-S Flip-Flop:
  • The R-S (Reset-Set) flip-flop is a simple memory circuit that can "remember" which of two inputs was most recently activated.
  • The R-S flip-flop has two stable states, allowing it to retain information even when its inputs are inactive.
  • The R-S flip-flop is the foundation for more advanced flip-flop circuits.
• The D-Type Flip-Flop:
  • The D-type flip-flop is a more versatile memory circuit that stores the value of its Data input when the Clock input changes.
  • Level-triggered D-type flip-flops store the Data input whenever the Clock is high, while edge-triggered D-type flip-flops store the Data input only when the Clock makes a transition from low to high.
  • Edge-triggered flip-flops are more useful for building counters and other synchronous circuits.
• Frequency Dividers and Counters:
  • By connecting the output of a flip-flop back to its input, a frequency divider circuit can be created, where the output frequency is half the input frequency.
  • Cascading multiple frequency dividers forms a binary counter that can count up to higher numbers.
  • Counters are essential components for measuring the frequency of an oscillator and keeping time in digital circuits.
• Clear and Preset Signals:
  • Flip-flops can be enhanced with Clear and Preset signals to force the output to 0 or 1 regardless of the other inputs.
  • These signals are useful for initializing counters and other circuits to a known state.

Let's Build a Clock!

• Building a Digital Clock:
  • Rather than a traditional grandfather clock, the clocks in this chapter will be digital clocks that display the time in numbers.
  • The first version will display the time in binary-coded decimal (BCD), where each decimal digit is represented by a 4-bit binary number.
  • This allows for easier conversion from binary to decimal, compared to displaying the time in pure binary.
• Binary-Coded Decimal (BCD):
  • In BCD, each decimal digit is encoded as a 4-bit binary number, ranging from 0000 (0) to 1001 (9).
  • The additional binary numbers from 1010 to 1111 are not used in BCD.
  • BCD is often used for displaying decimal numbers, even though it complicates basic arithmetic operations.
• Building the Clock Circuitry:
  • The clock is built using multiple stages of flip-flops, with each stage representing a decimal digit of the time.
  • NAND gates are used to clear the flip-flops when the appropriate BCD value is reached, allowing the clock to cycle through the valid decimal values.
  • Additional logic, including AND and XOR gates, is used to handle the transitions between seconds, minutes, and hours.
• Displaying the Time:
  • The clock can display the time using binary lights, but this is not very user-friendly.
  • Alternative display options include Nixie tubes and seven-segment displays, which require additional decoding circuitry to convert the BCD values to the appropriate signals for the displays.
  • Dot matrix displays can also be used, with the individual LEDs controlled through a diode matrix and transistor-based "sinker" circuits.
• Manual Time Setting:
  • The clock can be designed with two switches to manually increment the minutes and hours, allowing for easy time setting without complex button sequences.
  • This is achieved using XOR gates to invert the normal "1 minute period" and "1 hour period" signals when the switches are pressed.

An Assemblage of Memory

• Memory and Recollection:
  • As we wake up each day, memory fills in the gaps about our lives and surroundings.
  • Human memory is not perfectly orderly - it can be fragmented and prone to lapses.
  • Writing was invented to compensate for the fallibility of human memory.
• Storing and Retrieving Information:
  • Different media have been used to store information, from paper to magnetic tapes to computer memory.
  • A flip-flop circuit can store 1 bit of information, with a Write signal triggering the storage.
  • Multiple flip-flops can be wired together to store bytes and larger units of data.
• Random Access Memory (RAM):
  • RAM allows storing and retrieving data at any memory address, unlike sequential memory technologies.
  • A 3-to-8 decoder and 8-to-1 selector enable addressing and accessing individual bits in an 8-bit RAM.
  • Larger RAM arrays are built by combining multiple 8-bit RAM units and decoders.
• Addressing and Tri-State Buffers:
  • Tri-state buffers enable connecting multiple outputs without causing short circuits.
  • Increasing the number of address bits exponentially expands the memory capacity.
  • Memory sizes are described using binary prefixes like kilobyte, megabyte, gigabyte, etc.
• A Memory Control Panel:
  • A control panel with switches and lights can be used to interact with and manage the 64KB RAM array.
  • The Takeover switch allows the control panel to have exclusive control over the memory.
  • The control panel design is similar to early home computers like the Altair 8800.

Automating Arithmetic

• Automating Addition:
  • The Automated Accumulating Adder uses an 8-bit adder, a 16-bit counter, and memory to automate the addition of numbers.
  • Numbers are stored in memory, and the adder adds them sequentially while the counter provides the memory addresses.
  • Control signals like the Clock and Write signals are needed to coordinate the memory access and addition.
  • The Automated Accumulating Adder is limited to 8-bit values, which can only represent numbers up to 255.
• Expanding to 3-Byte Values:
  • The Triple-Byte Accumulator extends the design to handle 3-byte values, allowing for larger numbers up to $83,886.07.
  • It uses instruction codes (opcodes) to indicate whether to add, subtract, write the result, or halt the process.
  • The hardware includes latches, tri-state buffers, and control logic to manage the 3-byte values and instruction codes.
  • The design demonstrates the connection between hardware and software, where the hardware implements simple tasks and the software combines them into more complex operations.
• The Rise of Microprocessors:
  • The development of the first microprocessors, such as the Intel 4004, 8008, and 8080, and the Motorola 6800 and 6502, enabled the creation of personal computers.
  • These early microprocessors had 8-bit architectures and could access up to 64 KB of memory, allowing them to perform a variety of tasks.
  • The microprocessors differed in their instruction sets and endianness (the order of storing multi-byte values).
  • In the next chapter, the author will attempt to build a simplified version of the Intel 8080 microprocessor using the basic components covered so far.

The Arithmetic Logic Unit

• The CPU and its Components:
  • The CPU consists of memory, the central processing unit (CPU), and input/output (I/O) devices.
  • The CPU is often referred to as the "heart", "soul", or "brain" of the computer.
  • The CPU to be built in this book is an 8-bit processor that can address 64K of random-access memory.
• Arithmetic and Logic Operations:
  • The CPU must be able to perform basic arithmetic operations like addition and subtraction on bytes.
  • Addition and subtraction with carry/borrow are necessary to work with multi-byte numbers.
  • The Arithmetic Logic Unit (ALU) is the part of the CPU that performs these arithmetic and logical operations.
• Bitwise Logical Operations:
  • The ALU also needs to perform bitwise logical operations like AND, OR, and XOR.
  • These operations are useful for tasks like converting text to uppercase or lowercase.
  • The ALU has function inputs to select the desired operation (add, subtract, AND, OR, XOR, compare).
• Flags and the Compare Operation:
  • The ALU maintains flags like the Carry, Zero, and Sign flags to track the results of operations.
  • The Compare operation subtracts two values but only updates the flags, not the result.
  • Comparing values is important for tasks like searching for text on a webpage.
• The Complete ALU:
  • The ALU combines the Add/Subtract module and the Logic module with supporting circuitry for the flags.
  • The final ALU design is a self-contained component that can be easily integrated into the CPU.

Registers and Busses

• Importance of Moving Bytes:
  • Bytes are constantly moved within the CPU during operations like loading, saving, and processing data.
  • This movement of bytes is essential, even though it is not as glamorous as the number crunching of the ALU.
• Registers:
  • The CPU uses a set of 8-bit latches called registers to store bytes as they are processed.
  • The accumulator (register A) is the most important register, as it is always one of the inputs to the ALU.
  • Other registers include B, C, D, E, H, and L.
  • Registers H and L are often used together to form a 16-bit memory address (indirect addressing).
• Instruction Codes and Assembly Language:
  • Instruction codes are made up of patterns like 0011 1000 where the last 3 bits indicate the destination register.
  • Assembly language mnemonics like "ADD E" provide a more concise way to refer to instruction codes.
  • Move immediate (MVI), arithmetic/logical immediate, and move (MOV) instructions are introduced.
• Register Array Circuit:
  • The register array circuit uses decoders to select which register to read from or write to.
  • It requires additional logic to handle the accumulator and the H/L register pair independently.
• Address Bus:
  • In addition to the 8-bit data bus, the CPU also requires a 16-bit address bus to access memory.
  • The address can come from the program counter, instruction bytes, or the H/L register pair.
  • The register array needs to be enhanced to allow the H/L registers to access the address bus.
  • Additional 16-bit increment/decrement circuitry is required for the program counter and H/L registers.

CPU Control Signals

• CPU Components and Busses:
  • The CPU includes an ALU, register array, program counter, and instruction latches.
  • These components are connected via an 8-bit data bus and a 16-bit address bus.
• Control Signals:
  • Control signals enable values to be put on the busses and saved in the components.
  • Signals that put values on the bus enable tri-state buffers, while signals that save values control the clock inputs of latches.
  • Coordinating these control signals allows the CPU to execute instructions stored in memory.
• Instruction Execution Process:
  • The CPU goes through a sequence of machine cycles to fetch and execute instructions.
  • Fetch cycles read instruction bytes from memory and store them in latches.
  • Execution cycles use the instruction bytes to control the components and perform the desired operation.
• Decoding the Opcode:
  • The opcode in the first instruction byte is decoded to determine the instruction type and number of bytes.
  • Decoders and logic circuits generate control signals based on the opcode.
  • This allows the CPU to properly fetch and execute each type of instruction.
• Timing and Synchronization:
  • An oscillator provides the basic clock signal that drives the CPU's operation.
  • Flip-flops and counters generate the Cycle Clock and Pulse signals that synchronize the control signals.
  • The Cycle Clock determines when new machine cycles occur, while the Pulse signal controls when values are latched.
• Implementing Control Signals:
  • Diode ROM matrices are used to efficiently generate the control signals needed for each instruction.
  • Separate matrices handle the address bus and data bus control signals for the various execution cycles.
  • This modular approach makes the control logic easier to design and understand.

Loops, Jumps, and Calls

• Importance of Repetition in Computing: Repetitive tasks such as adding large numbers together are well-suited for computers. Ada Lovelace recognized the need for "cycles" or loops in early computing.
• Jump Instructions: The JMP instruction allows the CPU to jump to a different memory address, enabling looping behavior. Conditional jump instructions like JZ and JNZ allow the program to jump based on flags set by the ALU.
• Implementing Loops: Loops can be implemented using a counter variable that is decremented until it reaches zero, at which point the loop terminates.
• Multiplication Algorithm: Multiplication can be implemented efficiently using bit shifting and addition, rather than repeated addition. This algorithm reduces the number of operations required.
• Subroutines and the Stack: Subroutines allow reuse of common code segments. The CALL and RET instructions, along with the stack, enable subroutines by saving and restoring the return address.
• The Significance of Conditional Jumps: Conditional jumps are a fundamental feature that makes computers Turing-complete, enabling them to perform any computable task.

Peripherals

• The Central Role of Peripherals:
  • The CPU is the most important component, but it must be supplemented with other hardware like memory, input devices, and output devices.
  • These input/output (I/O) devices and other hardware components are collectively known as peripherals.
• Video Displays:
  • Video displays create images composed of rows and columns of pixels, each capable of displaying a different color.
  • The resolution of a display is defined by the number of horizontal and vertical pixels.
  • Video memory stores the pixel data, which is refreshed sequentially to prevent flickering.
  • Video display adapters control the process of refreshing the display.
• Memory-Mapped I/O:
  • Peripherals can be accessed through the same memory space as the CPU, a concept known as memory-mapped I/O.
  • Alternatively, a separate I/O bus can be used to communicate with peripherals, with special instructions like IN and OUT for accessing I/O ports.
  • Interrupts allow peripherals to signal the CPU, enabling more efficient and responsive I/O processing.
• Analog-to-Digital and Digital-to-Analog Conversion:
  • ADCs convert analog signals like sound and light into digital values, while DACs perform the reverse conversion.
  • These devices enable computers to interact with the analog real world.
• Bitmap and Audio Compression:
  • Bitmaps can be compressed using techniques like run-length encoding, GIF, PNG, and JPEG.
  • Audio can be compressed using techniques like MP3, which exploit psychoacoustic properties to reduce data while preserving perceived quality.
• Mass Storage Devices:
  • Computers require long-term storage for programs and data, provided by devices like floppy disks, hard drives, and solid-state drives.
  • Storage devices are divided into fixed-size sectors, which are used to store files in a non-contiguous manner.
  • The operating system manages the complex task of storing and retrieving files from mass storage devices.

The Operating System

• The Need for Software: When a computer is first powered on, the microprocessor begins executing random bytes in memory, which is not productive. Software is needed to provide the initial instructions for the computer to execute.
• Entering Machine Code: Early computers required manually entering machine code through a control panel, which was a tedious process. This led to the development of a keyboard handler and command processor to make code entry easier.
• The Role of ROM and File Systems: To avoid losing programs when power is turned off, machine code can be stored in read-only memory (ROM). File systems were later developed to organize programs and data on disk storage.
• CP/M Operating System: CP/M was an influential 8-bit operating system that provided a file system and standard interface for applications. It served as a model for later operating systems like MS-DOS.
• The Graphical User Interface: The pioneering work on graphical user interfaces (GUIs) at Xerox PARC influenced later systems like the Apple Macintosh and Microsoft Windows, which provided a more intuitive visual computing environment.
• The Influence of UNIX: The portable, text-oriented design of the UNIX operating system has had a lasting impact on operating system philosophy and the development of systems like Linux.

Coding

• Assembly Language and Machine Code:
  • Computers execute machine code, a series of simple instructions specific to a CPU.
  • Assembly language provides human-readable mnemonics that correspond to machine code instructions.
  • Assemblers convert assembly language programs into machine code that the CPU can execute.
• High-Level Programming Languages:
  • High-level languages, such as FORTRAN, COBOL, ALGOL, and BASIC, abstract away low-level machine details.
  • High-level languages allow programmers to write code in a more natural, human-readable way.
  • Compilers translate high-level language programs into machine code that can be executed by the computer.
• JavaScript and Web Development:
  • JavaScript is a high-level language used to add interactivity and dynamic behavior to web pages.
  • JavaScript code can be embedded directly in HTML files and executed by web browsers.
  • The chapter provides examples of simple JavaScript programs that demonstrate variables, loops, and mathematical operations.
• Floating-Point Arithmetic:
  • Computers represent floating-point numbers using the IEEE 754 standard, which defines a binary format for storing and manipulating real numbers.
  • The IEEE 754 format includes a sign bit, an exponent, and a significand (or mantissa), allowing for a wide range and precision of floating-point values.
  • Floating-point arithmetic can sometimes produce unexpected results due to the limitations of the IEEE 754 representation, such as the inability to represent certain decimal fractions exactly.

The World Brain

• H.G. Wells' Vision:
  • In the 1930s, H.G. Wells proposed the concept of a "World Encyclopedia" - a global repository of knowledge to guide humanity.
  • Wells envisioned this World Encyclopedia as a "mental clearing house" that would provide a "common interpretation of reality" and lead to the "mental unification" of the world.
  • He believed science, rationality, and knowledge were the best tools to shape a better future for the world.
• Vannevar Bush's "Memex":
  • In 1945, engineer Vannevar Bush proposed the "memex" - a machine that could store and interconnect information in an associative manner, anticipating hypertext and the internet.
  • Bush recognized the growing "mountain of research" and the need for better tools to navigate and access information.
  • The memex would allow users to "reacquire the privilege of forgetting" by providing easy access to information without the need to remember everything.
• Ted Nelson and "Hypertext":
  • In 1965, Ted Nelson built on Bush's ideas and coined the term "hypertext" to describe a system of interconnected information that could not be conveniently presented on paper.
  • Nelson envisioned hypertext as a way to increase the student's "range of choices, sense of freedom, motivation, and intellectual grasp" through a dynamic, growing system of knowledge.
• The Emergence of the Internet:
  • The internet, built on concepts of packet switching and digital communication, became the technological realization of the visions of Wells, Bush, and Nelson.
  • The decentralized, interconnected nature of the internet enabled the creation of vast repositories of knowledge, such as Google Books, JSTOR, and Wikipedia.
  • However, the internet has also given rise to the proliferation of misinformation, conspiracy theories, and a lack of a "common interpretation of reality" as envisioned by Wells.
• Evaluating the Internet's Impact:
  • While the internet has made information more accessible, it has not automatically led to the "mental unification" or guidance of humanity that Wells hoped for.
  • The internet remains a complex and sometimes chaotic reflection of human nature, with both the potential for expanding knowledge and the challenges of misinformation and fragmentation.
  • Ultimately, the internet's impact depends on how humanity chooses to use and shape this powerful tool to realize the visions of Wells, Bush, and Nelson.