When learning assembly, it’s easy to get lost in the “why” of CPU design, but this blog will stay focused on the x86 instruction set itself. The goal here isn’t to study computer architecture or dive into microarchitectural details — instead, we’ll build a working reference for how to write and understand x86 assembly code. Everything that follows is about the x86 family of processors, starting from the registers that form the foundation of all instructions. ...

When your x86 computer first starts up, it’s in a surprisingly primitive state: No operating system - Obviously, since we haven’t loaded one yet No memory management - No virtual memory, no protection between processes No file system - Can’t open files, no directories, no abstraction layer No network stack - No TCP/IP, no internet connectivity No device drivers - No USB drivers, no graphics drivers, nothing What Services Are Available at Boot Time? Despite the barren landscape, the BIOS (Basic Input/Output System) gives us a few essential tools: ...

Introduction: The Communication Challenges At its core, a CPU is designed for one primary task: processing data and executing instructions at incredible speed. But this processing power becomes meaningful only when it can interact with the rich ecosystem of peripheral devices that extend its capabilities. Why CPUs Need to Talk to Many Different Devices? Your CPU must read input from your mouse or keyboard, process that input to understand your intent, communicate with memory to load the browser application, send rendering commands to your graphics card, request data from your network interface to load the webpage, and potentially write temporary files to your storage device. Each of these interactions involves a different type of peripheral device, each with its own communication requirements, data formats, and timing constraints. ...

The 8086 Processor A Brief History The Intel 8086, released in 1978, marked a pivotal moment in computing history as Intel’s first 16-bit microprocessor. Designed by a team led by Stephen Morse, the 8086 was Intel’s answer to the growing demand for more powerful processors that could handle larger programs and address more memory than the existing 8-bit chips of the era. The processor introduced the x86 architecture that would become the foundation for decades of computing evolution. With its 16-bit registers and 20-bit address bus 1, the 8086 could access up to 1 megabyte of memory—a massive improvement over the 64KB limitation of 8-bit processors. However, it retained backward compatibility concepts that would prove both beneficial and constraining for future generations. ...

BIOS Boot Recap Previously, we saw that after the BIOS firmware is loaded, it searches for a bootable device from a list of storage options, such as a hard drive, SSD, USB, or network interface. The BIOS identifies a valid bootable device by checking for the 0x55AA signature at the end of the first sector. Once found, it loads the 512 bytes from this sector (LBA 0), which is known as the Master Boot Record (MBR). ...

1. Power‑On & Hardware Reset 1. Power‑Good Signal The power supply stabilizes voltages and asserts a “Power‑Good” (PWR_OK) line to the motherboard. All devices receive power and begin to initialize themselves. The Central Processing Unit (CPU) is initially held in a reset mode, meaning it’s not yet executing instructions. The memory layout is powered up, although the RAM itself has no content since it’s volatile. 2. CPU Reset Vector The reset vector is a predetermined memory address where the CPU begins execution after being powered on or reset. On x86 processors, this address is typically 0xFFFFFFF0 (near the top of the 4GB address space). When the CPU comes out of reset, its program counter (instruction pointer) is automatically set to this address. The motherboard’s memory mapping ensures that this address points to the BIOS/UEFI firmware ROM chip, so the very first instruction the CPU executes comes from the firmware. ...

Representing negative numbers in binary poses unique challenges due to the inherent nature of binary systems. Unlike decimal systems, which can easily use a minus sign to indicate negative values, binary systems must encode this information within a fixed number of bits. This requirement leads to various methods of representation, each with its own set of advantages and limitations. The main challenge lies in developing a system that can accurately represent both positive and negative values while ensuring that arithmetic operations remain efficient and straightforward. In the following sections, we will explore several common approaches to representing negative numbers in binary, including their respective challenges and trade-offs. ...

MIPS (Microprocessor without Interlocked Pipeline Stages) assembly is one of the RISC ISA’s. It was developed in the early 1980s at Stanford University by Professor John L. Hennessy. MIPS is widely used in academic research and industry, particularly in computer architecture courses due to its straightforward design and in various embedded systems applications for its efficiency and performance. History The first MIPS processor, the R2000, was introduced. It implemented the MIPS I architecture, which was one of the earliest commercial RISC processors. There are multiple versions of MIPS: including MIPS I, II, III, IV, and V; as well as five releases of MIPS32/64. MIPS I had 32-bit architecture with basic instruction set and addressing modes. MIPS III introduced 64-bit architecture in 1991, increasing the address space and register width. ...

The Fetch-decode-execute cycle or instruction cycle is how CPU executes programs. During this cycle, the CPU retrieves an instruction from memory (fetch), interprets what action is required (decode), and then carries out the necessary operations to complete the instruction (execute). This cycle is crucial for the CPU to perform any computational tasks, and it repeats continuously while the computer is powered on. What is Machine Code? Machine code is the lowest-level programming language that consists of binary instructions directly executed by a CPU. Any program is compiled to a binary executable is transformed into machine code. Machine code consists of set of instructions which varies for each CPU architecture and is decided by the CPU manufacturer, eg: ARM, MIPS, x86, etc. Machine code consists of a set of instructions defined by the Instruction Set Architecture (ISA) of each CPU. The ISA, determined by the CPU manufacturer, varies across different architectures such as ARM, MIPS, and x86. This architecture-specific design means that machine code written for one type of CPU cannot be directly executed on another without translation or emulation. ...

CPU Manufacturers publish several metrics related to CPU like clock speed, number of cores, cache sizes, ISA, performance per Watt, number of transistors and more. Measuring CPU performance is complex, and it cannot be summarized by a single metric. In this post, I’ll explore each of these metrics and discuss some standard benchmarking software and their limitations. What is Clock Speed, How does it Affects CPU Performance? All Synchronous digital electronic circuits require an externally generated time reference. This is usually a square wave signal provided to the circuit called as clock. A clock cycle is the fundamental unit of time measurement for a CPU. A clock cycle is a single electrical pulse in a CPU, during which the CPU can execute a fundamental operation such as accessing memory, writing data, or fetching a new set of instructions. A clock cycle is measured as the amount of time between two pulses of an oscillator. The clock speed of a CPU is measured in Hertz (Hz), which signifies the number of clock cycles it can complete in one second. Common units are Megahertz (MHz) and Gigahertz (GHz). ...